Planting Onion
Onion is a cool season vegetable in the Amaryllidaceae family. Getting the timing right is the difference between a strong stand and a disappointing one, so the windows below are given relative to your own last spring frost and first fall frost rather than a generic calendar date. Look up your local frost dates and count back or forward from there.
Onion is started indoors 8–10 weeks before your last spring frost date, giving seedlings a head start before they move outside.
Transplant young plants outdoors 2–4 weeks before your last frost — Onion tolerates cool conditions and benefits from an early start.
You can sow Onion directly into the garden 2–4 weeks before your last frost.
Onion can be grown by starting indoors, direct sowing, and transplanting. Starting indoors gives the longest, most controlled season, while direct sowing is simplest where the season is long enough.
Spacing and Planting Depth
Give Onion room to mature. The figures below come from verified extension and seed-supplier data for typical varieties.
| Spacing in row | 4 inches |
|---|---|
| Row spacing | 12 inches |
| Plants per sq ft | 3 |
| Planting depth | 0.25 inches |
| Sun requirement | Full sun |
Days to Maturity
Onion reaches maturity in 100–120 days from transplant.
Onion is ready to harvest after about 110 days. Harvest before summer heat or, for fall crops, before a hard freeze, to keep quality high.
Conditions and Care
As a cool-season vegetable, Onion does its best growing in the cooler weather of spring and fall and tends to bolt or turn bitter in summer heat. It is frost hardy and can shrug off light freezes, so it can stay in the ground later into the season than tender crops.
Onion needs full sun — give it at least six hours of direct light a day for the best growth and flavor. Sow seed about 0.25 inches deep — small seed is sown shallow and barely covered, then keep the soil evenly moist until seedlings establish.
Onion belongs to the Amaryllidaceae family; rotating where you grow members of this family each year helps limit the build-up of soil-borne pests and disease. Onion is generally grown as a single planting each season rather than succession sown.
Companion Plants
Pairing Onion with the right neighbors can improve growth and deter pests; a few combinations are best avoided.
Grows well with: Carrots, Lettuce (Loose-leaf), Tomato, Beets
Keep away from: Shelling Peas, Green Beans (Bush)