Green Man Publishing

Lettuce

Lactuca sativa

Introduction: Lettuce is probably descended from Lactuca serriola and originated somewhere around the Mediterranean or Near East. Some types have been grown since the time of the ancient Egyptians.

Lettuce is almost synonymous with salad. It is easily the most popular salad ingredient, as countless restaurant salads consisting of a bowl of lettuce with a couple of cherry tomatoes will testify. The lettuce available in supermarkets rarely measures up to those you can grow yourself.

Ease of growing: Lettuce is easy to grow if you give it the right conditions, which means fairly cool and moist weather. The challenge comes in getting it to grow when you want it, as it doesn’t like hot weather. The other problem is that once it is mature it doesn’t stay in prime condition for very long before it turns bitter and bolts.

Nutritional content: This varies a lot depending upon the type. Head lettuce is the least nutritious (though not negligible and leaf and romaine types are the most nutritious. They contain protein, calcium, vitamins A, C and K, as well as several B vitamins. They are also a good source of copper, magnesium, manganese, phosphorus, potassium and various antioxidants. They are pretty low in calories, with about 60 per pound.

Soil

pH 6.0 (6.8) 7.5

Lettuce needs to grow fast for best quality, which requires a good soil. It should be fertile, moisture retentive, well-drained and rich in organic matter. The pH isn’t particularly important. Light soils that warm up quickly are good for early lettuce. 

About Lettuce  
Seed facts
Germination temp: 35 – 80°F
Germination time: 2 – 15 days
49 days / 35°F (may rot)
15 days / 40°F
7 days / 50°F
3 days / 60°F * Optimum
2 days / 77°F
3 days / 86°F (only 12% germination)
Seed viability: 2 – 5 years
Germination percentage: 80%+
Time to grow transplant: 5 – 6 wks  

Planning facts
Hardiness: Half hardy
Growing temp: 45 (60 – 65) 75°F
Plants per person: 4 per sowing
Plants per sq ft: 4  

Planting:
Start: 6 – 8 wks before last frost
Plant out: 2 – 4 wks before last frost
Direct sow spring:
Leaf: 4 – 6 wks before last frost
Head: 2 – 4 wks before
Fall crop: 6 – 8 wks before first fall frost
Succession sow: every 2 – 3 wks  

Harvest facts
Days to harvest: 50 – 100 days
Yield per plant: 6 – 12 oz
Yield per sq ft: 1 lb  

Soil preparation: Lettuce has a weak root system and isn’t a very efficient feeder, so the soil needs to be quite fertile.

Its first requirement is for nitrogen (add compost or aged manure), but it also needs moderate amounts of potassium (add wood ashes or greensand) and phosphorus (add colloidal phosphate). It also likes calcium, so you might also want to give it some dolomitic limestone. To simplify things you might just add an organic fertilizer mix at the recommended rate.

Prepare the soil by adding 2˝ of compost or aged manure (unlike most plants it is also happy with fresh manure), along with any other amendments. This needn’t be dug in very deeply as lettuce is quite shallow rooted (the weak roots only penetrate about 4 – 8˝). For very early crops you might want to prepare the soil the previous fall.

Planning

Where: In cool climates lettuce needs full sun, but in hotter ones it will benefit from light shade during the hottest part of the day.

When: Lettuce germinates quite well in cool (40°F) soil and will continue to do so until the temperature gets up to 75°F (after this it gets erratic). With careful planning and a little ingenuity it is possible to have lettuce for 6 – 9 months of the year (though much depends on the climate).

Spring: The first spring sowing should be of leaf lettuce, as this is the hardiest kind. It can be direct sown 6 wks before the last frost, or started indoors 8 weeks before the last frost and planted out 4 weeks later. Properly hardened seedlings can take frost down to 20°F. For the earliest crops you may want to warm the soil with cloches and use transplants.

Head lettuce is less hardy than the leaf types, so is usually started indoors 6 weeks before the last frost date. Seedlings can be planted out 2 weeks before the last frost date.

Summer: Lettuce doesn’t like hot weather. At temperatures above 75°F only 50% of the seed may germinate and the plants may turn bitter and bolt quickly.

To have lettuce through the warmest part of the growing season you have to be creative. Use a heat tolerant variety, water every day and plant it in the shade of larger plants. If a spell of cooler weather is forecast, you can take advantage of it to sow some lettuce. See Unusual Growing ideas for more on this.

Autumn: Sow fall lettuce 4 – 8 weeks before the first fall frost date (or whenever summer temperatures start to moderate). Pests are very active at this time, so you may want to start them inside or in a protected place.

