Green Man Publishing

Potato

Solanum tuberosum

Introduction: Potatoes originated in the mountains of Central and South America and have been cultivated for over 6000 years. A huge number of varieties are still grown in that region.

The potato wasn’t initially a success when brought to Europe around 1580, probably because they were short day varieties and didn’t start to produce until late in the year (and because they were related to many poisonous plants). However better varieties were developed eventually and when people realized its many benefits its use spread throughout Europe.

The potato was introduced into North America in 1621, but didn’t become a significant food crop for almost a century until Irish settlers arrived in 1719 (in some places it is still called Irish potato to this day).

Potatoes are the fourth biggest crop in the world overall and by far the most important vegetable crop (over 320 million tonnes in 2010). This is mostly for local consumption as they are quite perishable and aren’t traded internationally like other major commodity crops.

In recent years there has been a huge increase in potato production in many third world countries. This isn’t surprising as they may yield from 10,000 to 30,000 pounds per acre and provide more usable protein per acre than any other crop (up to forty times more than cows).

Apparently only about 20% of gardeners regularly grow potatoes, I’m not really sure why this is, maybe people think they need a lot of space, are cheap to buy and don’t taste much better than shop bought.

I grow quite a lot of potatoes and find that they are actually one of the most space efficient crops you can grow, considering the amount of nutrition they provide. A 10 square foot planting can yield 20 pounds of tubers. Organic potatoes are never cheap where I live; they generally range from $1 to $2 per pound and at those prices growing potatoes becomes a pretty good deal.

Home grown potatoes also taste better than those you buy and are one of the great treats of the summer garden. Also over 80% of conventional potatoes contain some pesticide reside, so it’s good to eat organic ones.

Crop value: Potatoes are the single most useful garden crop from a self-sufficiency standpoint, because of the combination of their exceptional nutritional value, their productivity, ease of growing and ability to store well. They are unique as the only garden crop you could live on (at least for a while). At one time Irish peasants really did live on a diet of potatoes and milk and their population increased rapidly (until the potato famine). 

I enjoy growing potatoes and find them to be one of the most rewarding crops of all. They emerge quickly, don’t need much attention and harvesting is so much fun that even my children used to help.

Potatoes are also a beneficial crop for the garden, as their growth and harvest loosens the soil, improves its tilth and suppresses weeds. This feature makes them useful as pioneer plants for starting a garden, or for reclaiming rough land (see Unusual growing methods).

If you have a small garden I would say that they are only cost effective if you have a fairly fertile soil and treat them well. If you are getting less than a pound of potatoes per square foot, it may not be worth it.

If you are ambitious and intend to grow a lot of potatoes it is important that you have somewhere to store them all. They are fairly perishable and won’t keep well if you don’t give them the right conditions. The last thing you need is a ton of rotting potatoes (for some reason they smell particularly bad).

Ease of growing: Potatoes are easy to grow if they have the right conditions. The only time I have had major problems was in Washington state when they were afflicted with late blight, due to wet weather.

Nutritional content: The potato is a substantial and nutritious food. Most people know it is a major source of carbohydrates, but are less aware that it is an important source of protein too. It also contains a lot of vitamin C, several B vitamins and the minerals copper, iron and potassium. These all make an important contribution to the diet because it is eaten in quantity (this isn’t horseradish) Apparently blue / purple potatoes also contain valuable anthocyanin phytonutrients.

Potatoes are a major energy food, with about 350 calories per pound.

Climate: Potatoes are native to tropical mountains and prefer mild temperatures, ideally in the range of 60 – 70°F. They grow best in fairly dry climates and can be affected by a variety of diseases when growing in wet conditions. In mild winter areas, with few frosts, they are sometimes grown in late fall or early spring.

Potato plants don’t like frost or cold weather and don’t do well if it is too hot. Soil temperatures above 70°F inhibit tuber formation and this stops altogether if it gets much above 80°F).

Having said all that, potatoes are actually quite adaptable and can do well in a surprising range of situations.

About Potatoes  
Planning facts
Perennial
Hardiness: Half hardy
Soil temp for planting: 45 (55 – 65) 70°F
Growing temp: 60 – 70°F day
                           45 – 50°F night
Plants per person: 15 – 20
Plants per sq ft: 1
Plant: 2 – 4 wks before last frost date
Days to harvest: 90 – 140 days  

Harvest facts
Yield per plant: 1 – 2 lb
per sq ft: 1 – 2 lb    
          per 10 ft row: 6 – 17 lb  

Soil

pH 4.8 (5.5 ideal) 6.5

Potatoes will grow and produce well in most soil types, even those that are too acidic for most crops. For best results they prefer light, deep, well-drained sandy soils. They like a more acid soil than most vegetables, as it increases yield and decreases the incidence of scab (a disease that mostly occurs when the pH is above 6.0.)

Potatoes don’t like heavy clay or rocky soil because the tubers can’t easily expand as they grow, Nor do they like wet soil, which can cause all kinds of problems with disease and rotting. You can solve both of these problems fairly easily by creating raised beds of loose, well-drained soil.

Soil preparation: Potatoes aren’t a fussy crop, but they respond well to soil improvement. They like loose soil, so if yours is heavy or compacted, deep cultivation such as double digging is beneficial.

Deep digging also enables you to add the all important organic matter in the form of 3˝ of compost, leaf mold or aged manure (some people avoid manure saying it encourages scab, but I haven’t found any problem with composted manure). All types of soil will be improved by the addition of organic matter.

