Kohlrabi

Brassica oleracea var gongylodes

Introduction: This somewhat bizarre looking vegetable is relatively rare in American gardens and kitchens; in fact many people wouldn’t even recognise it.

The edible part of the kohlrabi is not a root, though people usually assume it is. It is actually the swollen above-ground portion of the stem, and is very much like a fat juicy broccoli stem. It is sometimes said to be a cross between a cabbage and a turnip, but that is just because it seems like one. More likely it was developed from a thick stemmed forage cabbage.

Kohlrabi (the name is German and translates as cabbage turnip) was developed in Italy or Germany around 500 years ago. It is still most popular there, so hasn’t really spread very much, though it is now popular in parts of Asia too.

Climate: Like all Brassicas this is a cool season plant, preferring to grow in temperatures of 60 – 70°F. In cool climates it can be grown all summer, but in warmer ones it must be grown in spring and fall (it grows best in the latter). It is quite hardy and can stand moderate frost.

Ease of growing: Kohlrabi is a very useful crop, tasty, fast growing, compact, nutritious and slightly more tolerant of warm weather than most Brassicas. Much that has been said about cabbage also applies to kohlrabi.

Crop value: Some people (those who are most familiar with it) consider kohlrabi to be the best tasting of all the Brassicas and prize it very highly (they also recommend it as a good plant for children to grow). Individual plants don’t produce much food, so you need to have quite a few of them to make it worthwhile. Fortunately it doesn’t require a lot of space and matures pretty quickly (60 days).

Nutritional content: This is much the same as cabbage, which means high vitamins A and C, folate, potassium, calcium and lots of antioxidants. It contains about 120 calories per pound.

Soil

pH 6.0 – 7.0

To grow to perfection, kohlrabi needs a light, rich, moisture retentive soil, with lots of organic matter.

Soil preparation: Incorporate 2˝ of compost or aged manure into the top 6˝ of soil (you could add fresh manure the previous fall), along with a fertilizer mix. It is a light feeding crop and doesn’t need a lot of nitrogen, but it does like potassium (add greensand or wood ashes). It also likes calcium and a fairly neutral pH, so lime if necessary.

Planning

Where: Kohlrabi grows best in full sun, though it will tolerate light shade.

Crop rotation: Kohlrabi should not be planted where any other

Brassica has grown in the previous 3 years, as this can lead to disease or pest problems.

When: Like most Brassicas, kohlrabi grows best in the cool (40 – 70°F) weather of spring and fall.

Spring: Sow the first spring crop 4 – 6 weeks before the last frost date (it will grow at 40°F). The fast maturing types work best in spring.

Fall: Kohlrabi generally does better as a fall crop, sown 6 – 8 weeks before the first fall frost date. Autumn kohlrabi can be allowed to get larger than 2 – 3˝, because cold weather seems to keep them tender for longer (it also increases their sweetness). Any type can be used at this time of year.

About Kohlrabi   Seed facts Germ temp: 40 (45 – 95) 100°F Germ time: 3 – 10 days 15 days / 50°F 9 days / 59°F 6 days / 68°F 5 days / 77°F * Optimum 4 days / 86°F Seed viability: 4 years Germination percentage: 75%+ Weeks to grow transplant: 5 – 6   Planning facts Hardiness: Half hardy Growing temp: 40 (60 – 65) 75°F Plants per person: 10 Plants per sq ft: 4   Planting: Start indoors: 6 – 8 wks before last frost Plant out 2 – 4 wks before last frost Direct sow: 4 – 6 wks before last frost Fall sow: 6 – 8 weeks before first fall frost   Harvest facts Days to harvest: 55 – 90 days 30 – 40 days from transplanting Yield per sq ft: 1½ Yield per plant: 6 oz

Succession sowing: In spring you can make several succession sowings, 2 – 3 weeks apart (very early sowings may be vernalized and bolt). In cool climates you can continue to succession sow all summer. 

Planting

Like most Brassicas it can be either transplanted or direct sown, both work well.

Transplants

Starting inside: Kohlrabi is such a fast growing crop, that starting indoors is probably only worthwhile if your growing season is very short, or if space is limited. Sow the seeds 3 – 4 weeks before planting out. It doesn’t really like transplanting so it’s best to use cell packs, soil blocks or plug trays.

Planting out: Transplant the seedlings outside 2 – 4 weeks before the last frost date. Plant them to the depth of their first true leaves.

Direct sowing: Kohlrabi is usually direct sown, Sow the seed ¼˝ deep in rows or broadcast it and cover with a thin layer of cover soil. Sow quite thickly initially and thin when they are around 4˝ high (eat the thinnings).

Spacing: Make sure the plants aren’t crowded, or they won’t size up properly.

Beds: Plant kohlrabi in offset rows across the bed, 4˝ – 6˝ – 8˝ apart (the exact distance depends upon the fertility of the soil).

Rows: When growing in rows plant 1˝ apart initially and slowly thin to 4 – 6˝ apart. Space the rows 12 – 18˝ apart.  

Care

Kohlrabi must grow fast for best quality, so give the plants all the water and nutrients they need.

Thinning: It is important to thin (and weed) the plants properly, so they have enough room. If they are crowded they won’t bulb up properly.

Weeds: Control weeds by hand weeding, as hoeing can easily damage the shallow roots and swollen stems. A mulch will help to suppress weeds.

Water: Keep the soil evenly moist, or the bulbous stems may turn woody. Fortunately this isn’t often an issue with this cool weather crop. The plants have their greatest need for water when the bulbs are forming, so make sure you keep the soil moist at this time.

Mulch: This is helpful to keep the soil moist and to suppress weeds (the leaves are fairly sparse and don’t create a lot of shade).

Fertilization: If the soil isn’t very fertile, you should give the plants a feed of liquid kelp or compost tea every 2 – 3 weeks.

Pests: The same pests that attack cabbage also go for kohlrabi, but they aren’t usually as bad (See Cabbage). In spring you may have to protect the seedlings from birds.

Harvesting

When: Start harvesting the bulbous stems when they are 1½ – 2˝ in diameter, as they are most tender at this stage. You can eat the larger 3˝ diameter roots, but in warm weather they often develop a woody core and their flavor deteriorates. In late fall and winter even the larger bulbs can be good.

 How: Cut the stem an inch below the bulb, or simply uproot the entire plant if they aren’t growing too closely together (don’t disturb neighboring plants). For storage you should cut off the leaves and roots.

Storage: Kohlrabi stores very well and will keep for several weeks in the fridge in a plastic bag. It may last for several months in a root cellar at 32 – 40°F and 90%+ humidity. It also freezes well.

Seed saving: This is the same as for cabbage. In very cold climates you may have to lift the plants in fall (keep their roots intact) and store them in moist sand in a root cellar over the winter. Replant them in early spring, somewhere they won’t be in the way

Unusual growing ideas

Intercrop: This compact and fast growing plant can be very useful for intercropping between slower growing plants.

Varieties

Up until recently there weren’t many varieties available in this country, but now new ones are appearing all the time, including a lot of hybrids from Europe.

Green Vienna (better flavored)

Purple Vienna (hardier)

Early White Vienna (what’s this with Vienna?)

Gigante (Superschmeltz): Can get very big without getting woody and tolerates low temperatures.

Eder F1: Fast growing (only 38 days).

Winner: Exceptional flavor.

Kossack: Stores well.

Kitchen use

The bulbous stem is good cooked, but is also mild enough to be eaten raw (sliced or grated) in salads. The thick skin is usually peeled off, though if the root is small and tender it may be eaten. The young leaves can be used like kale.