Cole Crops for Fall Harvest

Posted by Sarah Schweig, Broomfield County Extension

I grew kohlrabi in my home garden for the first time this spring. I opted for a purple variety, though more common varieties are light green to white. The plants were beautiful, tasty, and front runners for the Least Fussy Award. A spot of good luck? Perhaps, but this was a feat for a spring crop for reasons I'll describe below. My overwhelming feeling was “Why didn’t I start growing these earlier?” I'll be seeding for a fall crop soon, and if you haven't grown kohlrabi yet, you can still give it a try this season.

White and Purple Kohlrabi (Photo: University of Kentucky)

The name kohlrabi comes from German and translates to “cabbage turnip.” Turnip and kohlrabi are both Brassicas. Rather than the edible taproot of turnip (Brassica rapa), the prize of the kohlrabi plant (Brassica oleracae) is the stem, which enlarges at maturity, forming a turnip-like bulb just above ground. Kohlrabi has a more mild flavor than turnip and is ever so slightly sweet, but they’re prepared in the same way.

Kohlrabi is more closely related to - in fact, the same species as - heading cabbage, broccoli, cauliflower, brussels, and kale, collectively referred to as cole crops. Whereas turnips and kohlrabi will provide you with a similar experience once prepared, I have always loved thinking about the totally different experiences modern Brassica oleracae varieties offer. My favorite description of the species is having a “remarkable natural tendency for thickening of plant parts,” and it provides the perfect visual for their domestication and divergence from the same wild cabbage plant.

Photo from University of Nebraska

Kohlrabi was bred by selecting for thicker stems; cauliflower through selecting for thicker immature florets; broccoli was selected for thicker florets and floral stems. While selecting for a thick terminal bud gave us cabbage, selecting for many enlarged axillary buds (the "sprouts") gave us brussels.

I’m glad I successfully dipped my toe in growing kohlrabi this spring, but I’m partial to growing cole crops for a fall harvest for a few reasons. First, it’s easier to direct seed. Cole crops like soil temperatures around 80ºF for germination, but they can bolt or become overly tough or bitter if temperatures are too warm, especially around heading. Combined with relatively long time to maturity, it’s a challenge to get the timing right when starting these plants from seed in the spring.

Pest pressure can also be eased in planting for a fall harvest. I tend to have issues with flea beetles. Most flea beetle damage is caused by adults, which chew characteristic “shot holes” on the interior of leaves. While plants can withstand quite a bit of this type of injury once established, flea beetles can be tough on younger plants. In my fall crop, I get to avoid their most active time in vegetable gardens (late May and early June in my area).

With early kohlrabi varieties maturing in as little as 40 days, and most common varieties maturing in around 60, July to August is a good time to seed your fall crop for a harvest in September, depending on your location. They can withstand some frost, and bonus, you can get bigger, sweeter harvests with a fall crop. Kohlrabi is usually harvested when the bulbous stem is 2-3” wide. After this point, the stem can become woody in higher temperatures, but cooler temperatures in the fall will keep them more tender longer.

Like familiar Brassicas, kohlrabi is a heavy nitrogen feeder and requires ample moisture and space to develop properly. Use an organic mulch to help regulate moisture and soil temperature. Most varieties require a minimum of 12” spacing, but always check your seed packet for specific information. Planting too close together may result in elongating of the stems rather than the thickening you want to see. Harvest the whole plant (I’m a big fan of the greens) by cutting just above the soil surface. Kohlrabi can be stored for months in cold, moist conditions (around 36ºF and 95% humidity), but most of us can’t create these conditions at home. Kohlrabi will last a couple of weeks after harvest in your refrigerator. 

For more information on cole crops and other ideas for your fall vegetable garden, check out the new Colorado Vegetable Guide.