Winter: Some hardy varieties of lettuce can tolerate temperatures as low as 25°F and can be grown as a winter crop in milder areas. Start the plants (inside or out) about 4 – 6 weeks before you expect the first frost. Though they are quite hardy, they still do better when given protection from hard frost. The additional warmth of a cold frame or cloche can greatly boost growth.

In very cold areas you can grow lettuce in the greenhouse or cold frame. If you do this, make sure they stay cool (35 – 45°F), as low light levels and short days, combined with higher temperatures, can encourage bolting.

Succession sowing: Most lettuce is only harvested once and then it is gone, so you need a constant supply of new plants. To get this you need to sow a small quantity of seeds every 10 – 21 days (depending on time of year). There is only a short time between lettuce maturing and lettuce bolting (especially in warm weather), so you don’t want to have many mature plants at one time.

To spread out the harvest you can plant different varieties with different maturation dates. You can also start harvesting the plants while they are immature.

 Planting

Transplants

Starting inside: Lettuce is often grown from transplants, as this gives the fastest harvest, saves on bed space and avoids various garden hazards. Seedlings are easily raised and don’t mind root disturbance, so you can use flats, cell packs, plug trays or soil blocks.

When using flats space your seed 1˝ apart. When the seedlings are about 2˝ tall and start to touch each other, prick out into another flat, leaving 2˝ between the plants.

Cell packs, plug trays and soil blocks should be big enough that the plants don’t need transplanting to large containers. Sow 2 or 3 seeds in each cell and remove the weaker seedlings when the best one is 2˝ tall.

It is said that some kinds of lettuce need light for germination. This is easy to arrange, just don’t cover the seed with soil. Of course you must then take extra care to ensure it doesn’t dry out.

Planting out: Lettuce transplants easily in cool weather. In hot dry weather you must take precautions to ensure that the young plants are kept moist.

Direct sowing: This is simple enough in cool weather, as the seeds germinate easily and the plants grow rapidly. You may run into problems when the soil gets warm, as the seed doesn’t germinate very well above 75°F. In this situation you can pre-germinate the seed in the fridge as described below.

Planting in rows: Lettuce is commonly sown in rows, with 1˝ between the plants and 5˝ between the rows. Plant at a depth of ⅛˝ in cool soil and up to ¾˝ deep in warm soil.

Planting in beds: Lettuce can also be broadcast, spacing the seeds 1 – 2˝ apart.

Thinning: When the plants are 2 – 4˝ high, they can be thinned to the required spacing. The thinnings can either be eaten or replanted elsewhere at the final spacing. Transplanting will slow them down a little and can help to extend the harvest. Careful sowing of seed can help to reduce the need for thinning.

Raising transplants outside: Lettuce germinates readily in cool soil, so you can easily start your transplants outdoors in a nursery bed. This is commonly done in mild climates and saves on greenhouse or bed space. Just transplant the largest seedlings as space becomes available in the intensive beds.

Hot weather germination: In warm soil (75°F or above) lettuce seed will germinate poorly, if at all. You can get around this by pre-germinating the seed in the fridge on a paper towel. You don’t have to keep it in the fridge until it has germinated, just 5 days will be enough to break its dormancy. It will then germinate in warm soil.

Spacing: This varies depending upon the type of lettuce and the variety grown. Don’t crowd the plants as they won’t produce large heads and won’t grow rapidly, which is important if you are to grow the best tasting lettuce.

Head Lettuce: Plant this in offset rows: 15˝ – 12˝ – 10˝ apart, depending upon the variety and soil fertility.

Leaf Lettuce: Plant this in offset rows: 12˝ – 8˝ – 6˝ apart, depending upon the variety and soil fertility.

Care

Lettuce needs to grow quickly for best quality. This can only occur if you give the plants everything they need.

Weeds: The young plants are vulnerable to weeds, so keep well weeded. Their roots are shallow so be careful with the hoe.

Water: Lettuce is largely composed of water and it responds to irrigation by giving a larger and better tasting harvest. If you think the plants might need water they probably do.

Good watering practices can help offset the negative effects of summer heat, so it is important to keep the soil constantly moist. In hot weather this may mean watering every other day. At the same time don’t over water and try to keep soil from splashing on to the leaves.

Fertilization: If your soil is not as rich as it could be, give the plants a feed of compost tea or liquid kelp about a month before harvest. This is especially important with the crisphead varieties.

Mulch: This helps to conserve soil moisture, keeps down weeds and helps to keep the plants clean. If you apply it early, it can also help to keep the soil cooler in hot weather. On the negative side, it may also harbor lettuce loving slugs.

Problems

Pests: Lettuce can fall victim to quite a few pests, but they are not usually too serious. These include tarnished plant bugs, thrips, aphids, leaf miners, flea beetles: These small creatures may be controlled by using row covers.

