How to Sow Kale Seed: Timing, Depth and Thinning

Kale germinates over a very wide temperature range

Kale seed will come up anywhere between 45F and 85F (7C and 29C), which is a far wider window than most brassicas give you. At 68F (20C) you can expect the seed leaves to break the surface in four days. At 50F (10C) it takes ten to fourteen days, and at that end of the range the germination rate itself drops, so sow a little more thickly and accept that you will thin harder.

The catch is the top of that range. Above 85F (29C) germination falls away sharply and the seedlings that do emerge stretch and flop. Late summer sowings, which are the most useful ones for a long autumn and winter harvest, therefore need shade cloth or a board laid over the row until the seed chits, and daily watering to hold the surface damp.

Sow half an inch deep, no deeper

Kale seed is small, about the size of a pinhead, and it carries only enough stored energy to push through half an inch (1 cm) of soil. Sown an inch (2.5 cm) down in heavy ground a good proportion of the seed simply exhausts itself underground. Rake the bed to a fine crumb first, draw a shallow drill with the edge of a stake, and cover with sieved soil or seed-starting mix rather than clods.

Firm the covering with the back of a rake or the flat of your hand. Seed sitting in an air pocket dries out and stalls. After sowing, water with a fine rose and keep the top half inch damp until emergence, which for a spring sowing usually means every other day and for a late summer sowing every day.

1. Draw a shallow drill Press the edge of a stake into a raked surface to make a groove about half an inch (1 cm) deep. A groove is easier to keep damp than scattered seed.
2. Space seed one inch apart in the row Drop seed every inch (2.5 cm). That is deliberately thick: it covers gaps and gives you a choice of which seedlings to keep.
3. Cover, firm and label Cover with sieved soil, firm gently, and write the sowing date on the label. That date is what lets you judge next year whether you sowed too early or too late.
4. Water with a fine rose A coarse jet washes the seed out of the drill and buries it too deep. Use a fine rose and water until the drill darkens.

Direct sowing or transplants

Kale transplants willingly, which is unusual and useful. Its root system is fibrous rather than a single taproot, so a plant lifted at the three to four true leaf stage settles in within a week. That means you can raise seedlings in modules while the bed is still carrying a summer crop, and set them out the day the bed comes free.

Direct sowing gives you a slightly stronger, more drought tolerant plant because the roots are never disturbed. Use it in spring when there is no rush for the bed. Use modules for the late summer sowing, when the ground is hot and dry and getting even germination in the open is the hard part.

Whichever method you use, do not let a kale seedling sit past six true leaves before it goes in the ground. A pot-bound kale never builds the deep frame that carries a plant through winter.

Thin in two stages, and eat the thinnings

Thin once when the seedlings have their first pair of true leaves, taking them to three inches (8 cm) apart, and again at four to five true leaves, taking them to their final spacing. Thinning in one go at the final spacing is a gamble, because slugs will take a plant or two and you have nothing left to fill the gap.

Final spacing depends on how you intend to pick. For full-sized plants that you strip leaf by leaf from autumn through winter, allow 18 inches (45 cm) between plants and 24 inches (60 cm) between rows. For baby leaf cut with scissors, six inches (15 cm) each way is plenty and the crowding keeps the leaves tender.

Snip the thinnings off at ground level rather than pulling them, so the roots of the plants you are keeping stay undisturbed. Everything you remove from the four true leaf stage onwards is a salad leaf, so the second thinning is really a first harvest.

Time the sowing so cold weather arrives when the plant is half grown

Kale improves in flavor after frost, because the plant converts starch to sugar in its leaves as a defense against freezing. The aim of your sowing date is therefore to have a plant with eight to twelve true leaves standing in the bed when the first frosts arrive. Sown too late, a small plant crawls through winter and gives you almost nothing until spring. Sown too early, it is huge and coarse before the cold ever reaches it.

Count back from your average first frost. Kale needs roughly 55 to 75 days from sowing to the first real picking, so a sowing 90 to 100 days before the first frost puts a well-built plant into the cold at exactly the right size. A spring sowing after the last frost gives summer leaves, but those are always harsher in flavor than autumn ones.

1. Find your first frost date Use the long-run average for your own garden, not the regional figure. A sheltered town garden often runs two weeks later than open ground nearby.
2. Count back 90 to 100 days That is your main sowing window for the autumn and winter crop. Sowing across a two week spread rather than on one day hedges against bad weather.
3. Add a spring sowing for summer leaf Sow again once the soil passes 50F (10C) in spring for tender summer leaves. Expect flea beetle pressure and a shorter useful life on these plants.
4. Record what you did Write down the sowing date, the date of first emergence and the date you first picked. After one season those three numbers replace every general rule you will read.