Onions are cool-season bulb crops that take 60–140 days from seed to harvest, depending on variety and daylength. Sow indoors early in the season or direct-sow outdoors in spring, then transplant seedlings into rich, well-draining soil in full sun. With consistent watering and minimal feeding, you'll harvest mature bulbs in midsummer or early autumn.
Onions are a gardener's staple—versatile, reliable, and surprisingly easy to grow. Whether you choose sweet varieties like Walla Walla, peppery red types like Red Torpedo Tropea, or dual-purpose spring onions, you'll find a cultivar suited to your climate and taste. A long growing season and modest fertility demands make onions ideal for succession planting and crop rotation, rewarding patient growers with months of storage-friendly harvests.
Choosing the Right Variety
Onion varieties fall into three main groups: long-day types (best for northern latitudes), short-day types (southern regions), and day-neutral cultivars. Long-day varieties like Ailsa Craig Exhibition Onion and Hyred bulb reliably in cool climates. Short-day Walla Walla and Red Torpedo Tropea excel in warm zones. Spring onions (bunching types) mature faster and suit container growing. Italian heirlooms—Cipolla di Zapponeta, Rossa Lunga di Firenze, Cipolla di Certaldo—offer distinctive flavour and storage. Borettana Cippolini produces small, sweet bulbs ideal for pickling. Choose based on your latitude, intended use (storage, fresh, pickling), and flavour preference.
When to Sow and Plant Out
Start seeds indoors 6–8 weeks before your last frost date to raise transplants with a head start. Sow directly outdoors 2–3 weeks after the last frost, once soil is workable. Soil temperature should reach 10–15°C (50–59°F) for reliable germination. Transplant seedlings when they're 10–15 cm tall and have developed 2–3 true leaves. Succession-sow every 2–3 weeks for continuous harvests. Long-day varieties need 14–16 hours of daylight to bulb; short-day types require only 10–12 hours. Timing your sowing ensures bulbs form when daylength triggers are met.
Soil, Site and Containers
Onions demand full sun (6–8 hours minimum) and well-draining soil rich in organic matter. Aim for pH 6.0–7.0. Work in 5–7 cm of compost or aged manure before planting. Avoid waterlogged soil; raised beds or containers suit poorly-draining sites. Grow in 20–30 cm pots (one plant per pot for large varieties, 3–4 for spring onions). Space transplants 10–15 cm apart in rows 30 cm apart. Container-grown onions need regular watering; in-ground crops tolerate drier conditions once established.
Feeding, Watering and Care
Water consistently, providing 2.5 cm per week (or 1.5 cm if rainfall occurs). Reduce watering in the final 3–4 weeks before harvest to encourage bulb maturation. Apply a balanced fertiliser (10-10-10 NPK) every 3–4 weeks, or use a slow-release granule at planting. Avoid high-nitrogen feeds, which promote leaf growth at bulb expense. Mulch lightly with straw to retain moisture and suppress weeds. Remove flower stems (bolts) as they appear. Thin seedlings to final spacing within 2–3 weeks of emergence.
Common Problems and Solutions
Onion fly larvae tunnel into bulbs; grow under fine mesh and remove affected plants promptly. Thrips cause silvery leaf streaks; spray with insecticidal soap if severe. Downy mildew appears as grey-white leaf coating in cool, wet conditions—improve air circulation and avoid overhead watering. Fusarium basal rot and white rot are soil-borne; rotate crops and avoid replanting onions in the same bed for 3 years. Neck rot develops during storage; cure bulbs thoroughly (2–3 weeks at 20–25°C) and store in cool, dry conditions. Bolting (premature flowering) results from cold stress or day-length confusion; choose appropriate varieties for your region.
Harvesting and Storage
Harvest when foliage yellows and falls over, typically 100–140 days after sowing. Gently lift bulbs with a fork and leave on soil for 1–2 days to dry. For long storage, cure in a warm, airy place (20–25°C, 40–50% humidity) for 2–3 weeks until skins papery and necks fully dry. Trim roots to 1 cm and remove loose outer layers. Store in a cool (0–5°C), dry, dark location with good ventilation. Properly cured onions keep 3–8 months depending on variety. Spring onions are harvested young (30–40 days) and used fresh.
Companion Planting and What to Avoid
Plant onions alongside Beetroot, Carrot, and Lettuce—these companions don't compete for nutrients and onion's pungent oils may deter some pests. Marigold flowers attract beneficial insects. Avoid planting near Beans or Peas, which fix nitrogen that encourages excessive leaf growth in onions. Keep Garlic separate to prevent cross-pollination and disease spread. Brassicas (cabbage, broccoli) compete for similar soil nutrients. Fennel and Dill inhibit onion growth through allelopathic compounds. Rotate onion beds with legumes or brassicas to rebuild soil structure and nitrogen.
Frequently asked questions
How long does it take to grow an onion from seed?
Typically 100–140 days from sowing to harvest, depending on variety and daylength. Long-day types take longer; short-day varieties are faster. Spring onions mature in 30–40 days.
Can I grow onions in pots or containers?
Yes. Use 20–30 cm pots with drainage holes and quality compost. Space one large variety per pot or 3–4 spring onions per container. Water regularly; containers dry faster than soil beds.
Why are my onion leaves turning yellow?
Yellowing suggests nitrogen deficiency, poor drainage, or onion fly damage. Check soil moisture, apply balanced fertiliser, and inspect bulbs for soft spots. Improve drainage if waterlogging is present.
Do onions need full sun?
Yes, 6–8 hours of direct sun daily is essential for strong bulb development. Partial shade reduces yields and delays maturity. Choose a south-facing, unshaded site.
How often should I water onions?
Provide 2.5 cm of water per week via rain or irrigation. Water deeply but infrequently to encourage deep rooting. Reduce watering in the final month before harvest.
What's the difference between long-day and short-day onions?
Long-day varieties (Ailsa Craig, Hyred) bulb when days exceed 14–16 hours, suiting northern latitudes. Short-day types (Walla Walla) bulb at 10–12 hours, thriving in southern regions. Choose based on your latitude.
Can I store onions long-term?
Yes, if properly cured. Dry bulbs in warm, airy conditions for 2–3 weeks, then store at 0–5°C in a cool, dark place with ventilation. Well-cured onions keep 3–8 months.
Should I grow onions from seed or sets?
Seed offers more variety choice and is cheaper; sets (small bulbs) are faster and more reliable for beginners. Sets skip the transplant stage and mature 4–6 weeks earlier than seed.