If organic matter is in short supply then simply put some in the planting trench or in each planting hole. A fertilizer mix is also a good idea (or alfalfa or seaweed meal).

Never lime the soil when planting potatoes. If the soil is too alkaline then add sulphur, pine needles or another acidifying agent to lower the pH.

Potatoes are so adaptable and tolerant that they are often used as the first crop when establishing a new garden.

If your soil is really bad (or you don’t have any at all) you can grow potatoes in raised beds, pots or grow bags. 

Fertilization: Potatoes are fast growing plants and respond well to high fertility.

An old practice is to provide nutrients to potatoes in the form of fresh plant material. Wilted comfrey leaves or seaweed were laid in the trench along with the tubers (up to a pound per tuber), to feed the plants as they decompose. You can also plant the tubers directly into a newly incorporated green manure or cover crop.

Nitrogen: Potatoes aren’t big nitrogen users and too much can result in abundant top growth, but fewer (and inferior) tubers. A potato rotation is often scheduled to follow a heavy nitrogen user like corn.

Potatoes do need some nitrogen however, especially in the first few weeks, when they are putting on a lot of leaf growth. They use 75% of all the nitrogen they need in the first 4 weeks of growth. Too little nitrogen may result in the premature production of small tubers, so use your judgment.

Potatoes aren’t very good at getting nitrogen, so it should be in an easily available form (compost, seed meal, comfrey).

Phosphorus: Potatoes should have a good supply of phosphorus, though they aren’t heavy users. Use colloidal phosphate and compost.

Potassium: This is the most important primary nutrient for potatoes, increasing yield, improving quality and hastening maturation. Potatoes need a steady supply of potassium throughout their lives, but especially when their tubers are forming. Adding 5 pounds of wood ashes per 100 square feet of bed can increase yields by as much as 30%. If using wood ashes would raise the pH too much, you could use greensand, though this isn’t as easily assimilated.

Calcium: Potatoes also like calcium, though most sources run the risk of raising the ph, which isn’t good. Gypsum doesn’t do this though and can be applied at 5 lb/100 sq ft.

Planning

Potatoes are usually propagated vegetatively from tubers, or pieces of tubers. These are known as seed potatoes, though they aren’t actually seeds at all.

Crop rotation: Potatoes are susceptible to the host of diseases that affect members of the Solanum family. Don’t plant them where tomato, pepper or eggplant have been grown within the last 3 years. If scab is a problem avoid planting after beets too.

Where: Potatoes need full sun if they are going to be really productive, so make sure they have a prime garden spot.

If you plan to grow potatoes as a staple food you will have to devote a significant portion of your growing space to them. At the same time you also have to rotate them for at least 3 years, so they usually form a separate rotation group of their own.

When: Potatoes grow best in mild temperatures (below 70°F) and don’t tolerate heat well (especially when forming tubers). The soil temperature should ideally be between 55 – 70°F. In areas with hot summers they are usually grown as a spring or fall crop. In mild climates they can be grown all summer.

The chitted tubers can go in the ground as early as 6 weeks before the last frost (though 2 – 4 weeks is more common). The young plants will tolerate a light frost, especially if covered with straw. Even if they are damaged they should recover quickly.

Potatoes are tropical plants and evolved to grow with short day length, so they grow best in spring and fall. Long summer days may actually delay tuber formation somewhat. In warmer areas you may be able to plant them as early as February (if you can find seed potatoes to plant).

Succession sowing: You may want to grow several crops in succession to maintain a constant supply of new potatoes. You can also plant early, maincrop and late varieties at the same time.

Early crop: The minimum soil temperature for planting potatoes is 45°F, though 50 – 55°F is better.

The first crops can be started as early as 6 weeks before the last frost date, though you must take care to protect them from any frost. This is pretty easy when they are barely poking out of the ground; just cover them with soil, mulch, row covers, cloches, tarps or old blankets (some varieties can even take mild frost). 

Main crop: This is usually planted around the last frost date (2 weeks before to 2 weeks after). Don’t leave it too long as long days can slow down maturation.

Late crop: In many areas you can get two crops of potatoes a year. Time the second crop to mature around the time of the first fall frost (the shorter days actually hasten tuber formation).

It can be a problem to find seed potatoes for this second planting, as they generally disappear from stores after the spring planting season. You might have to buy them in spring and store until required. You can also store some of your spring harvest in the fridge (temperatures below 50°F will help to overcome their dormancy period). Chitting them isn’t really necessary when planting into warm soil.

Where I live finding seed potatoes isn’t that easy even in spring, unless you want to buy a pound of them in a little mesh bag for $5.00 (which has a significant impact on the economics of growing them.) Apparently a lot of stores are reluctant to carry them in bulk because they are so perishable. It is good to buy yours early, when you first see them for sale. They may as well sit in your house as in the store.

Before planting

Certified disease free tubers:

You can grow perfectly good potatoes using old potatoes from the market (supposedly they are often sprayed to prevent them sprouting, but they do sprout eventually). The problem is that these may be infected with virus diseases, which will then become established in your garden. Once a virus is established in your garden it is there to stay and can infect every subsequent crop (and maybe even tomatoes, peppers and eggplants as well).

