How to Grow Onions
A culinary staple, onions are an essential vegetable in American gardens. Onions, like other members of the Allium family, are biennials, producing seeds in their second year of growth. The bulb (or common) onion has brown, yellow, or red skin and is round, elongated, or flattened. Bunching onions (or scallions) are harvested small.
When and How to Start Indoors
Plant onion seeds indoors 8–10 weeks before transplanting them outside just before the average last frost date in your area. Seeds should be sown ¼ inch deep. Onions require an open and sunny site, fertile soil, and good drainage.
Time to Germination
Seeds will germinate in 4-10 days when started indoors.
When to Transplant
Transplant outdoors just before the last frost.
When transplanting your seedlings, space them at least 6 inches apart. Plant onion seedlings in the least weedy part of your garden; onion seedlings are small and do not compete well with weeds.
Before the last frost, make a large furrow in your soil, at least 4 inches deep . Water this furrow before planting your seedlings to make transplanting easier.
Common Pests and Diseases
Several bacterial, fungal, and viral diseases can affect onion growth. Insect pests can also be a problem to various degrees in different parts of the United States. Plant your onions in a well-drained space. Diseases and fungi such as Fusarium basal rot, white rot, and Botrytis neck rot can affect crops in storage. Crop rotation can help prevent these diseases.
When and How to Harvest
The tops of bulb onion plants fall over naturally once the bulbs have matured. When half of the tops in a planting have fallen over, lift all of the bulbs and place the pulled plants in a warm, dry place (away from direct sunlight) to cure.
Onions have limitless potential in the kitchen and are indispensable for flavoring savory dishes. Onions can be roasted, fried, pickled, sautéed, and combined into dishes in dozens of other ways. Bunching onions, especially, are perfect for salads, pastas, and soups. Onion jam or compote is a great way to use a flush of red onions that will not store as well as white or yellow onions.
Cure onions for two to three weeks after harvesting by storing them in a warm place away from direct sunlight. When the onions feel paper-dry on the outside, clip off the tops and roots, and lightly brush off loose soil before storing the onions in a cool, dry place. Arrange them in a single layer or hang them in mesh bags. Always handle onions very carefully; the slightest bruise will encourage rot. Properly cured onions will store for 6–8 months in a root cellar or cool basement.
How to Save Onion Seeds
Onion seeds are not typically difficult to grow or to collect, but keep in mind that they are a biennial crop, meaning that they seed once every two years.
Recommended Isolation Distance
When saving seeds from onion, separate varieties by at least 800 feet up to ½ mile. To produce seed from onion, select as many perfect onions as you can spare for seed production and store them through winter in a cool, dry, dark place. Replant them in early spring at the same bulb depth and spacing as when they were harvested.
Recommended Population Sizes
To ensure viable seeds, save seeds from at least 5 plants. To maintain a variety over time, save seeds from between 20-50 plants.
Assessing Seed Maturity
Watch first for flowers and then for seed heads to form during the late summer of the second season. Wait for the seed heads to dry. Most of the flowers will be dry, and the seeds will begin to fall out on their own.
After the plants bloom and seed heads begin to dry, gather the heads in a paper bag. Most of the seeds will fall out on their own; shake the bag to free the remainder of the seeds.
Cleaning and Processing
Separate the seeds from the stems and other matter that makes up the seed head. Allow the seeds to air-dry for a few days before storing the seeds in a cool, dry place.
Storage and Viability
When stored in a cool, dark, dry place, onion seeds will remain viable for 2 years.