Kohlrabi

Direct sow the spring crop 1-2 weeks before your last frost

Sow seed a quarter to a half inch deep, about an inch apart, in rows 12 to 24 inches apart, starting one to two weeks before your average last frost date. Sow a short row every 2 to 3 weeks for a run of harvests rather than one glut. Do not plant where cabbage, broccoli, kale, or other brassicas have grown in the last four years. Kohlrabi sizes up best between 65F and 75F and turns tough and woody once sustained heat arrives, so the crop has to do its growing in the cool window on the front end of the season. The rotation matters because clubroot, black rot, and blackleg all survive in soil and debris from previous brassica crops.

Sow the fall crop about 50 days before you want to harvest

Direct sow a fall planting so it matures roughly 50 days later, counting back from your first hard freeze date and using your variety's days to maturity. In most of the country that lands in mid to late summer. Pick a variety noted for holding in the field rather than the fastest one. The fall crop finishes as the weather cools, and cool nights are when kohlrabi is at its sweetest and most tender. It also sidesteps the summer heat that makes spring plantings turn woody, and light frost does not hurt it.

Set out transplants once they have 4-6 true leaves

Start seed indoors 5 to 6 weeks ahead at cool room temperature, 60F to 70F, and set plants out one to two weeks before your last frost date once they carry 4 to 6 mature leaves and a well developed root system. Space them 4 to 6 inches apart in the row and plant at the same depth they grew in the pot. Transplants started too warm or held too long in the cell get checked, and a checked brassica seedling that then meets a cold snap is the classic recipe for bolting straight to flower instead of swelling a stem.

Mulch and keep water steady once it turns hot

Once temperatures climb above 80F, put down 3 to 4 inches of weed free straw or herbicide free grass clippings around the plants, and give the bed 1 to 2 inches of water a week counting rainfall. Drip irrigation is better than overhead. Check moisture with a trowel rather than guessing, and water more often on sandy soil. Heat and dry spells are what turn the stem tough, woody, and bitter. Mulch cools the soil and evens out the moisture swings, and uninterrupted growth is the only thing that keeps the texture crisp.

⚠ Use only grass clippings from lawns that have not been treated with herbicide. Broadleaf weed killers carry over in clippings and will damage the crop.

Side-dress with nitrogen about 3 weeks after thinning

Roughly 3 weeks after thinning or transplanting, when plants are around 4 inches tall, band a half cup of a nitrogen fertilizer such as 21-0-0 per 10 feet of row. Place it about 6 inches to the side of the plants, not against the stems, and water it into the soil. Kohlrabi has a short season and only gets one shot at bulking up. A steady nitrogen supply during that stretch is what keeps the stem swelling fast, and fast growth is exactly what makes it sweet and tender rather than fibrous.

⚠ Keep dry fertilizer off the leaves and stems. Banding it against the plant burns roots and foliage.

Thin seedlings to 4-6 inches at the 3-4 leaf stage

When direct sown seedlings have 3 to 4 true leaves, thin them to stand 4 to 6 inches apart, or a full 12 inches apart for the giant storage varieties. The seedlings you pull can be transplanted elsewhere rather than composted. Thin fall sowings the same way. The edible part is a swollen stem, not a root, and it simply cannot expand to size against its neighbors. Crowded plants produce small fibrous bulbs no matter how well you feed and water them.

Check both leaf surfaces weekly for caterpillars

Starting right after planting, look over both sides of the leaves at least once a week, and more often as the season warms. You are looking for imported cabbageworm, cabbage looper, and diamondback moth larvae plus the ragged holes and dark frass they leave behind. Also watch for the tiny shot holes of flea beetles on young seedlings. All three caterpillars are far easier to kill while small, and finding them early is the whole game. Flea beetles matter most at the seedling stage, when a plant has few enough leaves that the feeding actually sets growth back.

Treat cabbageworms with Bt while the larvae are still small

Once scouting turns up small caterpillars, spray Bacillus thuringiensis subspecies kurstaki, covering both sides of the leaves since the larvae have to eat the material for it to work. Spinosad is a second low risk option. Repeat as the label directs while new larvae keep appearing. Bt is a bacterium that only poisons the gut of caterpillars that swallow it, and it works dramatically better on early instar larvae than on big ones. Waiting until the caterpillars are large means both a failed spray and a chewed crop.

⚠ Bt and spinosad are caterpillar and chewing insect specific and spare bees and natural enemies, unlike broad spectrum choices such as permethrin or lambda-cyhalothrin. Follow your product label's days to harvest interval before picking. Kohlrabi is harvested long before it flowers, but if plants bolt and open flowers, stop spraying anything that is not Bt while bees are working them.

Float a row cover over the bed at seeding or transplanting

Lay lightweight garden fabric over the row the day you seed or transplant and bury or anchor the edges all the way around, so it is in place before cabbage maggot flies begin flying. Leave it on through the first weeks of growth to keep cabbage maggot flies and cabbageworm moths from ever reaching the plants. Cabbage maggot flies lay their eggs at the base of the stem and in nearby cracks in the soil right as brassicas go into the garden, and the larvae then feed on the roots and root hairs below ground. There are no rescue treatments once maggots are already feeding, so excluding the adult is the only reliable control, and the same fabric stops the moths that would otherwise start the caterpillar problem.

⚠ Do not use a row cover on ground where brassicas, onions, or other root crops grew last year. Maggots overwinter as pupae in that soil, and the cover will trap the emerging adults underneath with your plants instead of keeping them out.

Cut at 2-3 inches across, no bigger

Harvest standard varieties when the swollen stem is 2 to 3 inches in diameter, about the size of a tennis ball, roughly 40 to 60 days from sowing depending on variety. Cut the stem just below the bulb. Check the planting every few days once it approaches size, and trim the leaves off before storing. Past about 3 inches the stem goes tough, woody, and bitter, and there is no getting the texture back. Size is the signal, not the calendar, and it arrives fast in warm weather. The giant storage varieties are the exception and hold their quality to 8 inches or more.