Because of the potential for disease problems most authorities recommend planting only certified disease free tubers. Its hard to get optimal yields from poor quality seed potatoes, no matter how good your soil and cultural practices. I used to think this was just a way to get you to buy much more expensive seed potatoes, but since I have had a problem from replanting infected tubers I know better. It can waste a lot more in time and effort than you save in money.

Selecting seed potatoes:

A tuber is not a root. It is a swollen stem adapted to be a food storage organ and has a small scar on one end where it was attached to the plant by the stolon. The other end of the tuber (the rose end) has a cluster of dormant buds known as eyes, which have the ability to grow into new plants. There are also eyes in other parts of the tuber and you can cut one tuber into several pieces. You just have to make sure that each piece has an eye that can grow (even a potato peeling can grow if it has an eye). However the rose end contains the most vigorous shoots.

There is much debate over the ideal size of a seed potato (this is the kind of thing that makes gardening so exciting), but it is smaller than many people think. Most books recommend 2 – 4˝ (3 – 4 oz) tubers, saying smaller ones produce smaller plants and hence smaller tubers and lower yields. They say that larger tubers produce bigger plants and hence larger tubers. However it is now thought that whole tubers of 1 – 3 oz are ideal.

Agricultural researchers in England obtained their highest yields by planting very small (⅓ oz) tubers very close together (only 9˝ apart). They found that larger tubers (spaced further apart) sent up several shoots that essentially became separate plants and eventually competed with each other. Using smaller tubers can also reduce seed potato costs significantly.

Of course many gardeners get the same results by cutting larger tubers into several pieces, each with 2 or 3 eyes. However cut pieces are more prone to rot unless they are left for a few days, so their cut surfaces can dry out and toughen. They can also be dusted with sulphur powder and left for 24 hours in indirect light. These measures usually work out okay, but take time. It would be simpler to just use smaller tubers (called single drops by farmers) in the first place.

If you do cut the tubers it is best done after chitting, as it results in some moisture loss (and of course you can cut the tubers according to the sprouts they have made). Some experts frown on cutting seed potatoes for early crops which will grow in cool soil because of the greater potential for infection and rotting.

When you buy seed potatoes they shouldn’t have started to sprout very much, as the brittle and delicate shoots are easily damaged once they start to elongate.

When you get your seed potatoes home, put them in a cool, humid place until you are ready to chit or plant them. To prevent premature sprouting store seed potatoes at 40 – 50°F. If they already have sprouts take care not to break them off (the first sprouts to appear are the strongest and best).

Sets: If you buy seed potatoes by mail order they may very well be in the form of sets. These consist of a single eye from a tuber with a small plug of potato attached.

Sets are not a very satisfactory way to grow potatoes. Even if you are fortunate enough to get sets in good shape, they won’t perform as well as whole tubers with their large reservoir of food to draw from. They may also be coated with fungicide for interstate shipment.

Chitting:

It is to your advantage to plant tubers that already have healthy shoots, because it gets them off to a faster start and so reduces the chance of rot. This is most important for early planting, as the tubers may sprout slowly in cool spring soil. Later plantings don’t need chitting as they will sprout rapidly in the warm soil.

About 2 – 3 weeks before you wish to plant the tubers, you should start chitting (sprouting) them. Do this by setting them out, rose end up, in indirect light (not direct sunlight, but still fairly bright- never dark) at a temperature of 55 – 65°F (warmer temperatures may cause them to shrivel).

The aim is to get 2 – 4 sprouts each 1 – 2˝ long on each tuber. If you can’t plant them out as soon as they get to this stage, you should then return them to cooler conditions. If they sprout very slowly you can speed this up by giving them warmer conditions for a while. Don’t worry if the tubers turn green, the solanine produced may help to prevent them rotting.

You should rub off any excess sprouts (above the 2 – 4 required), as soon as they start to sprout, so the tuber doesn’t put too much energy into them. Leaving too many sprouts can mean smaller potatoes. Of course if you are going to cut a tuber into pieces you need some sprouts on each piece, so don’t rub them off until after you cut them up.

Some gardeners allow the shoots to grow to 6 – 8˝ in length, claiming this increases yields by up to a third and reduces the time to harvest. However the long sprouts are easily damaged and must be handled very carefully.

It is actually possible to remove the sprouts from the tuber and plant them separately (or leave one or two on the tuber and plant the others separately). This might be worth trying if you only have a couple of unusual tubers. It is also possible to eat part of the tuber and plant the peel, so long as it has some eyes on it. Of course these techniques you give you lower yields and they will take longer to mature.

Planting

Planting depth

New tubers only form above the old one, so the deeper the tuber is set into the ground, the higher the potential yield. This is one reason potatoes were traditionally hilled up.

The actual planting depth varies according to soil and season. In England (where spring weather is cool) they tend to plant their potatoes quite shallowly (2 – 3˝) and hill them up later. They do this because the tubers can easily rot if planted too deep in cool soil.

In warm soil you can plant the tubers much deeper. They might be 4˝ deep in a heavy soil and up to 8˝ deep in a light soil.

Rows or beds: If your soil consists of very fertile and loose raised beds and you don’t plan on earthing them up, you can grow potatoes on equidistant spacing. If you plan on hilling them up you should grow them in rows.

Spacing: Potatoes are grown at a wide variety of spacings, depending upon the soil, water availability, and other factors.

Spacing is the biggest factor in determining the final tuber size. The closer the spacing, the more competition and the smaller the tubers (however you get more of them). Researchers have found the optimal spacing for highest yield per square foot (of fairly small tubers) is two plants per square foot, which averages out to be about 9˝ apart. This could be worth trying, unless you really want large tubers.

