Brassica oleracea var acephala
Introduction: This cool season biennial is the most primitive of the cabbage family crops and isn’t far removed from the wild Brassica oleracea. It is not very common on dinner tables in this country, which is unfortunate as it is one of the most nutritious vegetables. It hasn’t always been so neglected though; it was once one of the most widely used green vegetables in Northern Europe
Many Americans think of kale as an attractive curly garnish (it is often used in restaurants as a cheap substitute for parsley), which isn’t intended to be eaten. This is a shame because well-grown kale, harvested in cold weather, is very good. At the same time I should add that poorly grown kale, harvested in warm summer weather, can be tough and almost inedible.
Nutritional content: Kale is a highly nutritious plant, being rich in vitamins A, C and K, chlorophyll and many important phytochemicals (including isothiocyanates, sulfuraphane, lutein and zeaxanthin). It is said to contain even more antioxidants than other Brassicas (which are all a good source of them). They also contain almost 200 calories per pound, which is more than any other common leafy green vegetable.
Germ temp: 40 (45 – 95) 95°F
Germination time: 4 – 9 days
15 days / 50°F
9 days / 59°F
6 days / 68°F
5 days / 77°F * Optimum
4 days / 86°F
Seed viability: 4 – 6 years
Germination percentage: 75+
Weeks to grow transplants 5 – 6
Hardiness: Very hardy
Growing temp: 40 (60 – 65) 75°F
Plants per person: 10
Plants per sq ft: to 1
Height: 18 – 36˝
Spread: 18 – 36˝
Start: 6 weeks before last frost
Plant out: 2 weeks before last frost
Spring: 4 – 6 weeks before last frost
Fall: 2 weeks before first frost
Days to harvest:
50 – 70 days from transplant
80 – 100 days from seed
Harvest period: 16 – 26 weeks
Yield per plant: 1 – 2 lb
Climate: Kale is a cool season plant and really comes into its own in the depth of winter, when other crops die off. It can survive temperatures as low as 0°F and will continue to produce food when most other crops are just frozen sticks. It can even be gathered from under the snow when frozen solid (though of course it won’t be growing then).
Kale doesn’t just tolerate cold temperatures and frost, the flavor of the leaves is actually improved by it (they become sweeter and more tender).
Kale grows quite well in warm weather, but at temperatures much above 75°F, the flavor deteriorates and it can become tough.
Ease of growing: Kale isn’t far removed from a wild plant and shows it by being pretty easy to grow. It is even easier in cold weather, when it leaves most of its potential pests and diseases behind.
Crop value: Kale was a staple winter food for many Northern European peasants (Scottish vegetable gardens were commonly known as kale-yards). It is very nutritious and was especially important as it produced during late winter and early spring, when it was one of the few fresh foods available (one cultivar was actually known as Hungry Gap for this reason).
Another benefit of kale is that it can be harvested repeatedly, often sending out new leaves for 6 months or more.
pH 6.0 – 7.5
Kale is more tolerant of poor soil than other Brassicas, but the most palatable leaves are produced by rapid uninterrupted growth. For this the soil must be rich, well-drained and moisture retentive with lots of organic matter. Of course with a winter crop, drainage is usually more of a concern than water retention. If the soil is too wet in winter the roots may rot.
Soil preparation: Kale likes organic matter, so amend the soil by digging in 2˝ of compost or aged manure. It also likes a neutral pH, so add lime to raise the pH if necessary. Its nutritional requirements are similar to those of cabbage, in that it needs a significant amount of phosphorus and potassium, but not a lot of nitrogen. If your soil isn’t very fertile you may also want to add some fertilizer mix.
Where: Winter kale will be growing in the coldest part of the year, so should be planted in the warmest, sunniest, most sheltered part of the garden.
Kale is one of the most shade tolerant garden vegetables, though it won’t be quite as productive as when in full sun. In warmer weather it is usually happier in light shade. If this is from a deciduous tree, it will mostly disappear in winter, which is ideal.
Crop rotation: Kale should not be planted where another Brassica has grown in the past 3 years.
Spring: Kale can be planted in spring for harvesting in early summer. It is started indoors 6 weeks before the last frost date and planted out 2 weeks before the last frost date. If you plant too early the plants can be vernalized and may bolt soon after planting out.
