Onion

Choosing & planning

What is onion and where does it grow best?

Onion (Allium cepa) is a cool-season vegetable belonging to the Amaryllidaceae family with a flavor ranging from mild to strong depending on variety. It grows best in full sun and fertile, well-drained soil. Onions can thrive in a wide range of climates—from cool temperate regions to warm, dry areas—when grown properly with consistent moisture and rich organic matter.

What are the main types of onion, and how do they differ?

Onions are divided into three day-length categories: short-day varieties (needing 10–12 hours of daylight) grow best in southern regions at latitudes 25–35°; long-day varieties (needing 14–16 hours) suit northern climates at latitudes 37–47°; and intermediate-day or day-neutral varieties (needing 12–14 hours) work well across mid-latitudes of 32–42°. Onions are also grouped by bulb color—yellow, red, and white—each with slightly different flavor profiles and growing needs.

Which onion varieties are recommended, and how do I choose one?

For bulb onions, recommended varieties include Utah Sweet Spanish, Candy, Walla Walla, Sweet Sandwich, and Cabernet (red). For long storage, Copra (can keep 10–12 months), Patterson, Stuttgarter (6–8 months), and Redwing are proven keepers. For green onions, Evergreen White Bunching is a good mild choice. Choose based on your day-length zone and use: storage onions are pungent with low moisture content, while sweet types like Walla Walla are best eaten fresh within weeks of harvest.

What should I look for when buying onion seeds or transplants?

Use certified disease-free seeds or transplants, as infection often starts from contaminated bulbs. Onion sets—small dormant bulbs—are generally the easiest option for beginners since they have a higher establishment rate and grow more predictably than seeds.

Can I grow onion in my climate or hardiness zone?

Selecting the correct type for your latitude is critical. Short-day varieties grow best at latitudes 25–35° in USDA Zones 7 and warmer, bulbing at 10–12 hours of daylight. Intermediate (day-neutral) varieties suit latitudes 32–42° and Zones 5–6, bulbing at 12–14 hours. Long-day varieties do best at latitudes 37–47° in Zone 6 and lower, requiring 14–16 hours of daylight to form bulbs.

Can I grow onion in containers or pots, and what size do they need?

Yes—onions are well suited to containers because of their shallow root systems. An ideal container is at least 10 inches (25 cm) deep and several feet (at least 60 cm) across; containers 8–12 inches deep and wide also work. Each onion needs about 2–4 inches of lateral space. Trough-style planters and fabric grow bags with excellent drainage are recommended options.

Can I grow onion indoors or in a greenhouse?

Yes. Onions can be grown in containers on patios, balconies, or indoors; if growing indoors, supplemental grow lighting is needed to provide adequate hours of light for bulb formation. Transplants are also commonly started from seed in a greenhouse before being set out in the field.

Starting from seed & propagation

Should I start onion from seed or buy transplants?

Seed offers the widest variety choice and can produce the best bulbs, but it is the slowest method. Transplants and sets allow for earlier production and harvest; sets are generally the easiest and most reliable for beginners. For early production, use transplants or sets planted out in April.

When should I start onion seeds indoors?

Start onion seeds indoors 10–12 weeks before the last frost date. Transplants require about 8 weeks of grow time before being planted out in the garden.

How deep should I sow onion seeds?

Sow onion seeds ¼ to ½ inch deep when starting indoors or in a garden seedbed. When direct-seeding in field conditions, seeds may be planted 0.5 to 1 inch deep. Cover with a light layer of seed-starting mix or vermiculite to retain moisture.

What temperature do onion seeds need to germinate?

Onion seeds germinate at soil temperatures above 40°F, with an optimum soil temperature of 75°F. Cooler soil will delay germination and slow emergence, which is why planting too early in cold, wet soil is discouraged.

How long do onion seeds take to germinate?

Under good conditions, onion seeds emerge in roughly 1–2 weeks. In cooler or suboptimal soil conditions, germination can take 15–25 days. The first sign of emergence is a single looped leaf that pushes up, hooks over, and then straightens—this is normal.

Site, soil & timing

How much sun does onion need?

