Cutworms Dull grey or brown, 2.5-to-five-centimetre caterpillars that cut off seedlings at or just below ground level. They hide in the soil during the day and feed at night. Place a plastic or cardboard collar, such as half a toilet paper roll, around the stem of each seedling when planting.
Cabbage looper Three-centimetre-long, green caterpillars, which loop their bodies as they crawl, chew ragged holes in the leaves. Cover young plants and seeded rows with floating row covers. Spray infested plants, including the undersides of leaves, with Bt (Bacillus thuringiensis) every two weeks until caterpillars are under control.
Cabbage root maggot White, three-centimetre-long maggots burrow into roots and stems, spreading disease. Seedlings become yellow and stunted, wilt in the heat and eventually die. Protect young plants with floating row covers, or place a collar or mat around their bases.
Black leg Fungal disease that produces small, dark, sunken spots on leaves and stems. Leaf edges wilt and discolour; the whole plant may die. Remove and destroy infected plants.
Black rot Causes yellow, triangular areas with darker veins on leaves. As rot progresses, leaves wilt and plants may die. If this is a problem in your area, plant resistant 'Grand Duke'.
Club root Leaves yellow, and plants stop growing and may die. Roots swell, become club-like and eventually rot. Remove and destroy infected plants.
Imported cabbageworm Green three centimetre-long caterpillars that chew large holes in leaves. The adult white moths hover around plants of the cabbage family. Protect young plants with floating row covers. Spray with Bt (as above).
An early crop
For an earlier crop, start seeds indoors two to four weeks before the outside temperature is consistently above 5°C. Give each seedling its own pot to minimize transplanting shock, which may result in small, tough bulbs. Use a good potting soil. Plant two or three seeds per container, later thinning to the most vigorous seedling. Make sure soil is moist but not soggy, cover containers with plastic-it doesn't have to be clear plastic as the seeds will germinate in light or dark-to retain moisture and place in a warm location.
Seeds germinate in a week or less at 24 to 26°C. As soon as the seeds germinate, move them to a bright location with a cooler temperature. Grow lights are ideal; otherwise, place in a sunny southern window with supplemental light on overcast days. Feed weekly with diluted fertilizer or compost tea. Make sure seedlings don't become pot-bound before they're planted outdoors.
About a week before planting outside, start hardening off the seedlings outdoors in a sheltered location. Gradually acclimatize them to sunlight. Plant out seedlings at the same time you would normally plant kohlrabi seeds directly into the ground.
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