Slugs and snails: These molluscs love the tender young leaves and are the commonest problem you will face when growing lettuce.

Cutworms: These can be a real problem for young seedlings in spring. Some gardeners use individual cutworm collars of cardboard, newspaper (2 layers) or aluminum foil). If you find plants laying on the ground, dig in the soil around them and you can usually locate the small dark caterpillar that is responsible. If you find it you can prevent it doing further damage.

Mammals: Deer, rabbits and groundhogs can quickly devastate even a mature lettuce patch. A fence may be necessary if you have these problems.

Disease: Lettuce may be afflicted by various blights, spots, rots, rusts, mosaics, mildews and yellows, but none are particularly significant when growing on a garden scale.

Bolting: A mature lettuce will flower (bolt) when the day length gets up to 14 or 16 hours (the exact day length depends upon the variety), even if the weather is cool. Warm weather (above 75 – 80°F) frequently accompanies the long days of midsummer and may hasten bolting, but it isn’t the primary cause. Likewise crowding can contribute to bolting.

Bolting will also occur when a plant reaches full size and has all the resources it needs to flower. When the plant has enough large leaves they signal the plant that it is ready to flower. The onset of bolting may be retarded somewhat by the frequent picking of single leaves, but it won’t stop it.

When a plant starts to bolt, it turns bitter, the head elongates and the new leaves begin to take on an elongated shape. The plants can be quite beautiful at this stage and if left alone the flower stalk will appear and ultimately produce an abundance of seed. I often allow them to produce seed, as I use a lot for growing Salad mix and Micro-greens.

Bitterness: This is a characteristic sign of imminent bolting, but it may also be caused by water stress or unusually warm weather.

Tip burn: Burnt looking leaf tips may indicate a shortage of calcium, or night temperatures over 65°F.

Harvesting

When: Lettuce is most nutritious if used fresh and doesn’t keep well, so it’s best to harvest it right before a meal. When grown for sale they should be harvested in the cool of early morning.

You can start harvesting leaf lettuce only a few weeks after planting, as soon as there are enough leaves to be useful. Head lettuce is harvested when the heads are firm, or at least have formed. There is no reason to wait for them to reach full maturity, as they will bolt soon afterwards. It is always better to harvest too early than too late.

How: I commonly gather individual leaves as I need them for salads. This works out well, so long as you leave enough on the plant for it to recover. Picking individual leaves may even slow down bolting.

Traditionally the whole lettuce is cut off at the base. If you leave a few leaves on the stem, rather than cutting at actual ground level, the head will be cleaner. The stem remaining in the ground may then continue to grow and sprout new leaves. It may even grow some little lettuces.

Storage: Leaf lettuce has thin leaves and won’t keep for much more than a week in a plastic bag in the fridge.

Crisphead lettuce has stiff, fleshy leaves and keeps very well, in fact that is why it is so popular with commercial growers. It will keep for several weeks in a plastic bag in a refrigerator.

The other types are somewhere in between. Don’t wash any lettuce until you are going to use it.

Seed saving: It is fairly easy to save lettuce seed and if you save it from your best plants, you can develop better strains than you can buy (and have higher quality seed). The plants are mostly self-pollinated, though there may be some cross-pollination from insects. It is recommended that varieties be separated by 25 ft to keep them pure, which is simple enough. It will also cross-pollinate with wild lettuce (several Lactuca species) so remove any you see within 200 ft of your flowering plants.

I often save lettuce seed with no thought for purity, as I want it in volume for growing cut and come again lettuce. I don’t really care if the variety is somewhat mixed up (in fact I probably wouldn’t even notice if it was). 

I often gather seed from plants that have bolted, but you shouldn’t gather it from the first plants to bolt. Early flowering is not a trait you want to perpetuate.

Head lettuce can present a problem when it comes to seed saving. The head may be so dense that the flower stalk may not be able to get out. If this is the case, you may have to cut an X in the top of the head, to enable the flower stem to emerge (as you would with a cabbage). If the flower stem is very big you may have to stake it to prevent it from falling over when it gets loaded with seed.

The yellow flowers are followed, 2 to 3 weeks later, by fuzzy dandelion-like seed heads. Gather the seed as it ripens by holding a paper bag over the head and shaking (I really dislike the smell the plants leave on your hands when doing this).

The seed ripens sequentially, so you must collect it every few days to get all the ripe seed. Keep on collecting until you have all the seed you need, or until it is blown away by the wind.

Alternatively you can cut the entire head when about 50% of the seed has ripened and dry it in a paper grocery bag. Clean the seed and remove the fuzz, then dry and store it in a cool place.

