Kohlrabi is a delicious vegetable that is easy to grow in the intermountain climate, yet it is  relatively unknown.  It is a form of cabbage, in the mustard family, the Brassicaceae.   It is botanically classified as Brassica oleracea var. gongylodes.   Kohlrabi is more well-known in Europe and Asia than in the U.S. and well deserves better recognition and use here.  This vegetable takes less space than other cabbage family member, is easy to grow and it is more tolerant of heat.   Insect infestations do not directly affect the most desirable part of the plant, the swollen stem.  Yes, the sweetest, juiciest part of the plant is its stem, which swells with moisture and goodness as the plant grows.  The leaves are edible also, and can be used like cabbage, but the spherical stem can be peeled and cut into cubes; or shredded; or sliced to eat fresh alone or in salads.   The stem is also good cooked and can be used in casseroles and soups. 

Several varieties of kohlrabi are available today, several of them open-pollinated heirloom types.  ‘Early White Vienna’ (55 days) has been grown since the 1850s and is probably the most popular one in gardens.  Early Purple Vienna’ (60 days) from before 1860, is a purple variant of the white.  Both types are remarkably heat and cold hardy for Brassicas.  ‘Superschmeltz’ (65 days) is a giant kind of kohlrabi with stems weighing up to 10 pounds.  This last variety can be left in the garden longer than the other two as it does not tend to become “woody”.  Consistent watering will improve the sweetness and tenderness of this vegetable.  Mulching kohlrabi with 3 or 4 inches of rotted straw will preserve moisture in the soil and will enable you to have great results with less watering, while keeping soil microbes alive.   It is noteworthy that mulched soils are living soils, with abundant soil fungi and microbes that can capture carbon out of the atmosphere.   Keep in mind that bare, un-mulched soils dry and erode, and actually release carbon rather than capture it. 

Organic production of kohlrabi is not difficult.  If you end up with an abundance of cabbage loopers and aphids, the swollen stem will be peeled and so is less affected visually by insects.  However, production will be much higher if you place row covers with breathable insect fabric over your crop and mulch heavily.   Your other Brassicas will benefit from this technique also—there will be no holes in cabbage leaves or worms in the cauliflower and broccoli.   BT, or Thuricide  (Bacillus thuringensis) can be used, but it is better for the environment to simply cover all crops rather than spray.  Insects develop resistance to BT over a few generations, so it should  be reserved for use in special circumstances.  

In Western Montana, we direct sow kohlrabi out April 21-May 1 depending on weather, for harvest in July-early August.   Plants can be started inside about March 1 to be set out around April 15, and harvest would begin in late June.  A second crop (in the same space in the garden) could be direct-seeded around July 15-Aug. 1 following the first crop’s harvests.  Some gardeners plant a new row of kohlrabi every three weeks all season long.  It is ok to plant in the same space within one season, but remember to rotate your crops year to year.  Do not plant the any members of the cabbage family in the same place they grew the previous year; in fact for the previous three years.  A four-year rotation of vegetable crops in your garden will feed your soil and reduce insect and disease infestations.

The germination temperature for kohlrabi is 40-100 degrees F. with 45-95 F. being ideal.  Germination time is usually 3-10 days.  In my experience the percentage of seeds of kohlrabi that sprout is usually low, so plant extra seed in pans or outside when seeding direct.   Water regularly, steadily, and evenly; keep moist, not wet.  Be sure to thin the plants if you direct seed, and mulch when they are about three inches high.  Pests include gophers, root maggots, aphids, cabbage worms, cabbage loopers, diamond back moths, and flea beetles.  Diseases that can appear are: clubroot, alternaria blight, blackleg, black rot, downy mildew, fusarium wilt and wirestem.  It has been shown that soils with a higher pH will reduce the chances of some diseases.  The best soil pH for Brassicas is 6.0-7.5.   Kohlrabi grows best in cool summers, but we still had a great crop last year, which was during the hottest summer any of us remember here in Western Montana. 





Controlling Cut Worms


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In my organic garden of heirloom plants here in Oregon, we have several pests: deer, raccoons, dogs, squirrels, rats, slugs, cabbage looper larvae and cutworms.  Cutworms have been the most consistently damaging of the group.  Typically, they sever the stems of young transplants, a preferred food of theirs.  They also eat most of the plants I grow; all vegetables, but especially plants of the Brassica, Dianthus, Primula and Viola families.  Here, they are a year-round pest, perhaps because of the mild weather of the region.  Cutworms belong to the family Noctuidae.  Note that the Latin root word here “noctu” refers to night—the time when these insects feed.  They come out about two hours after dark to feast, then disappear under the soil for the day.  Some cutworm eggs overwinter and hatch in spring, but others can hatch at any time after, as long as it is not too cold.  Evidently there is one generation per year.  The adult moths emerge in late summer and lay eggs which can hatch at varying times of the year.  Several species exist: the Army cutworm, Black cutworm, Variegated cutworm, and a new species here in Oregon, which is active year round, the Winter cutworm.  The first picture above shows a cutworm that appeared on heirloom petunia seedlings while they were on a bench in my greenhouse.  The plants had been transplanted two weeks before, so I was surprised that the cutworm was able to climb so far!  the photo was taken in May, so this was probably the variegated cutworm, which we have spring, summer and fall.  The second picture shows another cutworm on the edge of a patio planter, three feet above the ground.  The other three photos reveal damage on in-ground plants made by Winter cutworms in December, January and February.  Cutworms typically leave black droppings on your plants; the photo of the cauliflower shows this.

An organic gardener’s best first defense against cutworms is prevention.  A week before plants are to be set out, prepare your bed by cultivating heavily to dislodge and destroy any worms in the soil, then cultivate again.  When you plant your transplants, place collars around the plants to make it difficult for the worms to get to your plants.  Collars may be made of paper (milk cartons work well), plastic, metal or cardboard.  Here it rains so frequently that I choose to use plastic milk jugs or cans with the bottoms cut out of them.  After planting, spread one or more of the following sharp-edged deterrent mulches over the soil, around the plants: diatomaceous earth, eggshells, wood ashes or sharp grit.  The rough surfaces of any of these will discourage cutworms from crawling to your plants.  Another preventive method is to wait until two hours after dark, take a flashlight, search out the larvae and use the squish-bug technique for control.  Continue to cultivate around your plants to dislodge hiding larvae.  One other way to fight the battle is to attract or purchase beneficial insects to control cutworms.  Ground beetles, parasitic nematodes and tachinid flies attack cutworms.  Pollen-laden flowers, (don’t forget about native wildflowers) will help draw beneficial insects to your garden.

A next recourse for severe infestations is to use biological pest controls.  The bacteria Bacillus thuringensis kurstaki (known as BTK) made into a liquid spray is effective in controlling cutworms as long as you do not have continuous rain, as we often do here in western Oregon.  The damaged cauliflower, cabbage and primrose in the photos above show the effects of cutworms in winter, during a three month-long period of rain.  Dry BTK can be mixed with moistened bran to act as a bait and will last a few days until washed away.  Of course one can use the handpick squishbug method, but it might be difficult to keep up!  BTK cannot be used over and over without enabling cutworms to develop resistance.  Take care not to spray anything other than the very plants you need to protect when the larvae of butterflies are present.  Organically approved pesticides containing pyrethrin may also be used for heavy infestations.