Onions require full sun—at least 6 hours of direct sunlight per day. Adequate day length is also essential because bulb initiation is triggered when daylength reaches a variety-specific threshold of hours of light.

What type of soil does onion prefer?

Onions grow best in loose, well-drained, loamy soil rich in organic matter with good water retention and aeration. Sandy loam is often cited as ideal. Compacted clay soils resist drainage and cause problems; raised beds or containers are recommended where native soil is heavy or compacted.

What soil pH does onion need?

Onions prefer a soil pH of 6.0 to 7.0 (slightly acidic to neutral), though they can tolerate up to 7.8. A pH below 6.0 limits nutrient availability—particularly calcium and phosphorus—and can cause deficiencies during the growing season.

How should I prepare and amend the soil for onion?

Before planting, conduct a soil test and work the recommended fertilizer into the top 6–8 inches of soil. Incorporate compost or well-rotted manure to improve organic matter, water retention, and fertility—no more than 1 inch of well-composted material per 100 square feet. Thoroughly loosen the soil to promote healthy root development, and in field production, shape beds in the fall to allow them to settle through winter.

When should I plant onion outdoors?

In most regions, plant onion seeds, sets, or transplants outdoors from late March through mid-April, once beds are dry enough to avoid soil compaction. Research shows most plantings made before April 15 yield the best crop results. For early production, transplants and sets can go out in April.

Does onion tolerate frost, heat, or drought?

Onion is a cool-season crop that tolerates mild frost, but it is sensitive to drought—water stress reduces yield, decreases bulb size, and affects flavor. Hot summer temperatures can trigger premature bulb formation before the plant has grown enough leaves, resulting in smaller bulbs. Planting early enough to give the crop a long, cool growing period is important in warm climates.

Planting

How far apart should I space onion plants?

Space onion plants 3–4 inches apart in the row for full-sized bulbs; 2–3 inches is acceptable for green onions. High-density plantings reduce bulb size unless adequate water and nutrients are supplied.

How far apart should the rows be?

Rows should be spaced 8–16 inches apart, with some sources recommending at least 18 inches between rows. In field production, beds of 26–44 inches (center to center) containing 2–4 seed rows per bed are common.

How deep should I plant onion?

Plant onion seeds ¼ to ½ inch deep in garden beds, or 0.5 to 1 inch deep in field conditions. Sets and transplants should be placed at the same final spacing depth—just deep enough to anchor the bulb or root ball firmly in the soil.

Are there special planting techniques for onion?

Onions need firm, finely textured soil for good germination; seedbed preparation ideally begins the previous fall so beds can settle through winter freezing and thawing. For storage-quality bulbs, plant transplants in late April—later plantings are less likely to produce seed stalks. Avoid wide in-row spacings, which promote large bulbs with thick necks that cure and store poorly.

What grows well next to onion (companion plants)?

Good onion companions include carrots (a symbiotic pairing: carrots deter onion flies while onions mask carrots from carrot flies), lettuce, beets, cabbage, broccoli, strawberries, tomatoes, peppers, chamomile, dill, and savory. Onions’ strong sulfur-like scent repels carrot flies, cabbage worms, aphids, Japanese beetles, and even deer and rabbits.

Watering

How much and how often should I water onion?

Onions require regular watering of approximately 1 inch per week to keep soil near field capacity. Moisten the soil thoroughly to a depth of 12 inches at each watering. Stop watering when tops begin to fall over as the crop approaches maturity—excess water at this stage delays curing and causes storage problems.

What is the best way to water onion?

Drip irrigation is recommended where possible because onions have shallow roots and water needs are critical. Consistent, deep watering is more effective than frequent shallow watering.

What are the signs of over- or under-watering onion?

Under-watering (drought stress) decreases yield, reduces bulb size, and negatively affects flavor. Overwatering or waterlogged soil leads to soft, mushy bulbs and rot; too much water late in the season also delays proper curing. Compacted soil with poor drainage compounds overwatering problems and can cause bulbs to turn mushy even without excessive irrigation.

Should I mulch onion, and with what?

Yes—organic mulches are recommended. They conserve soil moisture, supply extra nutrients as they break down, and reduce weed competition. Overwintered onion crops should also be mulched late in the year to minimize winter damage.

