Zone 8 gives onions a long season, but only if the planting date respects the frost calendar. Average winter lows are 10 to 20°F, the last spring frost averages Mar 1–15, and the first autumn frost averages Nov 15–Dec 1 — a long frost-free stretch in an average year. All of these are USDA/NOAA averages, not exact dates for a particular garden, so use your local extension service to confirm the dates for your own spot.
Zone 8 at a glance
In Zone 8, winter lows average 10 to 20°F and the growing season runs from the last frost (Mar 1–15) to the first frost (Nov 15–Dec 1). Onions needs 90–120 days to maturity and is hardy. The planting calendar hub gives the full Zone 8 picture, and the frost date calculator can work from your own local averages.
| Detail | Zone 8 guidance |
|---|---|
| Average last spring frost | Mar 1–15 (average/range) |
| Average first autumn frost | Nov 15–Dec 1 (average/range) |
| Winter low | 10 to 20°F |
| Days to germination | 7–14 days |
| Days to maturity | 90–120 days |
| Sowing | Sets/transplants Jan–Feb; seed indoors in autumn or early winter |
| Transplanting | Jan–Feb |
| Harvest | 90–120 days after planting; harvest May–Jun |
| Sun | full sun |
| Soil pH | 6.0–7.0 |
| Spacing | 4–6 inches |
| Water needs | moderate |
When to plant onions in Zone 8
Onions are a long-season crop in Zone 8, and the key is to start early. Set out transplants or sets from January into February, so plants have months of leaf growth before bulbing begins in spring. Seed can be started indoors in autumn or early winter and transplanted out in January. In Zone 8, choose varieties suited to southern gardens — short-day types that bulb as days lengthen in spring — because varieties bred for northern summers often fail to bulb here. With 90–120 days to maturity, a January planting is typically ready from May into June. There is no late-season sowing: onions need the long run-up. The Seed Starting Calculator turns your frost averages into sowing dates — see also Zone 8 January for what else is happening in the garden that month.
Starting onions in Zone 8
Sets are the simplest start: push small bulbs into the soil so just the tip shows, at the final spacing. Transplants need gentle handling — trim roots and tops lightly if needed, and plant them shallowly, no deeper than they grew before. Seed started indoors takes 7–14 days to germinate. Whatever the start, keep the bed weed-free: onions are poor competitors, and weeds at this stage cut the crop badly. Water steadily while bulbs swell, then ease off as tops begin to fall over.
Soil, spacing, sun and water
Site matters as much as timing. Choose the sunniest spot you have — full sun — and check the soil pH is in the 6.0–7.0 range. Space plants 4–6 inches apart so each plant gets its share of light and air. Onions needs moderate water: keep the soil evenly moist, water deeply at the base in the morning so leaves dry before night. Mulch after planting to hold moisture and keep soil off the leaves. Onions like a fertile, well-drained soil with a pH of 6.0–7.0 and plenty of compost. They are shallow-rooted, so the top few inches matter most. Avoid fresh manure, and keep the bed free of weeds throughout — onions will not push through competition.
Companion planting
Companion planting for onions is simple: marigolds, basil, onions, lettuce are all good neighbours. Planting onions near marigolds — see Zone 8 Carrots — can help you plan a bed that uses the season well. Onions are useful companions for many crops, including carrots, lettuce and brassicas, because their smell is said to confuse pests. They grow well with marigolds and basil. Keep them away from beans and peas, which onions can stunt. What matters most is rotation — do not grow onions or its Amaryllidaceae relatives in the same bed year after year.
Pests and problems
Onions is not trouble-free, and the usual suspects are aphids, slugs, cutworms. Onions are relatively trouble-free, but aphids can cluster on leaves and slugs attack young plants. Thrips can silver the leaves in hot, dry weather. The best defence is rotation — do not grow onions or garlic in the same bed year after year — and clean, weed-free soil. Healthy, well-spaced plants in good soil resist pests better than stressed ones, so spacing, watering and rotation are your first line of defence. Inspect leaves weekly, hand-pick larger pests and wash small ones off with water.
Harvest and storage
Harvest when most tops have fallen over and started to yellow — typically 90–120 days after planting. Lift bulbs on a dry day, leave them on the surface or under cover to dry and cure for a week or two, then trim tops and roots and store in a cool, dry, airy place. Well-cured onions keep for months. Use any thick-necked bulbs first, since they do not store well. Do not store onions near potatoes, which make them sprout. The Zone 7 Onions page shows how the same crop behaves in a shorter season.
Frequently asked questions
When should I plant onions in Zone 8?
Set out transplants or sets from January into February, so plants grow leaves all winter and spring before bulbing. Onions are hardy and need 90–120 days to maturity — check your local extension service for your own dates.
Which onion varieties suit Zone 8?
Choose varieties suited to southern gardens — short-day types that bulb as days lengthen in spring. Varieties bred for northern summers often fail to bulb in Zone 8, so check the variety description before buying.
How far apart should I space onions in Zone 8?
Space plants 4–6 inches apart, in full sun and soil with a pH of 6.0–7.0. Plant sets and transplants shallowly, and keep the bed weed-free — onions compete poorly with weeds.
When are onions ready to harvest?
When most tops have fallen over and started to yellow, typically 90–120 days after planting. Lift on a dry day, cure for a week or two, then store in a cool, dry, airy place.
What pests affect onions in Zone 8?
The most common are aphids, slugs and cutworms. Onions are relatively trouble-free; rotation and a weed-free bed are the best defences. Keep stored onions away from potatoes.