Closer spacings are used for early potatoes, or when you have ideal growing conditions, don’t mind smaller tubers, or when space is limited. They don’t work if the soil isn’t very fertile, as potatoes don’t like having to compete with each other.

Wider spacings are used for maincrop potatoes, when the soil isn’t very fertile, when water is limited, when you want large tubers, or if you have lots of room and don’t need high yields per square foot. You might be able to grow a fast growing intercrop (such as spinach or mustard) in between the widely spaced plants to help increase productivity.

Bed spacing: The traditional spacing ranges from 9˝ – 12˝ – 15˝ depending upon variety, tuber size and soil fertility. You may want to start with 12˝ and see how that works.

Row spacing: Traditionally they are planted 8˝ – 12˝ – 15˝ in the rows, with 18˝ – 24˝ – 36˝ between the rows. Start with 12 x 24˝ and see how that works.

Dry gardening: If you want to grow potatoes without irrigation you might try 18˝ between plants and 48 – 60˝ between rows.

Planting methods

Rows/ trenches: The traditional way to plant potatoes is in trenches and this is probably still the best way if you want to grow them in bulk as a staple food. One advantage of planting in rows is that it is easy to hill them up later. A potential disadvantage in wet weather is the soil in the trench may stay wet for longer.

Begin by digging a trench one spade deep and one spade wide (put the soil you remove evenly on both sides for later earthing up). Put all of the amendments that potatoes love (compost, wilted comfrey leaves, seaweed, wood ashes, alfalfa pellets, greensand, fertilizer mix) into the bottom of the trench.

The tubers are then placed in the trench at the desired spacing and about 4˝ of soil is pulled back into the trench to cover them. When the plants reach 6˝ in height you can fill the trench up with soil.

Beds: If you don’t want to use trenches you can simply dig holes in the wide beds, at the desired spacing (use a bulb planter to speed this up). You then simply place your tubers in the holes and cover with 3 – 4˝ of a mix of compost and soil. If the soil is cold don’t water the bed until the plants have emerged, to reduce potential rotting problems.

Growth pattern

The growth of a potato plant can be divided into four distinct stages.

Vegetative stage

For the first 30 – 70 days a potato plant produces main shoots and lots of foliage. The larger the plant at the end of this stage, the larger the eventual yield can be. Watch out for pests such as Colorado potato beetle during this stage. Vegetative growth goes along best during the long days of early summer.

Tuber formation

After 70 – 90 days of vegetative growth the main shoots stop growing and side branching occurs. At this time tubers start to form on stolons coming from the feeder roots. A soil temperature of 60 – 70°F is said to be optimal for tuber formation and it slows down as the temperature goes above this, until it stops altogether at above 80°F (hilling can help to keep the soil cooler).

Tuber formation usually coincides with the onset of flowering and is a good indicator that tuber formation has begun. It is not physiologically related however and in some situations flowers may not appear at all.

Tuber enlargement

As the plants come into full bloom the tubers enlarge rapidly and the plant has its greatest need for potassium. This is also the most critical time for water and for maximum growth they need a steady supply. You can start digging new potatoes at this stage and mine often get no further than this. Fungus diseases sometimes attack plants at this time.

Maturation

When the plants reach maturity the tops wither and die back and the skins on the tubers thicken (this is important for storage). When 75% of the foliage is dead, water them for the last time, wait 10 – 14 days and they are ready to dig.

Care

Hilling: If you planted the tubers at a shallow depth you should hill them (also known as earthing up) as this can greatly improve the crop. It increases the depth of soil for tuber formation and ensures they aren’t exposed to the sun (which would turn them green). It is also a good way to eliminate weeds and can help to keep the soil cool in hot weather. It also prevents the plants sprawling and gives the tubers nice loose soil to grow in.

Hill up the plants when they are 6 – 8˝ tall, by burying the bottom half of the plant. You should repeat this 2 – 3 weeks later and perhaps a third time several weeks after that (adding a couple of inches more soil each time, until the hill is 6˝ tall). Don’t over-do the hilling though or you can reduce their ability to produce food by burying food producing foliage.

Hilling up is easy if you planted in widely spaced rows, but not possible if you planted in beds (use mulch instead).

Mulch: Potatoes are commonly mulched with compost, shredded leaves, hay or seaweed (potatoes have a special affinity for seaweed). This conserves moisture and helps to keep the soil cool, which is important in warmer areas.

Of course you can’t use mulch if you plan on hilling up the plants, but a thick mulch can actually be used instead of hilling (see Unusual growing methods).

It is best to avoid mulch if you have problems with slugs.

Weeding: Potatoes are vigorous plants and can compete against weeds pretty well. However you should weed while the plants are small. Do this carefully so you don’t damage the shallow roots. When you earth up the plants you will eliminate all of the weeds in the bed.

Feeding: The young plants need nitrogen for fast uninterrupted growth. Give them a foliar feed of compost tea, comfrey tea or liquid kelp, 3 – 4 weeks after the shoots emerge from the soil and are 4 – 6˝ tall (or side dress with fertilizer mix). This could also help to remedy any minor nutrient deficiency.

For maximum yields (or in poor soil) they should receive a foliar feed every 2 weeks until they start to flower.