Newly harvested seed usually won’t germinate for a couple of months.

Lettuce mosaic virus can be seed borne so watch out for it if you save your own seed, or swap seed with others.

Unusual growing ideas

Intercrop: Lettuce is a compact and fast growing plant, perfect for intercropping between a slower maturing crop. It will be out of the ground before the other crop needs the space. This works well with slow growing crops such as parsnip peppers or tomato. If you have a ready supply of transplants, you can fit a few lettuces into any small vacant space that appears.

Cut and come again: If lettuce plants are packed very closely together, the plants don’t produce a head at all, they just produce an abundance of single leaves, the perfect size for using in salads. This gives you a completely different way to grow lettuce, either alone or in a salad mix.

The seed is broadcast directly on to the bed, so there is a seed roughly every ½˝. You may want to cover the seeds with a very light covering of soil, to stop it drying out.

Start harvesting when the leaves are 2 – 3˝ tall, leaving at least an inch of stem on the plant when cutting, so that it can regenerate. I like to make a small sowing every 3 weeks to maintain a steady supply of leaves.

When grown in this way lettuce needs only about half the space it would for growing heads. I like this method so much, I stopped growing individual lettuce for a while.

See Salad Mix for more on growing salad greens at close spacing.

Multi-planting: Leaf lettuce can also be multi-planted. Sow 2 or 3 seeds in each block or plug tray and allow them all to grow to maturity. Plant the clusters out 12˝ apart.

Containers: Lettuce does quite well in containers, so long as they contain a fertile soil mix and you keep them watered. Because containers are so portable you can move the plants to cooler locations in warm weather.

Hot weather growing: The best way to grow summer lettuce is as a cut and come again crop. Use a heat tolerant variety and grow it in the shade (use shade netting or interplant under taller plants). Apply cold water daily to keep the soil moist and cool. Cut the leaves frequently to make the most of the harvest and to slow down bolting. Of course you should also use a heat tolerant variety (see below for a few of these).

Volunteers: If you allow lettuce to flower it will often self-sow. You can aid this process by scattering some of the abundantly produced seed in suitable places. In spring you can often simply transplant these seedlings to where you want them. If they are of different varieties, they can give quite an extended harvest period.

Varieties

Four types of lettuce are commonly grown, looseleaf, crisphead, butterhead and semi-heads (romaines). There is a lot of variation within these types, with many cultivars bred for specific purposes. Some are especially good for a specific season, some are for growing under glass and some have even been bred for container growing.

There are now a huge number of lettuce varieties available, so I’m just going to mention a few from each type.

Looseleaf

Lactuca sativa

This is the easiest to grow, the most tolerant of heat or cold, the fastest to mature and the most nutritious. It comes in various colors and sizes and usually matures in 40 – 50 days.

Oakleaf

Black Seeded Simpson

Deer Tongue

Salad Bowl

Red Sails

Bronze arrow

Flame

Sunset

Butterhead / Bibbs

Lactuca sativa var capitata

These have soft, loosely packed heads and very good flavor. They are fairly easy to grow and tolerate some heat. These mature in anywhere from 65 – 80 days.

Buttercrunch

Tom Thumb

Merveille de Quatre Saisons

Romaine (Cos)

Lactuca sativa var longifolia

These produce heads of tender green leaves with crisp midribs. They are better flavored and more heat tolerant than most other lettuce. They mature at around 70 – 85 days.

Little Gem

Paris Green Cos

Valmaine

Winter Density

Crisphead / Iceberg

Lactuca.sativa var capitata.

These have dense heads of crisp leaves and need a long period of cool weather for best growth. Head lettuce is the most difficult to grow, the slowest, the most demanding and the least nutritious. It is very popular however, because of its crisp and crunchy texture. It is prized by industrial agriculture for its ability to survive handling, shipping and sitting on supermarket shelves. These take the longest time to mature, anywhere from 80 – 95 days.

Crispino

Great Lakes

Iceberg

Webbs Wonderful

Heat tolerant lettuce

There are quite a few of these when you start looking.

Black Seeded Simpson

Deer Tongue

Green Star

Jericho

Lollo Rossa,

Oakleaf

Merveille de Quatre Saisons

Red Sails

Webbs Wonderful

Cold tolerant lettuce

Lettuce is quite a hardy crop all around, but some varieties are more hardy than others.

Arctic King

Brown Dutch Winter

Merveille de Quatre Saisons

Red Oakleaf

Rouge d’Hiver

Valdor

Valmaine

Winter Density

Kitchen use:

We tend to use lettuce raw in salads and sandwiches, but it has other uses as well. It has been used as a potherb, stir fried, added to soups and used as wrapping for other foods.

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