Kale will actually grow right through the summer, but doesn’t taste very good in warm weather. It really needs cool weather to make it tender and sweet.
Fall: As I already mentioned kale is most useful as a fall crop, planted in mid-summer, at least 2 – 3 months before the first fall frost date. The plants need to be fairly big by the time of the first frost, so they are vigorous enough to keep growing. In mild climates it will continue to grow all winter without disruption and can be harvested continually for months (it will also be in peak condition in the cool weather).
It is a good idea to plant your autumn kale as an intercrop between an existing summer crop. It will gradually take over the space as the weather gets cooler and the summer crop fades.
Late kale can succeed a mid-season crop such as potatoes or beans.
Winter: Your fall kale crop will gradually become your winter kale crop, especially in milder areas. If you want to use kale as a staple winter crop you should probably grow quite a lot of it. It grows slowly in winter, so you want to have quite a few plants to harvest from (so you don’t stress any one plant too much).
Direct sowing: The easiest way to grow kale is to sow it directly into the garden. The seed germinates easily and grows quickly, even in fairly cool soil. Plant the seeds ¼ – ½˝ deep and 2˝ apart. Start harvest thinning when all the seedlings have emerged and are of sufficient size.
Starting inside: Kale is often started indoors to get an earlier crop in spring. It will germinate and grow much faster in warmer conditions, enabling you to plant out a thriving seedling, rather than a seed. Transplants may also be used where insects or other pests are a problem.
Kale doesn’t mind transplanting so can be started in flats, as well as cell packs and soil blocks. Plant the seed 1˝ apart in a flat and when the seedlings are big enough prick out into another flat, leaving 2˝ between the plants. The plants are ready to go outside when they have 3 – 5 true leaves, a stem diameter of about ⅛˝ and are 3 – 5˝ tall.
Hardening off: If transplants are to go outside while it is still cold, they should be hardened off first. They will then tolerate temperatures as low as 25˚F. You do this by putting the plants outside for 2 hours on the first day, then 4 hours on the second day. Add 2 hours every day for a week and then plant out.
A simpler alternative is to put them in a cold frame, which is opened for longer periods each day and closed at night.
Planting out: Transplant the seedlings slightly deeper than they grew in the flats, up to the depth of their first true leaves. Water well after planting (of course).
Protection: Early growth is often slow in cool weather. You might want to use cloches to keep the plants warmer and so speed this up.
Bolting: In spring a potential problem with using transplants may occur if you are too successful and grow big healthy transplants. If these are then exposed to cold weather they may be vernalized, which will cause them to bolt. To avoid this your transplants should have stems no thicker than ⅛˝.
Starting transplants outside: In terms of bed space, a kale seed takes up as much space as an 8 week old transplant, so direct sowing isn’t very space efficient. You can use bed space more efficiently by sowing your fall kale in an outdoor nursery bed (this only works in warm weather of course). Leave them there until they are of transplant size and then plant them out at their full spacing in early fall.
Spacing: Kale plants can get quite big, so give them plenty of room:
12˝ apart in excellent soil
15˝ apart in good soil
18˝ apart in poor soil
Rows: If you want to plant in rows, space them 18 – 24˝ apart, with 12˝ between the plants.
Weeds: Kale is pretty independent once established, so you only really need worry about weeds while it is young.
Watering: Kale has large leaves and can lose a lot of water in warm weather (another reason not to grow it then). It is actually moderately drought tolerant, but for the best quality (texture and flavor) you must keep it well supplied with water.
Fertilization: If the soil isn’t very fertile, feed the plants with compost tea or liquid kelp. Start feeding them as soon as they have recovered from transplanting and every month thereafter. This is especially important if you are going to be harvesting for weeks on end.
Mulching: Use a mulch in summer to keep down weeds, keep the soil cool and conserve soil moisture. In winter a mulch can help to protect the roots by moderating soil temperature and preventing frost heaving.
Pests and diseases: Kale is less vulnerable to pests than other Brassicas (even to clubroot), but it still has its share, especially in warm weather (aphids are the commonest). See Cabbage for more on these pests and how to deal with them.
