Onions Allium cepa

Reviewed June 2026 · how we check this

A cool-season biennial grown as an annual for its swelling, layered bulb. Onions are easy to grow but exacting about two things — day length and a steady, weed-free start — and reward patience with a long-storing harvest from a single planting each spring.

Light

Onions are sun-hungry and need full sun — at least 6 to 8 hours of direct light a day, and more is better. Every extra hour of sunlight feeds bigger leaves, and because each leaf becomes a ring in the bulb, more leaves mean a larger onion. Just as important is day length: long-day varieties bulb when daylight hits 14–16 hours and suit northern gardens, short-day types bulb at 10–12 hours for the South, and day-neutral kinds work almost anywhere. Plant the wrong type for your latitude and the plants will grow lush tops but never form a proper bulb. Choose by region first, then give them the sunniest, most open bed you have.

Watering

Onions have shallow, sparse roots and can't chase down moisture, so aim for a consistent 1 inch of water per week from rain or irrigation. Even, steady moisture during bulb formation produces large, sweet onions; drought stress makes them small and pungent, while a soak-then-bake cycle can split bulbs or trigger bolting. Water at the base to keep foliage dry and discourage disease, and mulch lightly to hold moisture and smother weeds. The crucial change comes at the end: once the tops begin to flop and yellow, stop watering entirely so the bulbs firm up and cure for storage rather than staying soft and rot-prone.

Soil & potting

Onions want loose, fertile, well-draining soil rich in organic matter, with a near-neutral pH of 6.0–7.0. Their fine roots struggle in heavy clay and compacted ground, so work in plenty of compost and loosen the bed deeply before planting; raised beds or mounded rows help where drainage is poor. Remove stones and clods that would deform the swelling bulb. Onions are weak competitors against weeds, so start with a clean bed and keep it that way. A soil that crumbles easily lets the bulb expand without resistance, which directly translates to bigger, rounder onions at harvest.

Humidity & temperature

Onions are a cool-season crop that grows its best foliage in the mild 55–75°F range and tolerates light frost, which is why they go in early. Plant sets, transplants, or seed as soon as the ground is workable in spring; the goal is to build a big, leafy plant before warm days trigger bulbing. Hot, dry weather late in the season is fine — it actually helps the bulbs ripen and cure. Avoid wide temperature swings on young plants: a hard cold snap after a warm spell can fool a fall- or early-planted onion into bolting and sending up a flower stalk instead of storing energy in the bulb.

Fertilizing

Onions are heavy nitrogen feeders during their leafy phase and reward steady feeding with bigger bulbs. Work a balanced fertilizer or rich compost into the bed at planting, then side-dress with a nitrogen source every 2–3 weeks once the plants are established and growing actively. Keep feeding through the leaf-building period, because every leaf the plant makes now becomes a ring later. The signal to stop is the start of bulbing: once you see the bulb swelling and the necks thickening, cut off the nitrogen. Feeding past that point pushes soft top growth and thick necks that won't cure or store well, so let the plant pour its energy into the bulb instead.

Pruning & maintenance

Onions need almost no pruning — leave the green tops fully intact, since each leaf powers a layer of the bulb, and trimming them only shrinks your harvest. The one exception is flower stalks: if a plant bolts and sends up a firm, round seed stem, snip it off promptly, but use that bulb soon because bolted onions store poorly. Harvest is the real 'pruning' moment. When about half to three-quarters of the tops have naturally flopped over and yellowed, stop watering and let the necks dry for a few days, then lift the bulbs on a dry day. Cure them in a warm, airy, shaded spot for two to three weeks until the necks are papery before trimming and storing.

Propagation

Onions are grown from seed, sets (small dormant bulbs), or nursery transplants. Starting from seed gives the widest variety choice but takes patience: sow indoors 8–10 weeks before your last frost in shallow trays, keep the soil around 70°F for germination, and trim the grassy seedlings back to a few inches to build sturdier stems. Harden off and transplant out a few weeks before the last frost, spacing plants 4–6 inches apart in rows. Direct-seeding works in long-season climates but needs careful thinning. Sets and transplants are faster and more forgiving for beginners; whichever you choose, match the variety's day length to your latitude or the plants will never bulb.

Common problems

Onions bolting (flowering early)A flower stalk means cold stress or a swing in temperature — harvest those bulbs first.→Small bulbs at harvestUsually crowding, wrong day-length variety, weed competition, or too little nitrogen early on.→

Through the year

Spring

Prime planting time — set out sets, transplants, or direct-sow as soon as the soil is workable, keep the bed weed-free, and feed nitrogen to build big leafy tops.

Summer

Bulbing season. Keep moisture even early on, stop feeding once bulbs swell, then stop watering when the tops begin to flop and start lifting and curing the crop.

Fall

Finish curing the last bulbs and store them. In mild regions, plant overwintering short-day onions or garlic to establish before cold weather.

Winter

Onion beds rest. Plan next year's varieties by day length for your latitude and start onion seeds indoors late in the season for an early spring transplant.

Companion planting

Onions earn their keep in companion planting — their pungent scent helps deter aphids, carrot rust fly, and many other pests, making them excellent neighbors for carrots, lettuce, and members of the cabbage family. Keep them well away from peas and beans, however, since onions inhibit the growth of legumes.

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