Kohlrabi: an experiment with transplants vs. direct sown

Kohlrabi 'Kolibri' 7 weeks after transplanting, in early June at the start of harvest season

Kohlrabi (Brassica oleracea Gongylodes group) is a member of the cabbage family grown for its swollen stem. Kohlrabi is crisp and juicy, with a mild, slightly sweet cabbage flavor. I like kohlrabi because it is one of the earliest cooking vegetables to mature in the spring, with varieties ready for harvest in 40-55 days after planting in the garden.

I’ve had the most success with kohlrabi by starting seeds inside around 3/21 and transplanting the seedings into the garden 4-5 weeks later. Harvest time is the entire month of June.

I’ve been teaching a vegetable gardening class in which I discuss the advantages of transplants compared to direct sowing. I explain that transplants usually result in earlier harvest, but I wondered how big the benefit is. After all, transplants can suffer ‘transplant shock’, as top growth stops and they push their roots out of the potting media into the garden soil. That is why Days to Maturity is measured from the time of transplanting or the day the seeds are planted in the garden.

This year I decided to do an experiment with kohlrabi to measure time to harvest for transplants compared to direct sown plants.

Method: I started the seeds of kohlrabi  ‘White Vienna’ indoors on 3/24/11. On 4/21 I transplanted the seedlings to my plot in the community gardens at Vassar Farm. The following day I direct sowed seeds of ‘White Vienna’ into a nearby bed of the same garden. BTW direct sowing is the recommended way to grow kohlrabi.

Direct sown White Vienna kohlrabi at 3 weeks

Expectation: the transplanted White Vienna kohlrabi would be ready for harvest a bit earlier than the direct sown kohlrabi.

Results: The seed packet says White Vienna matures in 55 days.  The first transplants were ready in mid-June, roughly 55 days after transplanting. The direct sown White Vienna was ready in 160  days – yes, in October – because the stem didn’t swell until the weather cooled in September.

Discussion: the direct sown plants sprouted quickly and soon had their first true leaves. But then they were attacked by flea beetles. This was a surprise because flea beetles prefer hairy leaved plants like turnips, radishes and Napa cabbage. The kohlrabi transplants, being bigger, were more able to withstand the flea beetles but the direct sown plants were badly stunted. This was the worst infestation of flea beetles I’ve ever experienced.

I planted the direct sown kohlrabi on the edge of a squash bed because I expected to harvest it before the squash plants started running all over. Unfortunately the direct sown plants were very stunted there was nothing to harvest and the squash overran the kohlrabis in late June, shading them until the squash died off in early September. By then, the kohlrabi plants were tufts of leaves on the ends of slender 8-10″ stems.

White Vienna kohlrabi finally forming a bulb after 6 months in the garden

Another surprise: with the cooler weather of September the kohlrabi stems started to swell.

I started harvesting them in early October. The long, skinny stem was tough and woody but the new top growth was tender and crisp – a perfect kohlrabi.

Perhaps I’ll try this test again next year – but I’ll sow the seed under a floating row cover to protect against the flea beetles. Although bigger plants can withstand flea beetle damage, they overwhelmed the seedlings.