Onions are a winter crop in Zone 9, and the secret is variety choice: short-day onions, planted in autumn and winter, are the ones that bulb up properly in the South's short winter days.
Zone 9 at a glance
USDA Zone 9 covers the warmest parts of the southern United States, including much of Florida, southern Texas, southern Louisiana, southern Arizona and coastal California. Winters are mild, with average winter lows of 20 to 30°F, and the growing season runs nearly all year.
The average last spring frost in Zone 9 is Jan 30–Feb 15, and the average first autumn frost is Dec 1–15. These are zone averages drawn from NOAA frost records, not exact dates for any one garden — always check your local extension service and the forecast before planting tender crops outside. The average last spring frost in Zone 9 is Jan 30–Feb 15, and the average first autumn frost is Dec 1–15. These are USDA zone averages drawn from NOAA frost records, not exact dates for any one garden. Always check your local extension service and the forecast before planting tender crops outside. Use our frost date calculator for your zone's averages.
When to plant onions in Zone 9
Plant sets or transplants in autumn through early winter (October to January), using short-day varieties; bulbs mature in late spring. The average last spring frost in Zone 9 is Jan 30–Feb 15, and the average first autumn frost is Dec 1–15. Because Zone 9 has a long, nearly year-round growing season, the question is rarely whether onions will grow, but whether you plant it in the right season for the crop — heat lovers go in after the last frost, cool-season crops go in for autumn and winter.
Onions planting at a glance
| Detail | Zone 9 guidance |
|---|---|
| Best planting window | Plant sets or transplants in autumn through early winter (October to January), using short-day varieties; bulbs mature in late spring. |
| Start indoors | Start seed indoors in late summer, or buy sets/transplants |
| Direct sow / transplant | Set out transplants or sets October–January |
| Days to germination | 7–14 |
| Days to maturity | 90–120 |
| Spacing | 4–6 in |
| Soil pH | 6.0–7.0 |
| Sun | full sun |
| Water needs | moderate |
| Cold tolerance | hardy |
All figures are broad published ranges from standard horticultural references, and they vary by variety, soil and weather. Check your local extension service for advice on your own garden.
Starting onions
Onions take 7–14 days to germinate and 90–120 days to mature, so starting from seed needs a long runway — sow seed indoors in late summer and transplant in autumn, or buy sets or young transplants, which is simpler and more reliable for most gardeners. Set transplants about an inch deep, with the roots down and the tip just above the soil. In Zone 9, choose short-day varieties: they bulb up when days are short, and long-day types grown in the South often stay small and never form proper bulbs.
Soil, spacing and sun
Onions need full sun and fertile, well-drained soil with a pH of 6.0–7.0. Work compost into the bed and keep it weed-free — onions have shallow roots and hate competition. Space plants 4–6 inches apart in rows, and do not plant them deep: the bulb forms at the surface, and buried bulbs stay small. A light mulch helps hold moisture, but keep it away from the necks of the bulbs. In containers, shallow wide pots suit them well.
Watering and feeding
Onions have moderate water needs and want steady moisture while bulbs are swelling, then drier conditions as they finish. Water regularly through winter and spring, aiming for even moisture — drought while bulbs are forming gives small onions. Stop watering once the tops start to fall over and yellow; wet soil at that stage causes rot in storage. In Zone 9's dry spring spells, a weekly deep soak is usually enough.
Companion planting
Good companions for onions are marigolds, basil and lettuce. Onions themselves are useful companions: their smell is said to deter carrot fly and other pests, so they are often interplanted with carrots and lettuce. Marigolds help with soil pests. Avoid planting onions near peas and beans, which grow poorly alongside alliums. Keep the bed weeded, because onions compete badly with weeds for light and water.
Pests and problems
The pests to watch for are aphids, slugs and cutworms. Aphids cluster on leaves in spring; wash them off with water. Slugs can damage young transplants, especially in wet winters — check after rain and keep the bed tidy. Cutworms may sever young plants at soil level; collars around transplants help. The bigger risk in Zone 9 is onion rot from wet soil late in the season, so good drainage and stopping watering as tops fall over matter more than insects.
Harvesting and storing onions
Harvest onions when the tops fall over and start to yellow, usually in late spring, 90–120 days after planting. Lift them on a dry day and cure them in a warm, dry, airy place for two to three weeks until the necks are dry and papery — curing is what makes them store well. Then trim tops, brush off loose soil, and store in mesh bags or braids in a cool, dry, dark place. Well-cured short-day onions keep for several months; check them regularly and use any that soften first.
Frequently asked questions
When should I plant onions in Zone 9?
In autumn through early winter, October to January, using short-day varieties. Onions bulb up in late spring — check your local extension service for the best planting dates in your area.
What are short-day onions and why do they matter?
Short-day onions form bulbs when daylight is short, which suits the southern United States. Long-day varieties need long summer days to bulb and usually stay small in Zone 9, so always choose short-day types.
Why are my onions small?
Usually planting too late, too deep, or in weedy, dry soil. Onions need a long season, shallow planting, steady water while bulbs swell, and a weed-free bed — they compete very badly with weeds.
How long do onions take to grow in Zone 9?
Seed germinates in 7–14 days and bulbs mature in 90–120 days from transplanting, so onions planted in autumn are typically ready in late spring.
For the full year, browse the planting calendar hub. Nearby: october in Zone 9, Onions in Zone 7 and Garlic in Zone 9. Use the seed-starting calculator and the vegetable yield estimator to plan your own dates.