Onion Farming: When to Plant for the Best Bulbs (2026)
Quick Answer
The right onion planting time depends far more on day length than on temperature alone — short-day varieties need planting in autumn or early winter in warm/tropical climates, long-day varieties need planting in early spring in temperate climates, and day-neutral varieties are more forgiving of timing. Get the variety-to-daylight match wrong and you'll get small bulbs or premature bolting no matter how well you otherwise care for the crop.
What Does It Look Like?
🧅
Determine your day-length category first
Before choosing a planting date, identify whether you're growing (or should grow) short-day, long-day, or day-neutral varieties based on your latitude. Growers within about 25° of the equator should almost always use short-day varieties; growers above about 35° latitude should use long-day varieties; the zone in between can often use either, with day-neutral as the safest default.
🧅
Time planting to your variety's season
Short-day varieties: sow or transplant in autumn to early winter so bulbing begins as days lengthen toward spring. Long-day varieties: sow or transplant in early spring, as soon as soil can be worked, so plants build enough leaf growth before the long summer days trigger bulbing. In both cases, onions are cold-hardy as seedlings and can tolerate a light frost.
🧅
Monitor bulbing onset through the season
Bulbing typically begins visibly (the base of the stem starting to swell) 60-100 days after planting depending on variety and conditions. From this point, avoid disturbing the root zone or piling soil over the bulb — unlike potatoes, onion bulbs should sit at or slightly above the soil surface as they mature, not be hilled up.
How to Treat It
Start from sets or transplants for reliability, seed for economy
Onion sets (small dormant bulbs) and transplants establish faster and are more forgiving of imperfect timing than direct seed, making them the more reliable choice for smallholders. Direct seeding is cheaper per plant but requires an earlier start and more precise timing to reach transplant-sized seedlings before the outdoor planting window.
Timing: Sets/transplants: plant directly at the seasonal window for your day-length category. Seed: start 8-10 weeks before that window if growing your own transplants.
Prepare loose, well-drained, compost-enriched soil
Onions form bulbs best in loose soil free of large clods or compaction, which restricts bulb expansion and can cause misshapen bulbs. Work in 5-8cm of finished compost before planting to improve structure and provide a broad nutrient base for the several months onions take to mature.
Timing: 2-4 weeks before planting.
Space correctly for bulb size, thin if from seed
Space plants 10-15cm apart in rows 30-40cm apart for full-sized bulbs; closer spacing produces smaller bulbs suitable for green onions/scallions instead. If direct-seeded, thin seedlings to final spacing once they have 2-3 true leaves — crowded onions never size up properly even with good care otherwise.
Timing: At planting for sets/transplants; thinning at the 2-3 true leaf stage for direct-seeded crops.
Stop watering and fertilizing once tops begin to fall over
When roughly half the leafy tops naturally flop over, the bulb has finished sizing and is entering the curing stage — continued heavy watering or feeding at this point can promote rot rather than further growth and should be stopped to allow the neck to dry down properly.
Timing: As soon as tops begin falling over naturally, roughly 2-3 weeks before expected harvest.
Best for: small farms, organic certification, home gardens
Balanced starter fertilizer at planting
Sulfur-based soil amendment (for alkaline soils)
Best for: large-scale farming, severe outbreaks
Match variety to latitude before anything else
No amount of good soil, water, or fertilizer will fix a fundamental day-length mismatch — if bulbs are consistently small or plants bolt prematurely across multiple seasons, the first thing to check is whether you're growing a short-day, long-day, or day-neutral variety appropriate for your region.
Avoid planting too late in the short-day/autumn window
Short-day onions planted too late in autumn don't have enough time to build leaf growth before winter, resulting in small bulbs the following spring — earlier planting within the recommended window generally outperforms later planting.
Don't hill soil over onion bulbs like potatoes
Unlike potatoes, onion bulbs mature partly exposed at the soil surface; piling soil over them doesn't increase yield and can promote rot in humid conditions by trapping moisture against the bulb.
Rotate away from alliums for 2-3 years
Planting onions, garlic, or leeks in the same soil repeatedly builds up soil-borne diseases like white rot and pink root that can persist in soil for years — rotating to unrelated crops breaks this cycle.
The best treatment is prevention
When This Problem Occurs
🌡️
Temperature
Onions germinate and grow best between 13-24°C; established plants tolerate light frost but hard freezes can damage exposed bulbs nearing maturity
💧
Humidity
Moderate humidity during growth is fine; high humidity combined with wet soil near maturity significantly increases bulb rot risk during curing
🌧️
Rainfall
Regular moisture needed during bulb sizing; reduce watering as tops begin falling over, and avoid harvesting or curing bulbs during wet weather
📅
Season
Short-day varieties: autumn to early winter planting. Long-day varieties: early spring planting. Day-neutral: flexible, generally spring in most climates
For Onion Farmers
For smallholders unsure which onion category to plant, the safest starting question isn't 'what does the seed packet season say' but 'what is my approximate latitude' — day length, not the calendar month by itself, is what actually drives bulbing, so a planting calendar written for a different latitude than yours can be actively misleading even if the month names match your local season. If you've had consistent problems with small bulbs or premature bolting despite good soil and care, day-length mismatch is the most likely cause and switching variety category (not adjusting fertilizer or water) is usually the fix.
Farmers Also Ask
How do I know if I have short-day, long-day, or day-neutral onion seed?
Reputable seed suppliers label this directly on the packet or in the variety description — look for 'short-day', 'intermediate-day' (a synonym for day-neutral), or 'long-day' in the variety name or description. If unlabeled and you're unsure of the source, treat it as a risk and prioritize known, correctly-labeled varieties for your latitude instead.
Can I plant onions any time of year in a tropical climate?
Not really — even in warm, frost-free tropical climates, day length still varies enough across the year to trigger or prevent bulbing in short-day varieties, so the autumn-to-early-winter planting window still applies for reliable bulb formation, not just avoiding cold.
What happens if I plant the wrong onion variety for my region?
Most commonly, the plant either bolts (sends up a flower stalk) prematurely without forming a usable bulb, or forms only a small, undersized bulb regardless of otherwise good growing conditions — this is a day-length response, not something that better fertilizing or watering can correct.
How long does it take onions to mature from planting?
Roughly 90-120 days from transplant to harvest for most varieties, though this varies by variety, climate, and whether started from seed, sets, or transplants — sets and transplants generally mature somewhat faster than direct-seeded crops since they start with more established growth.
Get the Complete Farming Toolkit
AI disease scanner • Pest identification • Weather alerts • AI chat expert • Works offline
3 free scans per day • 10 languages • No internet needed