Growing onions from seed offers a wider variety, is less expensive, and gives you more control over growing conditions than using starter plants.
Bulbing onions' bulb growth is triggered by day length, which varies with latitude. Onion bulbs are actually an extension of the above-ground leaves. Starting onions seeds early in the season produces larger leaf growth prior to day length, triggering bulb growth, which means better potential for large bulbs. Understanding what varieties grow best in your area is the first step to success.
Choosing a Bulbing Onion
Long-day varieties grow well in the north (above the 37th parallel), as they need 14 to 16 hours of daylight to trigger bulb formation. Intermediate-day varieties overlap the long and short day ranges a bit and cover the middle of the country (32nd to 42nd parallels). These varieties start the bulbing process when day length is 12 to 14 hours.
Short-day varieties are best sown in fall in the south (below the 35th parallel), for a late winter/early spring harvest. These varieties need 10 to 12 hours of daylight to trigger bulbing. Depending on soil temperature, southern gardeners may choose to sow onion seed directly into the garden. In the north, short-day onions may be grown over the winter in a greenhouse, or transplanted out in the spring; this method produces earlier but smaller bulbs.
Among the above types of onions, cultivars will also vary in storage length. Storage Onions have thicker skin, stronger flavor, and can be stored for 2 months or longer. Sweet Onions are softer, sweet, and best used within a few weeks after harvest.
When to sow outside: 4 to 6 weeks before average last frost, or as soon as soil can be worked; when soil temperature is at least 45°F. In mild climates, sow in late summer or early fall.
When to start inside: RECOMMENDED. Bulbing Onions: In cold climates, seeds should be started indoors 10 to 12 weeks ahead of your average last spring frost. In mild climates, sow seeds indoors 6 to 8 weeks prior to transplanting in late summer or early fall. Leeks and Shallots: Leeks and shallots are similar to onions-the bigger the transplant, the better potential for larger product-so start these early (8 to 10 weeks before average last spring frost). Shallots are cold hardy and can also be transplanted out in the fall, and over-wintered in any USDA zone. Bunching Onions: Sow bunching onions indoors 8 to 10 weeks before average last frost.
Use a lightweight seed-starting mix/media (sterile, and lighter than potting mix), and sow seeds 1/2" deep about 1/2" apart. Thinning is not necessary. When seedlings get tall, they may begin to flop over; if this happens, trim them to 3" tall to keep them upright. Read about more indoor sowing tips.
Onions can be sown by broadcasting seeds in a wide, shallow container with drainage or in cell packs.
In cold climates, transplant onion seedlings 4 to 6 weeks before your average last spring frost. In mild climates, transplant onions in late summer or early fall.
After hardening off, transplant onions, shallots, and leeks up to 4" deep. This may leave only a tiny bit of green poking up through the soil; this leaves room for the bulb to grow and in the case of leeks, it blanches stems (keeps them white with better flavor). Don't worry; leaf growth will quickly catch up. Scallions and bunching onions can be grown densely; all others should be separated into individual plants. Bulbing onions are transplanted 3"–4" apart. Shallots are transplanted 6" apart and leeks are transplanted 6" apart.
How to sow
Sow a group of 2 seeds every 4" at a depth of ¼". When 2" tall, thin to 1 every 4".
Optimal Growing Conditions
Light, well-drained soil that is high in nitrogen and rich in organic matter and free from rocks, sticks and clods.
Keep evenly moist; do not allow to dry out. Water stress will reduce yields and bulb size. Always water immediately after fertilizing. The closer to harvest time, the more water the onions will require, but when the tops begin to fall over, stop watering.
Choose an area with full sun (6 or more hours per day).
Onions are heavy feeders and benefit from soil rich in organic matter, phosphorous, and nitrogen. Performing a soil test is the best way to know what, if any, additional nutrients should be added to your soil. Once onions begin pushing soil away from the "neck" (at the soil surface) the bulbing process has begun, and fertilization should cease.
Once greens are 8"–10" tall, beds can be heavily mulched or hilled with soil to reduce weed pressure and conserve water.
Bulbing onions and shallots can be harvested at any time, but to get the largest bulbs and longest storage time look for a few cues from the plant. When bulbing onions and shallot tops have fallen over and turned yellow or brown, they are ready for harvest. Harvest in the morning, lifting onions with a garden fork. Dry them in the garden in the sun for 2 to 3 days, lightly covering the bulbs with straw, or the tops of other onions to prevent sunscald. Cure onions for 3 to 7 days in a dry area with good air circulation. Once dry, cut the roots to 1/4", and the greens to 1" to create a seal to prevent decay.
Bunching onions and leeks can also be harvested at any time and both are frost tolerant but should be harvested before a hard freeze (bunching onion species Allium fistulosum can actually overwinter). Many gardeners prefer to harvest leeks after a light frost, as the sugars produced to prevent freezing make the onions sweeter.
Common Pests and Diseases
Botrytis Blight, also called gray mold, is a common fungal disease of onions. The first signs of infection are tiny, water soaked spots on leaves, stems, and fruit. The spots enlarge and become soft and watery, and plant tissues turn light brown and crack open, allowing fuzzy gray spores to emerge. The botrytis fungus overwinters on plant debris, and in the spring, spores are transported to new sites by wind, water, and dirty garden tools. The spores reproduce rapidly during periods of very high humidity or wet weather, and temperatures of 60°–76°F. Plant in well-drained soil, and keep air circulation high with proper spacing. If possible, avoid leaf wetting when watering. To reduce the spread of the disease, remove and discard infected plant parts (do not compost), clean up garden debris in the fall, and sanitize your garden tools regularly.