Birds In my garden quail have a particular affection for Brassicas such as kale and they may strip whole leaves from the plants. In my last garden they got so bad in winter that I had to net the 4 ft tall plants.
When: Kale produces an abundance of foliage right through the growing season, but it is at its best during cold weather. This is because cold weather stimulates the conversion of starches in the leaves into sugars (a similar thing happens in Jerusalem artichokes and parsnips). A few nights of freezing temperatures are enough to make this happen. The cold also makes the leaves more tender.
You can gather kale leaves for as long as they are produced, sometimes right through the winter. In extreme cold you might cover them with mulch for extra protection. Even the frozen leaves can be eaten and are actually very good.
The new shoots, produced when the plant first starts growing again in spring, are also good.
When kale bolts in spring, the flower buds can (and should) be gathered and used like broccoli. They are a nutritious and tasty treat, that is not to be missed. If they are infested with aphids, blast them off with a jet of water.
How: For a longer and bigger harvest it is best to gather single leaves as they get large enough. Don’t take them from the growing point and only take 1 or 2 leaves from a plant at one time.
You may be able to stimulate an old plant to put out tender new growth, by stripping off all of its leaves.
Seed saving: Plants over-wintered in the ground will flower the following spring. Kale will cross-pollinate with any other Brassica crop (broccoli, Brussels sprout, cabbage, collards), so only one variety can be flowering at one time. Save the seed in the same way as you would cabbage.
The plants will produce an abundance of seed. In fact they sometimes get so top heavy with seed they need staking to stop them falling over.
If you save kale seed you will end up with a lot, especially as you should save the seed from at least 5 plants to maintain some genetic variability. This is far more than you will ever need for planting. You can sprout most of it like alfalfa, or use it to grow micro-greens.
Unusual growing ideas
Winter indoors: In very cold areas kale can be grown in winter in an unheated greenhouse or cold frame. It has even been grown indoors as a winter houseplant.
Ornamental use: Some kales have very attractive foliage and can be used in the flower garden. The specially bred ornamental kales are edible too and can be quite good.
Container growing: Kale does well in containers, so long as it has enough room, good soil and plenty of water.
Growing as a perennial: You can get some of the hardier kale varieties to survive for several years, by not allowing them to flower and set seed. You can propagate them by taking green stem cuttings in summer.
Cover crop: Kale is sometimes planted as a green manure or winter cover crop. In areas with mild winters it will produce a lot of foliage over the winter and has the additional benefit that it is edible. In spring you can eat the tender new flower shoots, before incorporating the rest of the plants into the soil.
The drawback to using kale as a cover crop is that it is a member of the Brassica family and so prone to all of the same pests and diseases.
There are several different types of kale:
Curly kale: This is the commonest and most familiar type.
Dwarf blue curled – Frilly blue leaves (55 days).
Redbor F1 – Frilly purple leaves (50 days).
Plain kale: These have smooth flat leaves and include the hardiest and most trouble free varieties. They are getting to be hard to find though.
Lacinato – This Italian variety is known in Italy as Cavolo Nero or black cabbage. It is more tender and delicately flavored than most types.
Thousand headed kale: This very hardy variety produces more than one growing point, hence its name. It can be persuaded to grow as a perennial in some areas.
Ornamental Kale: This multi-colored and quite spectacular plant is commonly used to add fall color to gardens. Though not considered as tender or tasty as other types, it can sometimes be surprisingly good (it depends on the variety and growing conditions).
In cool weather the tender young leaves are good enough to eat raw. Older leaves can be steamed, stir-fried, used in soups or dried for chips. They are quite substantial and don’t shrink down nearly as much as some other greens, so you don’t need to gather as much. If the main leaf stem is tough, it is easily removed by folding the leaf in half and slicing it off.
These are a genuinely nutritious snack that tastes so good they encouraged me to grow Scotch kale again. They can also be made with collards, They are pretty simple to make, you just sprinkle the leaves with sea salt and dry them.
The simplest (and most nutritious) way to dry the leaves is in a dehydrator at 110°F until dry (usually overnight). Put the dried leaves / chips in a bowl and sprinkle with olive oil and nutritional yeast.
If you don’t have a dehydrator you can do it in an oven by using the lowest temperature and turning it off occasionally.