Water: It is important to keep the soil evenly moist (but not wet) for best growth, as lack of water results in poor yields of small tubers. It is also important to water uniformly, making sure it penetrates through the dense foliage and down to the full root depth of 18˝ (or at least the top 12˝ where the greatest proportion of roots are found).

Potatoes should get at least an inch of water per week, though the exact quantity will depend upon the weather. Continue watering regularly until the tubers are almost ready for harvest and then stop.

If water is in short supply, just give them 4 gallons per square yard at the crucial time when the tubers start to form (when the flowers appear). In humid climates many gardeners stop watering when tuber formation starts, so they don’t grow too fast. Excess water may cause hollow heart, where the interior grows so rapidly it cracks (this is most often seen in the bigger potatoes).

Try not to get the foliage wet when watering as potatoes are very vulnerable to fungus diseases. Drip irrigation is best for this reason. Otherwise water in the morning or early evening, so foliage can dry out quickly (you don’t want it to stay wet all night).

Problems

Pests

The potato has more than its fair share of insect pests (aphids, blister beetles, nematodes, leafhoppers, tuberworms, wireworms, flea beetles and more). Fortunately these aren’t found everywhere; in some favored areas potatoes have few problems and are very easy to grow.

The severity of potato pests also varies from year to year, with different growing conditions. In some years they do little harm, in other years they can be devastating. Warm humid conditions are the worst for potatoes.

The best thing you can do for your plants is keep them well fed, so they can (hopefully) deal with pests and diseases as they arise.

Colorado potato beetle: Both adults and larvae feed on potato leaves and they can be a big problem if they get out of hand. On a small scale you can simply hand pick off any beetles you find and scrape off the tiny orange egg masses from under the leaves (and any newly hatched larvae). The larvae are eaten by many predators, though the adults are fairly poisonous.

Diseases

The potatoes is prone to more than its fair share of diseases, a few of which are listed below.

Viruses: Virus infection may show itself as pale, distorted or mottled leaves and stunted plants, but often there are no obvious symptoms except reduced yield (this can progressively decrease each time they are planted). You may avoid these by using certified seed and not saving your own tubers for planting. 

Removing viruses: It is possible to get a virus-free plant from an infected tuber. You plant the tuber in a container of sterile potting mix and keep it in a warm place to grow. When the shoot reaches 6 – 8˝ high you cut it off 2 – 3˝ above the soil line (it should never touch the soil or the rest of the tuber). The shoot can then be rooted in another container of sterile potting mix. It will then hopefully be virus-free, just hope that your garden is also, when you plant it out.

Verticillium wilt: This fungus shows itself by the tops dying off prematurely (it’s also known as early dying fungus). You may still get a small crop of potatoes from affected plants, but they won’t store well. This disease may last for 7 years in the soil and to eliminate it you can’t plant potatoes in the same spot for at least 4 years. Other members of the Solanum family are also affected, so they can’t be grown either (except for a few resistant varieties).

Scab (Streptomyces scabies)

This very common disease is caused by a fungus in the soil. It is undetectable above ground and the damage is mainly cosmetic, so it is not very serious unless you are growing for market (it reduces their marketability).

Alkaline soil (above 6.0 pH) and lack of moisture are the main causes of scab. It persists in the ground for several years and can also infect other root crops such as carrot, beet and turnip. The best ways to prevent scab is to rotate annually and to keep the pH of the soil somewhat acid (below 5.6 pH), so don’t lime it. Abundant water may reduce damage from scab.

If your garden has conditions that encourage scab, you may want to use a resistant cultivar.

Late blight (Phytopthora infestans): This is the disease that caused the famine that depopulated Ireland, by killing one and a half million people and causing another million to emigrate. It is called late blight because it likes warmer weather and usually occurs after tomatoes (which are also susceptible) have flowered. It doesn’t much bother early crops, so planting early is a good preventative.

This fungus first manifests itself as spots on the lower leaves in cool, wet weather, but then the leaves die and brown patches appear on the tubers. The only thing you can do is dig the tubers 2 weeks after the tops die down and use them. This disease affects yield, but doesn’t affect storability (of course you wouldn’t use infected potatoes for seed. Many modern varieties have some resistance to late blight.

Early Blight (Alternaria solanii)

This fungus disease occurs earlier in the season and isn’t as big a problem as late blight. It appears as irregular shaped dark brown concentric spots on the shaded lower leaves. These slowly enlarge and merge until badly infected leaves eventually die (those at the bottom of the plant first and then progressing upward). If you recognize it early enough you may be able to treat with Bordeaux mixture.

You can minimize its effects by keeping plants well fed and watered, by removing crop debris, keeping leaves dry, preventing soil from splashing onto leaves and removing infected plants.

Blackleg (Pectobacterium carotovorum): This bacterial disease most often occurs in cool/warm wet weather when the plants are growing well (or even flowering). It shows itself as black slimy decay around the base of the stem (hence Blackleg). Leaves turn yellow and then brown and eventually die. The tuber rots from the stem end and becomes a slimy, smelly mass.

Though infection most often results from infected seed potatoes, the bacteria may also enter the plant through wounds in the tuber (such as from scab or insect damage).

To avoid this disease use certified disease-free seed potatoes. Use whole tubers (rather than cutting them up) and rotate them annually. Also remove any volunteers from the ground.

