How to Sow Kohlrabi: Depth, Spacing and Thinning

Soil temperature decides how fast it comes up

Kohlrabi germinates over a wide range, from about 45F (7C) up to 85F (30C), but the speed changes enormously across that range. At 68F (20C) it is through in four days. At 50F (10C) it can take twelve, and slow germination is when seed rots and slugs find the seed leaves.

That is the whole reason to care about soil temperature: not whether it will come up, but whether it comes up quickly enough to get past the vulnerable fortnight. If you can only sow into cool soil, start in modules somewhere warm and set the plants out later.

Kohlrabi is a fast crop. From sowing to a bulb you would want to eat is roughly 55 to 70 days for a spring crop and 70 to 90 days for one grown into cooling weather. Knowing which of those you are aiming at changes how you space it.

Germination itself is rarely the problem with kohlrabi. Nearly every failed sowing is flea beetle shredding the seed leaves in the first ten days, not seed that never came up.

Spacing sets the size of the bulb

The swollen stem has to have room to expand sideways on the soil surface. Crowd the plants and you get tall, thin stems with no bulb at all, because the plants compete for light and put their energy into leaves.

Six inches (15 cm) each way is the standard for a bulb the size of a tennis ball, which is the size most people are aiming at. If you want them larger, or you are growing into cool autumn weather where they sit for longer, go to eight or nine inches (20 to 23 cm).

Rows closer than ten inches (25 cm) make hoeing awkward and hold damp air around the leaf bases, which is where rot starts on a crop that sits on the surface.

How to sow

Kohlrabi seed is small and round, and it wants shallow covering. Half an inch (1 cm) is right. Deeper than that and the seedling arches under the soil and comes up bent or not at all.

1. Rake the bed level and firm it The bulb forms at the soil surface, so a lumpy surface gives you lopsided bulbs later. Break the top two inches (5 cm) down fine and firm it lightly with the back of a rake.
2. Water the drill, not the surface Draw a shallow drill, water along the bottom of it, and let it soak in before you sow. Watering after sowing washes small seed sideways and gives you clumps and gaps.
3. Sow three seeds at each station Station sowing at the final spacing saves thinning work later. Drop three seeds at each mark and cover with half an inch (1 cm) of fine soil, pressed down with your palm.
4. Cover the row before it emerges Put fine mesh or row cover over the row the day you sow, edges buried. Flea beetle finds seedlings within hours of them appearing, and a cover put on afterwards traps the beetles in with the crop.
5. Mark the sowing date at the end of the row Kohlrabi is judged by days from sowing as much as by size. A stake with the date on it is worth more than a guess in eight weeks time.

Thin in two stages, and do not delay

Kohlrabi will not recover from being left crowded. Once the plants have drawn each other up into thin stems they will not go back and make a bulb, however much room you give them afterwards. Thinning late is the single most common reason for a bed of leaves and no bulbs.

Thin twice rather than once. The first thinning is insurance against losses to slugs and flea beetle, the second sets the final spacing.

1. First thinning at the first true leaf When the first proper leaf appears beyond the two seed leaves, cut each station down to two plants. Snip with scissors at soil level rather than pulling, so you do not disturb the roots of the one you are keeping.
2. Second thinning at three or four true leaves Take each station down to one plant, keeping the one with the shortest, thickest stem. A tall seedling with a long neck below the leaves has already been drawn up and will make a poor bulb.
3. Eat the thinnings Kohlrabi thinnings are good raw. The leaves are edible at every stage, so nothing is wasted, and it makes it easier to be ruthless about thinning on time.
4. Firm the survivors back down Press the soil around each remaining plant after thinning. A rocking plant makes a lopsided bulb and gives cabbage root fly an easy route down the stem.

Do not draw soil up around the plants when you firm them. The bulb has to sit on top of the soil, and a buried stem rots instead of swelling.

A cold check on young plants makes them bolt

Kohlrabi is a biennial being grown as an annual, and it decides to flower when it thinks it has been through a winter. Two weeks below about 50F (10C) on a plant that already has a pencil-thick stem is enough to trigger that. The plant then runs to a flower stalk instead of swelling, and no amount of feeding or watering brings it back.

This is why a spring sowing that goes out too early bolts and a slightly later one does not. The safe rule is to sow so that the plants are still small seedlings during the last of the cold, or to delay until the risk of a fortnight of cold nights has gone.

1. For a spring crop, sow under cover Sow in modules four to five weeks before you intend to plant out, and keep them above 54F (12C). Plant out once nights are reliably above 45F (7C).
2. Do not plant out big transplants into cold A module plant that has been held too long and has a thick stem is exactly the plant that bolts. Plant out at three or four true leaves, before the stem thickens.
3. For an autumn crop, sow in late summer Sowing in the heat of late summer and growing on into cooling weather avoids the bolting trigger entirely, because the cold arrives when the bulb is already forming.
4. Sow a short succession Sow a few stations every two to three weeks rather than the whole bed at once. Kohlrabi holds its quality for only a couple of weeks, so a whole bed ready together mostly goes woody.

If a plant does bolt, pull it. The flower stalk takes everything the bulb would have had, and the swollen stem on a bolted plant is fibrous whatever size it reached.