When to plant kale

Kale is a long crop that you eat in winter, so there is no rush in spring. The main sowing time is May, and anything from April to June works. UK sources allow sowing from March, but an early start mostly gives you plants that are ready long before the cold weather that makes them taste best.

Raise the young plants in a seed tray, modules or a spare corner of the garden, then move them to their final spot in June or July. Planted by mid-July, kale still has around 100 days to grow into a sturdy plant before winter.

If you only want tender baby leaves for salads, you can sow thickly in rows or pots almost any time from spring to late summer and cut the leaves when they are a hand high.

Sow the seeds 0.4–0.8 in deep. In a tray, two seeds to a spot about 2 in apart work well; pull out the weaker seedling once both are up. Germination takes about a week in mild weather.

The young plants are ready to move once they have three or four true leaves and stand 4–6 in tall. That takes three to four weeks in modules and up to six to eight weeks in an outdoor seedbed.

Give kale a sunny spot; it copes with a little shade. Like all cabbages it likes deep, firm soil with plenty of lime. Aim for a pH around 7, which also makes clubroot less likely. Dig in compost well before planting, then tread the soil firm rather than leaving it fluffy.

Set each plant a little deeper than it grew in the pot and water it in well. Deep planting helps tall varieties stand up to winter wind. Allow 18–20 in between plants and 24–28 in between rows; dwarf varieties can go as close as 12 in. In a 120 × 80 cm raised bed that means four tall plants, or six to eight compact ones.

Kale is less fussy than cauliflower or broccoli. In a bed that has had compost for years, a generous layer of compost before planting is usually all it needs. In German terms it is a Starkzehrer, a heavy feeder, like the other cabbages; on thin soil give a nitrogen-rich organic feed in summer.

Water young plants until they have rooted and in long dry spells after that. Keep weeds down while the plants are small.

The biggest threat in summer comes from insects: cabbage white caterpillars, cabbage root fly and flea beetles. Covering the plants with insect mesh straight after planting keeps nearly all of them off. More on that below.

You can start picking as soon as the plants have a good number of full-sized leaves, often from September. Most gardeners wait until late autumn, though, and pick from about October or November through to March.

Take a few of the lower, older leaves from each plant at a time and leave the crown alone. The plant keeps growing from the top and can be picked many times over the winter. In early spring it sprouts again and gives you tender side shoots before it flowers.

Does kale really need frost to taste good? Not quite. What makes it sweeter is a spell of cool weather: the plant keeps making sugar in its leaves, while the cold slows down how fast it uses that sugar up. Frost itself plays no part, which is why putting picked kale in the freezer does not copy the effect.

Kale is very hardy. It copes with about 14°F without trouble, and more under a blanket of snow. Hard frost on bare ground with a cold wind does the most damage.

Curly kale is the classic type, with tightly frilled leaves. Varieties range from dwarf plants about 12 in tall to old tall types over a metre high. In northern Germany it is also called Braunkohl.

Tuscan kale (cavolo nero, 'Nero di Toscana') has long, dark, blistered leaves and grows like a small palm. It is grown and harvested the same way. Red-leaved varieties such as 'Redbor' look good enough for the flower border.

Cabbage white caterpillars chew holes in the leaves from about May to October, worst in high summer. Check the undersides of the leaves every week and squash the yellow egg clusters and small caterpillars. Better still, keep the butterflies off with insect mesh.

Cabbage root fly lays its eggs at the base of young plants from about late April. The maggots eat the roots, and freshly planted kale wilts and dies. The first generation in late spring is the most damaging. Mesh laid over the plants straight after planting, or a collar 3–6 in across fitted snugly around each stem, stops the eggs being laid.

Mesh with holes of about 1.3 mm is enough against cabbage whites and root fly. To keep out flea beetles and mealy cabbage aphids too, use 0.8 mm. Tuck the edges into the soil so nothing can crawl underneath, and keep the mesh on until the end of summer.

Kale is also prone to mealy cabbage aphid and cabbage whitefly. Whitefly is at its worst in warm, dry late summers; blasting the undersides of the leaves with a hose helps. Whitefly overwinters on old brassica plants, so pull up and clear old stems in spring.

Clubroot is the disease to fear. The roots swell into solid, distorted clubs, and plants wilt on warm days and stay small. Its spores last in the soil for many years, up to 20. Keep the pH around 7, raise plants in pots so they go out with a strong root system, bin infected roots rather than composting them, and grow cabbage-family crops in the same bed at most every four years; after an outbreak, wait seven or more.

Kale belongs to the cabbage family (Brassicaceae), together with cabbage, broccoli, kohlrabi, radishes, turnips, rocket and mustard. They all count for rotation, including mustard as a green manure.

Planting charts pair kale with tomatoes, beans, spinach, celery, lettuce or leeks and keep it away from onions, potatoes and other cabbages. We found no solid research behind these pairings; they are garden tradition.

One idea does have evidence: underplanting brassicas with low clover makes them harder for cabbage root fly to find, because the fly looks for bare soil around a plant. Mixed plantings in general tend to carry fewer pests than a solid block of one crop.