Bacterial ring rot (Clavibacter michiganense ssp sepedonicus)

The first symptom of this bacterial disease is yellowing and browning of leaves and wilting of some stems (though usually not all). In mild cases there may not be any obvious symptoms until you cut a tuber in half and see the characteristic ring of discoloration (in mild cases) or rot (in severe cases). Though it doesn’t kill the plants it is significant because it makes the tubers unsalable (and in severe cases unusable).

This disease can only live in living plants (it can’t survive in the soil) and infection is usually the result of planting diseased tubers (it can be spread to healthy tubers by the process of cutting them into smaller pieces with an infected knife). If you save your own seed potatoes it is important that you don’t plant any that contain this disease (the best safeguard is to plant only certified seed potatoes). You should also remove any volunteers from the garden, as well as any plants that show signs of wilting, before they can spread infection to other plants.

Harvesting

When: You can start harvesting new potatoes 70 – 90 days after planting (after the plants have been flowering for a while). Just root around beneath the living plants until you find some sizeable tubers.

New potatoes taste great, but taking them reduces the final yield, so only take a couple from each plant (if you really like them, then grow some plants specially for this). The skins of new potatoes are very thin and they are high in sugar, so they don’t store well.

The main potato harvest begins when the leaves start to lose their green color and die back (late in the season they may be killed by frost before this happens). If you want to store the tubers you should leave them in the ground for 2 weeks after the tops turn yellow and die down. This allows the skins to toughen up. If the skin rubs off easily with a finger they are not ready to store. Once the tubers are mature you should dig them, otherwise they may eventually start to sprout again.

It is a lot easier and less messy to harvest potatoes when the soil is fairly dry.

How: Digging the tubers is a very rewarding activity. It feels like digging for buried treasure (which it kind of is), but is a lot more fruitful. Dig the tubers with a spade or spading fork, starting at least a foot away from the plants to minimize accidental spearing. Tubers will always be found above the seed potato (which is usually still recognizable), but may be some distance to one side.

Some people like to dig a hole alongside the first plant and then pull the plant over into it. The second plant then goes into the hole left by the first one (this method ensures thorough soil cultivation). 

Always handle the tubers gently to minimize damage. Even the slightest skin abrasion can cause a tuber to rot in storage and this can spread to nearby tubers.

When you have finished digging, let them dry out and sort the tubers into three piles: badly damaged (speared or chopped) ones for immediate eating, grazed ones for use in the near future and perfect ones for storage. You will also sort out any partially green potatoes that have been exposed to light and are inedible.

Green tubers: Any tubers that are not fully covered with soil or mulch will turn green from exposure to light. These are mildly toxic and the common advice is that they shouldn’t be eaten (though you can often cut off the green parts). However recent studies show that most of this is concentrated in the skin, rather than the flesh. This suggests that they could be peeled and eaten in an emergency – though a better idea might be to save them for planting.

Storage

Careful storage is very important with potatoes. If they are not given ideal conditions they will soon become inedible, due to rotting, turning green or sprouting. Even under the best conditions they will gradually deteriorate over time (your job is to maximize this period).

Unlike most other root vegetables potatoes can’t be left in the ground until needed (a few weeks is okay). If the soil is fairly warm and moist they will sprout as soon as their dormancy period of 2 – 3 months is over. If it is cold and wet they may rot, or develop diseases such as scab. If the soil freezes they will probably rot.

Temperature is the most critical storage factor. If conditions are too warm (above 50°F) they will sprout as soon as their natural dormancy period is over in 2 or 3 months (of course this won’t matter if you only have a 2 months supply of tubers to store). If it is too cold (below 40°F) their starch may turn to sugar and give them an off flavor (apparently you can convert this back into starch by storing them above 65°F for a couple of weeks). The fridge isn’t a good place for potatoes as it is too cold.

Prepare the tubers by air-drying in a dark place for several days (don’t wash) and then store them at 60°F for two weeks to cure. They should then be stored at 40 – 50°F with high humidity (90%). Keep them in wooden boxes, or sacks, with good air circulation (never in plastic bags) and check periodically for rot. Keep them in the dark of course, or they will turn green.

Properly stored potatoes can last for at least 5 – 6 months, though you should keep checking them for signs of rot or deterioration. Potatoes that are sprouting can still be eaten, just rub off the sprouts.

Other storage options for potatoes are somewhat limited, as they can’t easily be frozen, dried or canned.

 Clamp: Large quantities of potatoes can be stored over the winter in a clamp. This works best in light, well-drained soil and should be in a sheltered position.

Start by digging out the soil in the area of the clamp to a depth of 10˝ and then lay down a 3 – 6˝ layer of straw or dry leaves (you might first lay down a layer of gopher wire to foil rodents).

A piece of perforated pipe is arranged in the center and the roots are placed around it to form a cone or prism shaped pile (a vent can also be constructed from straw). The pile is then covered with a 6˝ (more in very cold climates) layer of straw, or leaves.

Finally the straw is covered with a 6˝ layer of soil, which is packed down with a spade. Some of this soil comes from the original excavation; the rest is obtained by digging a drainage trench around the clamp. Keep the vent open on top of the clamp, unless it gets very cold, in which case it should be closed up with straw.

Saving seed (potatoes): When we talk about seed saving with potatoes we are usually talking about seed potatoes, rather than actual seed. Saving your own seed potatoes can save you a bit of money if you grow a lot, as well as the trouble of having to find them to plant. It also helps you to another level of food self-sufficiency, as you don’t have to depend upon anyone else. It is also appealing if you want to grow unusual varieties that are hard to find.

