Onions and their close relatives–chives, garlic, shallots, and leeks–are among the oldest of home garden plants. Allium is the genus for these crops. All varieties of Allium require loose, well-drained soil rich in nitrogen.
There are hundreds of varieties of onion family plants. All suffer from similar pest, disease, and cultural problems
Here is a troubleshooting list of possible onion family growing problems with control and cure suggestions:
• Plants produce many leaves but no bulbs. Planting time incorrect or temperatures are too warm. Bulbing onion and garlic must be exposed to temperatures of 32° to 50°F for 1 to 2 months before planting to induce bulb formation. Place garlic cloves in the refrigerator for 4 weeks before planting or plant early in season so that cloves are chilled.
• Plants are stunted; worms boring into roots.Wireworms are the soil-dwelling larvae of click beetles; they look like wirey-jointed worms. Check soil before planting; flood the soil if wireworms are present. Remove infested plants and surrounding soil. Keep the garden clean and free of plant debris.
• Leaves turn silvery and white streaked or blotchy; leaves may become distorted. Onion thrips are most common during dry warm, weather. Keep the garden clean. Blast thrips with water to wash them away. Use insecticidal soap.
• Leaves die back from tips; root turn pink to red to yellow; yields is reduced. Pink root is a soilborne fungus. Plant in well-drained soil; rotate crops to reduce disease in soil. Plant resistant varieties: Sweet Spanish, Excel, Granex.
• Leaves have yellow or white spots; stalks wilt, bend and die. Gray to purple mold forms on leaves. Downy mildew is a fungus that attacks during wet, humid weather. Remove and destroy old plants debris. Keep the soil well drained. Allow plants to dry out between irrigations. Keep the air circulating in the garden. Plant resistant varieties.
• Leaves fade, wilt, and yellow; leaf tips turn brown. Tunnels and cavities in bulb; plant may die. Onion maggot is a white legless larva of an adult fly. . Destroy disfigured plants after harvest. Destroy flies during the growing season.
• Leaves yellow and wilt; leaf tips die back. Seedlings thicken and become deformed. Older plants are stunted limp; bulbs are swollen at the base. Stem and bulb pest nematodes are microscopic wormlike animals that live in the water that coats soil particles; they enter plant roots and secrete a toxic substance. Do not plant garlic or onions in areas where onions, garlic, leeks or chives grew in previous years; parsley and celery are also hosts. Remove and destroy infested plants immediately. Use certified seed.
• Onion necks are thick; plant growth is stunted. Phosphorus or potassium deficiency is likely. Side dress plants with compost tea or aged compost.
Neck of bulb becomes spongy and water-soaked and gray or brown mold develops. Botrytis rot or neck rot is a fungal disease. Remove and destroy infected plants. Keep weeds out of garden where fungal spores may harbor.
• Onions set flower and go to seed; bulbs are hollow. Nip off flower stalks and flowers so that plant will put energy and nutrients into bulb formation not seed production.
• Bulbs are small but look white and normal. Wrong variety planted or seed or plants planted at the wrong time. Plant a variety suited for your region at the proper time. Keep garden free of weeds; onion family members do not compete well with weeds.
• Leaves yellow, bulbs have soft, watery rot and decay; bulbs may be speckled black. Bulb rot also called white rot is a soilborne fungal disease. Remove and destroy infected plants. Rotate crops. Keep weeds out of garden where fungal spores may harbor. Plant resistant varieties: Elba, Globe, Grandee, Hickory.
• Onion bulbs split into two or three sections. Watering is uneven. Water so that soil is fully moist and then allow the soil to dry to 4 inches deep before watering again. Mulch to keep soil evenly moist. Stop feeding plants 7 weeks before harvest.
• Flavor of sweet onions is pungent. Heat stress and water stress can cause onions to become pungent flavored. Sweet onions are best grown in cool weather with even watering.
• Elongated blisters and streaks on seedlings and bulb scales. Smut is a fungal disease that resulting in dark, slightly thickened areas on leaves. Black lesions appear on the scales of forming onion bulbs. Remove and destroy infected plants. Plant resistant varieties: Evergreen Bunching, White Welch, Winterbeck.
Onion Growing Success Tips:
Planting. Plant onions in full sun. Onions grow best in light loam or sandy soil rich in organic matter. Onions can be grown from sets–small bulbs–or seed. Onion sets are easier to start than seed. Place the set in the soil so that the top of the small bulbs are level with the soil surface. Small sets are less likely to bolt.
Plant time. Generally, onions are planted in the fall in warm, southern regions and in the spring in cool northern regions.
• In southern regions, plant short-day onions in the fall, allow them to root and grow foliage before they go dormant as temperatures drop in winter. Mulch over-wintering onions and remove the mulch in spring. These onions will form bulbs in late spring.
• In northern regions, plant onions in the spring. Sow seed indoors 8 to 10 weeks before transplanting onions into the garden. Set out transplants as early as 4 to 6 weeks before the last average frost date in spring.
• Several days below 50°F or one or two days below 30°F will cause sets to bolt, so do not plant too early.
Care. Keep onions evenly moist. When the tips of leaves turn yellow bulbs are nearing maturity and watering can stop. Keep planting beds weed free. Side dress onions with aged compost during the growing season, up to about a month before harvest.
Harvest. Green onions can be harvested at any size suitable for use. Bulb onions are ready for lifting when the leaf tops begin to yellow and die back. Cure onions for storage for 1 or 2 weeks, and then store them in a cool, dry place.