This guide answers when to plant red onions zone 7 with timing worked out from USDA Zone 7 frost-date averages, not guesswork. Red onions are the sweet, mild onion for salads, grilling and pickling, with purple-red rings that hold their colour when cooked lightly. In Zone 7 they grow as a spring-sown crop: seed or sets planted in March mature by early summer, and the long days of late spring and early summer are what trigger bulbs to swell. Choose a variety suited to your day length — your local extension can advise — because day length matters more for onions than for most vegetables.
Zone 7 at a glance
Zone 7 covers areas where average annual minimum temperatures run 0 to 10°F. The average last spring frost falls in the Mar 15–Apr 1 range and the average first autumn frost in the Nov 1–15 range — these are USDA zone averages from USDA and NOAA frost records, not exact dates for any one garden, so check your local extension service before planting tender crops outside. Use the frost date calculator to work out dates for your own garden, and browse the planting calendar hub for the full Zone 7 year.
When to plant red onions in Zone 7
| Step | Timing in Zone 7 |
|---|---|
| Start indoors | January to February, 8 to 10 weeks before the last frost |
| Direct sow outdoors | March, as sets or seed, about 2 to 4 weeks before the last frost |
| Transplant outdoors | March to April |
| Autumn / second sowing | September to October for overwintering sets in mild spots — check your local extension |
| Harvest | June to July |
The table above is the short answer. The detail matters because red onions is hardy — a hardy crop that tolerates light frost, so it can go out weeks before the last frost. Start indoors when the table says so, using the seed-starting calculator to count back from your last frost, and harden seedlings off for a week before they go outside. The Zone 7 March guide shows what else is happening in the garden in that month.
How to plant red onions
Sow seed indoors in January to February, or plant sets — small dormant bulbs — directly in March, about 2 to 4 weeks before the last frost. Plant sets about an inch deep, 4 to 6 inches apart, in loose, well-drained soil, and keep the bed weed-free: onions have shallow roots and lose badly to weeds. Onions need full sun and soil in the pH 6.0 to 7.0 range, with plenty of compost worked in before planting.
Give red onions full sun — at least six hours of direct sun a day for full-sun crops, or morning sun with afternoon shade where part shade is listed. Prepare the bed by forking in compost and removing stones and weeds, and check the soil pH: red onions prefers 6.0–7.0. Space plants 4–6 inches apart — the spacing in the table below — and sow seed at the depth on the packet, keeping the seedbed moist until germination, which takes 7–14 days.
| Fact | Range |
|---|---|
| Sun | full sun |
| Days to germination | 7–14 |
| Days to maturity | 90–120 |
| Spacing | 4–6 inches |
| Soil pH | 6.0–7.0 |
| Sowing method | both |
| Cold tolerance | hardy |
| Water needs | moderate |
Watering and feeding
Onions have moderate water needs, needing steady moisture while bulbs form but drier soil as harvest approaches — too much water late in the season causes soft bulbs that store poorly. Water about an inch a week in dry weather and stop watering once the tops start to fall over. Feed lightly with compost at planting; avoid heavy nitrogen late, which grows thick necks that do not cure well.
A layer of mulch — straw, shredded leaves or compost — helps on both fronts: it holds moisture in the soil, keeps roots cool, and stops soil splashing onto leaves and fruit. Keep mulch a few inches away from stems so it does not trap damp against them.
Companion plants and rotation
Good companions for red onions include marigolds, basil, onions, lettuce. Use the companion planting finder to plan combinations for your whole bed. Rotate crops each year: do not plant red onions where the same family grew last year, and keep a simple note of what grew where so you can rotate properly next season. Also see our guide to when to plant white onions in Zone 7, which pairs well with red onions in a Zone 7 bed.
Pests and problems
The common pests for red onions are aphids, slugs, cutworms. Onion fly is the main pest: cover young plants with fine mesh if it is a problem in your area. Check plants weekly, looking under leaves as well as on top — most problems are easy to deal with if caught early. Good air flow helps: do not crowd plants, water at the base rather than over the leaves, and clear away dead leaves and debris where pests hide.
Harvesting and storing
Harvest when most of the tops have fallen over and started to yellow, usually 90 to 120 days after sowing. Pull bulbs on a dry day, leave them on the soil surface or under cover for a week or two to cure, then trim the tops and store in a cool, dry, airy place — a mesh bag or braid in a shed or garage. Well-cured red onions keep for several months. Eat any thick-necked or damaged bulbs first, as they will not store.
For the best flavour, harvest in the morning, when plants are cool and their moisture and oils are at their best, and use a sharp knife or scissors for a clean cut that heals quickly.
Frequently asked questions
When should I plant red onions in Zone 7?
March, as sets or seed, about 2 to 4 weeks before the last frost. As a rule, work back from the average last frost range of Mar 15–Apr 1 for spring planting, and back from the average first frost range of Nov 1–15 for autumn planting. These are zone averages, so check your local extension service for your area.
Can I grow red onions in containers?
Yes, in most cases — choose a pot at least 12 inches deep with drainage holes, fill it with good potting mix, and water more often than beds, since pots dry out faster. Container growing suits red onions well, especially where garden soil is heavy or space is limited.
How long does red onions take to grow?
About 90–120 days to maturity from sowing or transplanting, depending on variety and weather. Germination takes 7–14 days. Warm, sunny weather speeds growth; cold or cloudy weather slows it, so treat these as ranges, not exact dates.
What soil pH does red onions need?
A pH of 6.0–7.0 is ideal. Test your soil before planting and amend it if needed — your local extension service can test a sample and advise on what to add.
What grows well with red onions?
Good companions include marigolds, basil, onions, lettuce. Avoid planting it where the same family grew last year, and give it the spacing and sun listed in the table above so companions do not crowd it out.