Unfortunately there is a problem with saving your own tubers and it is frowned upon for the same reason as using supermarket tubers: it can lead to problems with disease. Plant an infected tuber and every tuber the plants produces will be infected at least as badly and perhaps worse. The disease may also spread to healthy plants via insects or infected tools.

The advisability of saving your own seed potatoes largely depends upon where you live and how much disease you have encountered. It worked well when I lived up in Washington, but down here in warmer California I have encountered serious problems with disease, notably bacterial ring rot. After one totally unproductive attempt I went back to buying certified seed potatoes.

If you want to save your own seed potatoes, just choose the best tubers, check them for disease symptoms and store them very carefully instead of eating them.

Saving true potato seed: You may want to experiment with saving true potato seed as well (see below for more on these). The fruits are produced readily in many cases, so all you have to do it allow them to ripen (this takes about 2 months from flowering). Squeeze out the seeds into a bowl and wash them. The good seeds sink and bad ones float. 

If you grow seed from a vegetatively propagated variety they won’t come true to type, but will produce entirely new varieties. If you like any of these you can propagate them vegetatively and name the new variety after yourself.

There are some true potato seed varieties out there that come true to type, but they are not easy to find.

Companion plants: Some gardeners interplant marigolds or beans with potatoes as a way to repel Colorado beetles. Researchers have found that the presence of these plants in a stand of potatoes does seem to confuse the insects (and most importantly there are fewer beetles to be found).

Of course any time you plant anything among the potatoes, it is going to be disturbed and uprooted while harvesting. This means that any edible interplanted crop must be harvested before the potatoes are ready. It will also take up space, water and nutrients that the potatoes could have used.

Unusual growing ideas

True potato seed: When I first started gardening many years ago there was much fanfare around the introduction of true potato seed (the first variety was named Explorer I believe), as the wave of the future. As a young and inexperienced gardener at the time, I thought it was a silly idea that wouldn’t catch on. I reasoned that one of the advantages of growing potatoes is that you don’t have to start with tiny seeds. I was right that it didn’t catch on, but wrong in thinking it was a silly idea. Growing from true potato seed actually has some significant advantages.

Advantages of true potato seed   The seed will be much less likely to be carrying any kind of disease   You can start the seed at any time you need potatoes (often seed potatoes are only readily available in spring).   They are a lot cheaper. A packet of seed (that might produce 100 plants) would only cost the same as a pound or two of seed potatoes.   True potato seed can also be stored for several years, whereas seed potatoes are very perishable. (They are an obvious choice for preppers and their survival gardens).   You can save your own seed quite easily.  

Potatoes are no harder to grow from seed than the related tomato and are treated in pretty much the same way. Often you don’t get a very big harvest the first year, but use these to grow larger crops in subsequent years

I think true potato seed will become important in the future, as it allows you to save them from year to year without having to worry about disease.

New potatoes: If you have lots of seed potatoes you could grow some plants specifically to produce delicious new potatoes. These can be planted as close as 6˝ apart, as they will be harvested as soon the new potatoes form.

Mulch planting: Potatoes will grow quite happily in a layer of mulch instead of soil. This also makes it easy to take a few new potatoes from the living plants; you simply pull the mulch aside. The only real problem with growing in mulch is getting the large quantity of mulch material needed.  

You can just put the seed potatoes on the ground and lay a 3˝ layer of mulch (compost, straw, chopped leaves, aged manure) on top of them. Add more mulch as the plants get taller, until it is 8 – 12˝ deep. This is necessary to keep light from turning the tubers green and because tubers only form above the seed potato.

Another method starts the previous fall, when you pile chopped tree leaves where you want the potato patch to be (do it as you clean them up). The following spring you plant sprouted tubers 6˝ deep in the leaves. Hill up the plants with more mulch as the pile settles and the plants grow.

You can also combine mulching with trench planting. Lay the tubers at the bottom of a 12˝ deep trench and cover with 3˝ of chopped leaves. As the plants grow keep filling up the trench with more leaves.

Land clearing: Potatoes are the best vegetable crop for starting a new garden on uncultivated ground. Simply dig trenches and plant as described above. The amending, digging, hilling and harvesting will loosen and improve the soil and make it more suitable for other crops. 

You can also use mulch to start a new garden on grass or weed infested land. Simply put the tubers on the ground and cover with a thick 3˝ layer of mulch. As the plants grow you add more mulch. The combination of a thick mulch, deep shade and the considerable soil disturbance will eliminate existing plants and leave you with a nice clear bed of loose soil.

Container growing: Potatoes are well suited to growing in containers and if you do it right they can be surprisingly productive. This is the way to go if you only have a small space (or just a patio or balcony)

Container growing isn’t just a novelty, or for patio gardeners. It can also give you a useful way to multiply one special tuber. It could also be used to grow your own seed potatoes, as it gives you a much greater control over the spread of disease. It is also a good way to get very early or very late potatoes.

Yellow Finn and Red Pontiac both work well for this. Early varieties don’t work so well, as you want types that continue to produce more tubers, rather than setting them all at once.

How: You can grow a single potato to be enormously productive in the following way. Obtain a large garbage can, put drainage holes in the bottom and fill it with a foot of good soil mix (try equal parts compost, good garden soil and sand).

Plant one large seed potato in the soil. As the plant grows, slowly fill the can with more fine compost, always covering only a third of the plant. With a little luck the end result will be one very large plant, completely filling the whole can with tubers. When the plant dies back, empty out the can and collect the tubers.

