When to Plant Onions: Perfect Timing by Region

Reading time: 6 min

Key takeaways

• Soil temperature is non-negotiable — aim for 7–10°C (45–50°F) at 10 cm depth. That’s your green light, not a calendar date.
• Region rules everything: if you’re in the UK or northern Netherlands, add 2–3 weeks to the “standard” March timing. Southern France? Start sets in February.
• Day length matters more than you think — choose the right onion variety for your latitude (short-day vs. long-day) or you’ll get tiny bulbs and big disappointment.

Don’t rely on the calendar alone

What most people get wrong is thinking there’s one universal “right day” to plant onions. In my experience, the soil is the real boss. I’ve seen gardeners in the same town plant two weeks apart and get wildly different results — all because the soil hadn’t warmed up yet.

Get a simple soil thermometer, and check at 10:00 AM on a dry day. Stick it 10 cm deep. If you see 7°C consistently for at least three mornings in a row, you’re good to go. Below that? Wait. Planting into cold, wet soil is the fastest way to rot your sets before they even sprout.

Month-by-month guide for temperate climates

Let me give you the practical breakdown. These are based on what I’ve learned from the trial gardens at Wageningen, combined with real feedback from nursery clients across Europe.

• February — Only for the mildest zones (Cornwall, Zeeland, southern Portugal). Start sets in modules indoors if you must. Outdoors is a gamble I don’t recommend.
• March — Main window for most of the UK, Netherlands, northern Germany. Wait until mid-March in colder valleys. Southern gardeners start earlier.
• April — Perfect for Scandinavia, Scotland, high-altitude regions in Austria. Soil is finally waking up.
• May–June — Best for autumn-harvest onions from seed. Sets planted this late will give you storage onions for winter. My grandmother taught me that patience here pays off.

If you miss the spring window entirely, don’t panic. You can still plant onion sets in early autumn for a green onion harvest the following spring — especially the overwintering types like “Senshyu Yellow.” Here’s what I’d do: choose shorter-day cultivars, and mulch well before frost.

Why your region’s climate beats any generic advice

I get emails from readers in the Dutch polders asking me whether to plant on the same day as gardeners in Limburg. The answer is no — because the clay soil in the polders stays cold and heavy much longer than the sandy loam of Limburg.

Use your local frost date as an anchor, not a rule. Count back four weeks from your average last spring frost — that’s your earliest safe planting time. But again, the soil thermometer is your real friend. My rule of thumb: when daffodils are about 15 cm tall, the soil is usually ready. The plant will tell you.

Onion variety matters just as much as the date

Don’t overthink it, but do get this right. Onions are photo-sensitive — they bulb according to day length. If you plant a long-day variety (like “Sturon” or “Red Baron”) in a short-day region, you’ll get tiny bulbs.

• Short-day varieties — Best for southern France, Spain, Italy, Portugal. They start bulbing when days reach 10–12 hours. Popular: “Texas Grano,” “Red Creole.”
• Long-day varieties — For the Netherlands, UK, northern Germany, Scandinavia. Need 14–16 hours of daylight. “Stuttgarter,” “Sturon,” “Red Baron” are my go-tos.
• Day-neutral varieties — Flexible for most European climates. “Candy” or “Ailsa Craig” work beautifully everywhere I’ve tested them.

I once had a client in Edinburgh insist on using short-day onion seeds from a holiday in Spain. The leaves grew lush and tall, but by August we had bulbs no bigger than a walnut. The plant will tell you — or rather, it will show you when you’ve made a mistake.

The most common timing mistake (and how to fix it)

Let me be honest: planting too early is the biggest problem I see. The temptation to get out there in early March, after a mild winter, is huge. But if the soil is still cold and wet, the onion sets can rot before they root. I’ve lost entire rows this way in my own garden in Haarlem one year — I learned the hard way.

Here’s what I’d do if you’re itching to plant but the soil isn’t ready:

• Start sets in module trays indoors for 3–4 weeks, then transplant after hardening off.
• Warm the soil with black plastic or cloches for two weeks before planting.
• Choose sets over seeds if you want to squeeze a few extra weeks out of the season.

My quick monthly checklist for 2026

Since we’re in June 2026 right now, here’s what you should be doing if you haven’t planted onions yet — or if you want to add a late batch:

• June — Plant long-day sets now for small firm bulbs by October, ideal for storage. Water well when dry.
• July — Too late for most sets unless you live in a warm microclimate. Direct-sow overwintering varieties like “Elektra” or “Senshyu.”
• August — Last chance for overwintering sets in many temperate zones. Mulch after planting.
• September — In mild coastal zones, you can still get autumn-planted sets established for very early spring harvests next year.

The plant will tell you if it’s happy — perky leaves and steady growth are good signs. If they look yellow or squishy at the base, you’ve waited too long or planted too early. I promise you: with a soil thermometer and a little local knowledge, you’ll never guess the timing again.