If you are a gardener, you know that timing is everything. Whether you are a beginner or a seasoned pro, the question “when to plant onions?” is one of the most critical factors in determining your harvest size. Unlike some vegetables that you can “set and forget,” onions are highly sensitive to daylight and temperature.
In this guide, we dive deep into the best planting windows, soil requirements, and regional tips to ensure you have a pantry full of bulbs by the end of the season.
The Golden Rule: Timing Based on Your Climate
The best time to plant onions depends largely on your local USDA Hardiness Zone. Because onions are frost-tolerant, they can go into the ground much earlier than heat-lovers like tomatoes.
1. Planting in Northern Climates (Zones 3–5)
For those in the North, the window for when to plant onions usually opens in late March or April.
- The Rule: Aim to get your onions in the ground as soon as the soil can be worked and is no longer frozen.
- Top Tip: You want your onions to have as much “green growth” as possible before the long days of summer arrive, as this dictates the eventual size of the bulb.
2. Planting in Central Regions (Zones 6–7)
In these middle zones, mid-March is the sweet spot. You have the luxury of a longer growing season, which allows for a wider variety of onion types.
3. Planting in Southern Climates (Zones 8–10)
In the South, the strategy flips. You should be asking “when to plant onions” in the autumn or winter.
- The Window: October through December is ideal.
- The Reason: Onions planted in the fall will over-winter and bulb up in the early spring heat before the intense summer sun arrives.
Matching Your Variety to the Sun
You cannot talk about when to plant onions without talking about Day Length. Onions are triggered to form bulbs based on how many hours of daylight they receive.
| Variety Type | Daylight Needed | Best Location |
| Long-Day Onions | 14–16 Hours | Northern States (Long summer days) |
| Short-Day Onions | 10–12 Hours | Southern States (Short winter days) |
| Intermediate-Day | 12–14 Hours | The “Middle Belt” (Versatile) |
Seeds vs. Sets: Does the Timing Change?
How you choose to start your onions also dictates your calendar:
- Onion Seeds: These require a head start. If you are starting from seed in the North, sow them indoors 8–10 weeks before the last frost.
- Onion Sets (Small Bulbs): These are much hardier. You can plant sets directly into the garden 4 weeks before your last frost date.
- Onion Transplants: If buying “starts” from a nursery, plant them in early spring once the soil reaches about 50∘F.
Pro-Tips for a Plentiful Harvest
To maximize your SEO and your garden yield, keep these “legit” gardening facts in mind:
- Don’t Plant Too Deep: Onion sets should be planted only about 1 inch deep. If you bury them too far, the bulb won’t be able to expand.
- Fertilize Early: Onions are “heavy feeders.” Use a nitrogen-rich fertilizer during the first few weeks of growth to encourage leafy tops.
- Watch for Bolting: If a cold snap hits after a period of warmth, onions may “bolt” (produce a flower). Harvest these onions immediately, as they won’t store well.
Conclusion: When Should You Start?
In short, the answer to when to plant onions is: as early as the soil allows. For most of the country, this means late winter or very early spring. By getting your onions in the ground early, you allow them to establish a strong root system and plenty of foliage before the “bulbing” process begins.
Ready to start your garden? Grab your sets and your spade, and get planting!
