How to Sow and Plant
Onions may be grown from seed, from young bare root plants or from sets (small bulbs). Make sure to choose the correct variety for your day length. Southern gardeners should select Short Day varieties; Northern gardeners do best with Long Day varieties; gardeners in the middle of the country should select Intermediate Day varieties, but can use some Short Day varieties.
Sowing Seed Indoors
- Onion seed may be started indoors in small flats in seed starting mix 6-10 weeks before the last frost.
- Sow thinly and cover with ¼ inch of seed starting formula. Keep moist and maintain a temperature of about 60-65 degrees F.
- Seedlings emerge in 7-14 days.
- As soon as seedlings emerge, provide plenty of light on a sunny windowsill or grow seedlings 3-4 inches beneath fluorescent plant lights turned on 16 hours per day, off for 8 hours at night. Raise the lights as the plants grow. Incandescent bulbs do not work because they get too hot. Most plants require a dark period to grow, do not leave lights on for 24 hours.
- Seedlings do not need much fertilizer, feed when they are 3-4 weeks old using a starter solution (half strength of a complete indoor houseplant food) according to manufacturer’s directions.
- After danger of a heavy frost plant the seedlings in the garden when they are about the thickness of a pencil. Before planting in the garden, seedling plants need to be “hardened off”. Accustom young plants to outdoor conditions by moving them to a sheltered place outside for a week. Be sure to protect them from wind and hot sun at first. If frost threatens at night, cover or bring containers indoors, then take them out again in the morning. This hardening off process toughens cell structure and reduces transplant shock and sun burn.
- Space 3-4 inches apart in rows 1-2 feet apart. Plant more closely if you plan to harvest scallions.
Soil Preparation in the Garden
- Choose a location in full sun where you did not plant onions the previous year.
- Apply a balanced fertilizer and work into the soil prior to planting. Onions prefer a pH of 6.0 – 7.0.
- Onions prefer an organic soil that drains well. Work organic matter into your soil at least 6-8 inches deep, removing stones, then level and smooth.
Sowing Directly in the Garden
- Sow onion seeds in average soil in full sun after danger of frost in spring. In frost free areas, sow in fall.
- Sow thinly in rows 1- 2 feet apart and cover with ¼ inch of fine soil. Firm lightly and keep evenly moist.
- Seedlings emerge in 7-14 days.
- Thin to stand about 3 inches apart when seedlings are 1- 2 inches high.
- Burpee ships small onion plants about 10 to 12 weeks old in early spring. Plant onion plants as soon as possible after you receive them, as soon as the soil can be worked, before the last frost.
- Plant onion plants 1 inch deep, 5 – 6 inches apart, or 2 – 3 inches if you prefer to thin later for green onions or scallions. Water well.
- Just press sets into the soil up to their tops, barely covered with soil 3-4 inches apart in rows 1-2 feet apart. If sets are planted too deeply they will take longer to develop.
Common Disease Problems
Bacterial Soft Rot: This is mostly an issue with mature bulbs. Affected scales first appear water-soaked and pale yellow to light brown or bleached gray to white. As the disease progresses, infected tissue becomes soft and sticky with the interior of the bulb breaking-down. A watery, foul-smelling thick liquid can be squeezed from the neck of diseased bulbs. It is spread by water and some insects through wounds on the plant. Burpee Recommends: Avoid overhead watering where possible, and control insect pests, such as onion maggot, that can spread it. Allow onion tops to mature before harvesting and avoid damaging bulbs during harvest. Store onion bulbs only after they have been properly dried, with good air circulation to prevent moisture condensation from forming on the bulbs.
Neck Rot: This fungus mostly affects onions in storage. It enters plant tissue at the neck of the bulb through a wound, most likely from cultivating or harvesting. The tissue becomes soft and spongy with brown water soaked lesions. Burpee Recommends: Avoid overhead watering, increase air circulation around plants and harvested bulbs. Avoid neck injury when working with plants. Make sure bulbs are properly cured and are stored dry. Remove infected plant debris.
Onion Smut: This fungus attacks mostly young seedlings as the new leaves emerge. It causes blister-like lesions near the base of the bulb and streaks on leaves, sheaths and bulbs. The streaks mature into black powdery spores. The fungus stunts the growth of the plants. More mature plants are not as susceptible. It is most prevalent in temperatures under 75 degrees F. The disease lives in the soil for several years. Burpee Recommends: Rotate crops and do not plant in the same area for at least three years. Encourage rapid growth with watering and fertilizer to get plants past the susceptible stage. Some fungicides are effective, check with your Cooperative Extension Service for recommendations.
Pink Root Rot: A fungus that attacks onion roots causing them to turn a light pink, then red and eventually purple-brown and causing them to shrivel. Infected plants show signs of nutrient deficiencies and drought because the roots cannot take up water and nutrients. Plants are stunted. The disease lives in the soil for several years and thrives in warm temperatures. Burpee Recommends: Plant as early as possible so the bulk of the plant growth will be in cooler temperatures. Rotate crops and plant resistant varieties.
Smudge: This fungus occurs late in the season and can extend into storage. It causes dark green to black lesions in concentric rings around the neck and on the surface of the skin of the outer scales and can spread to inner scales and cause the bulb to shrivel. It thrives in high humidity. Burpee Recommends: Practice crop rotation as the disease can live in the soil for several years. Harvest only in dry conditions and cure properly. Some fungicides are effective, check with your Cooperative Extension Service for recommendations.
Common Pest and Cultural Problems
Aphids: Greenish, red, black or peach colored sucking insects can spread disease as they feed on the undersides of leaves. They leave a sticky residue on foliage that attracts ants. Burpee Recommends: Introduce or attract natural predators into your garden such as lady beetles and wasps which feed on aphids. You can also wash them off with a strong spray, or use an insecticidal soap.
Leafhoppers: Leafhoppers cause injury to leaves and stunt growth. They also spread disease. Burpee Recommends: Remove plant debris. Use insecticidal soaps. Consult your Cooperative Extension Service for other insecticide recommendations.
Leafminers: These insects bore just under the leaf surface causing irregular serpentine lines. The larvae are yellow cylindrical maggots and the adults are small black and yellow flies. They do not usually kill plants, but disfigure the foliage. Burpee Recommends: Remove affected foliage.
Cutworms: These insects cut off the seedlings at the soil level. Burpee Recommends: Place a paper cup collar (use a coffee cup with the bottom cut out) around the base of the plant. They are usually mostly a problem with young seedlings. You can also control by handpicking and controlling weeds, where they lay their eggs.
Wireworms: These insects live in the soil and kill seedlings by girdling their stems at the soil line, bore into stems, roots and tubers. They may be found around the stems in the soil are and ¼ to ¾ inch long, thin, yellow brown worms with a shiny skin. The adults are called click beetles, and are about 1/3 inch long, reddish brown with a hard shell. Burpee Recommends: Rotate crops. Check with your Cooperative Extension Service for pesticide recommendations which must be applied prior to planting.
How many onions do I get per plant or set? One.
Why are my onions not bulbing? A northern onion variety is being grown in the south (or vice versa) OR transplants were planted too deeply OR plants are planted too closely together OR the soil is too clayey OR plants are stressed from a water or nutrient deficiency, pest or disease.
Can I grow onions over winter? Southern and frost free gardeners can grow onions all winter.
How long will my onions store? Sweet onions store for a maximum of 3 months; storage onions store for a full winter season.
Can any onion be harvested as scallions? Yes, there are varieties that will only be grown as scallions, but any onion variety may be harvested early for scallions.