Kohlrabi is not as familiar to most Americans as broccoli, cauliflower, cabbage, collards and kale, but it belongs to the same species: Brassica oleracea. What distinguishes kohlrabi from other members of the species is the part that we eat.
Cabbage, collards and kale are grown for their leaves, and broccoli and cauliflower for their immature flower heads, but kohlrabi is grown for its stem. It forms a round, crisp globe just above ground, with leaves growing above that. Although the plant is a bit odd-looking, the stem is delicious and can be eaten raw or cooked. In flavor, it is like a mild, sweet broccoli stem or turnip.
Last summer, several Napa County Master Gardeners tried growing kohlrabi in our gardens. Most of us had never grown it before. We chose three varieties: the purple Azure Star and two green varieties,Superschmelz and Kossak F1.
Harvest typically begins when the stems have reached a diameter of two to four inches. The Superschmelz was advertised as being tender up to eight inches in diameter.
Like its better-known siblings, kohlrabi is a cool-season plant. We grew it as a fall crop, starting seed in late summer so the crop would mature in late fall.
Kohlrabi can be seeded directly in the ground or started indoors for transplanting later. Planting seeds directly in the garden in midsummer presents some challenges. It’s hard to keep the soil evenly moist, a must for good germination. Also, insect pests seem to be at their highest numbers in warm weather, and little seedlings are vulnerable. Most of the gardeners decided to start their seeds indoors in containers.
One dilemma for plants started indoors is getting enough sunlight. A sunny window is often not enough. As soon as my seeds sprouted, which took less than a week, I moved the containers outside to a sunny deck with a high patio umbrella.
I monitored them daily for moisture and put them in shade on hot afternoons. This strategy turned out to be successful for me, although another Master Gardener lost all the plants on his deck to a critter. We suspect the culprit was a raccoon.
Commercially grown plants are sold at about six weeks old. I started my seedlings in recycled six-packs and transplanted them at four weeks, as they were getting too big. I amended the garden soil with compost and organic fertilizer and made sure the irrigation line was working. Kohlrabi needs a minimum spacing of 12 inches.
I planted 12 of each variety. Since they had been living outside on my deck, the seedlings did not need hardening off. If they had been started in a greenhouse like commercial plants, I would have kept them outside for a week before planting. As you do with tomato plants, you can bury the seedling’s stem deeply to encourage rooting.
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As soon as plants are in the ground, pests become an issue. Small plants in my garden are magnets for birds such as towhees and quail, so I used bird netting over the bed.
Kohlrabi attracts the same insect pests that afflict cabbage and broccoli, including several kinds of caterpillars. A floating row cover of lightweight spun fabric allows light and rain to reach plants but excludes insects. Check the underside of leaves for egg cases, and inspect stems for caterpillars. Remove by hand. With attentive monitoring and the help of natural predators, you can keep damage to a minimum.
Aphids can also plague kohlrabi, but a row cover will help. Monitoring for aphids and washing them off the plant with a spray of water keeps the population under control. Since aphids attack the soft parts of the plant, primarily the leaves, you can still get a nice crop in spite of them.
I felt fortunate in my results, eventually harvesting 31 of the original 36 transplants. My first kohlrabi was harvested in October, 74 days after planting the seeds and 54 days after transplanting. These results were consistent with seed-packet information.
The Azure Star variety produced kohlrabi two to three inches in diameter, while both green varieties got larger the longer they were in the ground. I noticed no difference in flavor among varieties. I harvested one or two kohlrabi a week through the winter. The last one, large but still tender, was harvested in March.
I found some amazing salad and side-dish recipes online, and because the harvest was so well paced, I never got tired of it. Kohlrabi is definitely going to be in my cool-season garden this year.
Napa County Master Gardeners will host a self-guided garden tour, “Down the Garden Path,” on Sunday, Sept. 22, from 10 a.m. to 4 p.m. Visit seven gardens in and around downtown Napa, all maintained by Master Gardeners. Tickets: $25 advance/$30 day of event. Purchase tickets online at http://bit.ly/16VOo2L. For more information about the tour and tickets, visit http://ucanr.org/ucmgnapa/, or call 707-253-4147.
Napa County Master Gardeners will host a workshop on “Cool Season Veggies” on Sunday, Aug. 18, from 2-4 p.m. at the Yountville Community Center, 6516 Washington St., Yountville.Register through Town of Yountville, Parks and Recreation: Mail in or Walk in registration (cash or check only).