The most important thing when growing in containers is to water carefully, too much, or too little, water will cause problems.

A refinement of this is to use tires (or slatted wooden bins). Start with one tire filled with soil and as the plant grows add more tires and soil. The advantage of this method is that the plant gets maximum light at all times and is never growing in the bottom of a can.

Another variation on this is to use a bottomless wooden box, or even a wire cage 18 x 18˝ in size (these can even be stood on driveways).

You can also use a standard 15 gallon plastic plant pot or a large fiber pot.

Grow bags: Many European urban gardeners have discovered the joys of growing potatoes in plastic bags. They often buy bags of prepared mix, but its easy to make your own. This is very similar to container growing.

Start with a large tough garbage bag, put some holes in the bottom for drainage and half fill it with a mix of equal parts sandy soil and compost. They need to get good sun, so fold down the edges of the bag. You then plant two chitted potatoes in the bag and add water. As the plants grow you fill the bag with more of the soil / compost mix. Be careful to keep them well watered (not too little or too much).

Dry gardening: In most areas a spring potato crop can be produced without irrigation, as there is usually a lot of moisture in the soil from the winter. You simply space the plants further apart than normal (18 – 24˝). Your yields may be slightly smaller, but the tubers will contain less water and so be more nutritious and better flavored. They may also store better as their skins tend to be tougher. This can give you an easy, low cost way to grow an important food crop.

You need to keep moisture robbing weeds under control when doing this. A mulch is good if you have enough of it.

Autumn planting: In mild areas any tubers overlooked during the harvest will survive the winter underground and volunteer the following year. This shows that it is possible to plant potatoes in autumn for a spring harvest. You might use some of the small tubers harvested earlier in the year. Plant them in October or November for harvest in spring.

Fall planting is something of a gamble depending upon the amount of frost and rain you get. It didn’t work for me because the plants became horribly infected with powdery scab (a disease I had never even seen before).

Varieties

Though only a half dozen varieties are widely cultivated commercially, there are an enormous number of potato cultivars, with different shapes, sizes and colors (white, yellow, red, blue) and other attributes (waxy, starchy or all purpose). Some do better on heavy soils, some are more resistant to cold or disease, some contain more vitamin C, protein or antioxidants and some taste better.

Generally the heavy yielding commercial varieties tend to contain more water, but farmers like them because they get paid just as much for water as for potato. Older, lower yielding types are often more nutritious.

Russets: These have characteristic brown russeted skin and are best for baking because they are high in starch.

Butte – Classic russet, very high in vitamin C

Russet Nugget – Good flavor. Late,

Yellow:

Yukon Gold – One of most popular potatoes, yellow flesh. Early.

Yellow Finn – One of best flavored potatoes, yellow flesh. Mid

Red:

All Red – Red skin, pink flesh. Mid

Red Cloud – White flesh, Heat tolerant. Mid.

Red Norland – White flesh, early.

Sangre – One of best tasting red potatoes. Mid

White:

Kennebec – High yielding, tolerant of adverse conditions. Mid

White Rose – High yielding, does well with irrigation. Early

Caribe – Red/purple skin, white flesh.

Purple Viking – This has purple skin and white flesh. Mid

Blue:

I once grew some amazingly productive blue potatoes, but I never did find out what variety they were. Unfortunately my family wouldn’t eat them, claiming they tasted weird. I don’t believe they did, but I have to admit that eating blue mashed potato was strangely disconcerting.

These potatoes may have some additional health benefits in that the color of their flesh is caused by anthocyanin pigments which are antioxidants.

All Blue

Purple Majesty

Purple Peruvian – Deep purple flesh, late.

Fingerlings:

French Fingerling – Small pink tubers with excellent flavor. Mid

Russian Banana – One of the most popular fingerlings, easy to grow. Late.

Early potatoes: These fast maturing (less than 90 days) varieties are usually eaten immediately. They are good for areas where spring is short and summer is hot. They are also good for new potatoes.

Early Rose

Mountain Rose

Purple Viking

Red Norland

Red Pontiac

White Rose

Midseason potatoes: These mature in around 100 days.

Colorado Rose

Desiree

French Fingerling

Maris Piper

Red Lasoda

Rose Finn Apple

Yukon Gold

Yellow Finn

Maincrop potatoes: The late maturing varieties are commonly stored for winter use. They produce large crops of tubers that store well. 

Bintje

Butte

German Butterball

Russian Banana

Katahdin

Kennebec

Kitchen use

Potatoes are famous for the infinite number of ways in which they can be prepared: boiled, baked, fried, chipped, casseroled, stewed, roasted, mashed, scalloped, twice baked and more.

Starchy or floury potatoes are prized for baking and frying. Waxy potatoes are firmer and hold their shape, so are used for boiling whole and for potato salad. All purpose types can be used for all purposes.

Potato latkes   8 potatoes 2 eggs 1 onion 1 tsp salt ½ cup flour oil   Grate the washed potatoes (don’t peel) into a bowl. Beat the egg into another bowl, then mix in the grated onion, along with the flour and salt. Squeeze the excess moisture from the potatoes and add them to the egg mix.   Heat some oil in a skillet until it sizzles and drop in large tablespoonfuls of the mix. Flatten the mix and allow it to cook until golden brown. Then flip it over and cook the other side. Serve with apple sauce.  

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