Feeding & fertilizing

How should I fertilize onion?

Work a complete fertilizer into the soil before planting based on a soil test. Then side-dress with nitrogen in May and June to sustain vegetative growth. During the growing season, a high-nitrogen feed every 2–3 weeks supports top growth; switch to a potassium-rich feed or compost tea once the tops begin to swell to support bulb development.

What fertilizer or nutrients does onion need?

Onions are heavy feeders requiring nitrogen, phosphorus, potassium, and micronutrients including sulfur, calcium, iron, zinc, and boron. A typical commercial crop uses about 200 pounds of nitrogen, 80 pounds of phosphorus, and 240 pounds of potassium per acre. For home gardens, an organic 10-20-10 starter fertilizer applied 2–3 inches below the planting area is a practical starting point.

When and how often should I feed onion?

Apply a complete fertilizer at planting, then side-dress with nitrogen in May and June. During vegetative growth, a high-nitrogen feed every 2–3 weeks is beneficial. Stop high-nitrogen applications when tops begin to swell and transition to a lower-nitrogen, potassium-rich feed to support bulbing.

What are the signs of nutrient deficiency in onion?

Older leaves yellowing first indicates nitrogen deficiency. New growth that is yellow or stunted points to iron or sulfur deficiency. Interveinal yellowing suggests other micronutrient shortages. Low soil pH (below 6.0) can also cause calcium and phosphorus deficiency even when those nutrients appear adequate on a soil test.

Pests

What pests commonly attack onion?

The most common onion pests are thrips (especially in mid-summer), onion fly/onion maggot, allium leaf miner, aphids, cutworms, and bulb mites. Thrips leave silvery, speckled foliage; onion maggot larvae burrow into roots and bulbs; aphids cluster on new growth; cutworms chew seedlings at soil level; bulb mites cause powdery decay inside outer scales.

How do I identify onion pest damage?

Thrips cause silver streaks and white speckled flecking on leaves. Onion maggots produce sudden wilting; digging up an affected plant reveals small white legless larvae at the bulb base with soft, rotten bulbs. Allium leaf miners leave distinctive rows of puncture marks on leaves and cause distorted stems. Aphids produce sticky honeydew and sooty mold with curled, yellowing leaves. Cutworms leave clean-cut, missing seedlings at soil level.

How do I prevent onion pests?

Rotate onions out of the same bed for at least 3 years to break pest life cycles. Apply floating row covers for the first 4–6 weeks to physically block egg-laying flies. Use certified disease-free seeds or transplants. Remove volunteer alliums and crop debris promptly. Plant insectary flowers such as alyssum to attract beneficial predators. Time planting to avoid peak onion maggot activity windows.

How do I control onion pests organically and chemically?

Organically, neem oil, targeted baits, and BT (Bacillus thuringiensis) for caterpillar pests are effective. Physical removal of infested plants and fine mesh netting or floating row covers block adult flies from laying eggs. Monitor with yellow sticky cards at canopy height and act when thresholds are reached—for example, an average of 4 or more thrips per leaf or 5% of plants showing maggot damage.

Diseases

What diseases commonly affect onion?

Common onion diseases include black mold, botrytis leaf blight, downy mildew, fusarium basal plate rot, fusarium damping-off, and pink root. Additional problems are white rot (a persistent soil-borne fungus), neck rot (often developing in storage), and rust. Most diseases are favored by warm, moist conditions, poor drainage, or insufficient plant spacing.

How do I recognize onion disease symptoms?

Black mold shows black discoloration and shriveling on outer bulb scales. Botrytis leaf blight produces small white lesions with light green halos expanding into full leaf blight. Downy mildew causes pale elongated patches and gray-purple fuzzy growth on leaves. White rot appears as white fluffy growth on roots with yellowing, wilting leaves. Neck rot softens the neck and causes grayish mold in storage. Rust produces orange, powdery spots on leaves. Pink root turns roots light pink, then darkens to purple, with plants becoming stunted and producing undersized bulbs.

How do I prevent onion diseases?

Rotate onions to a new field every 5–7 years (or at least 4 years for fusarium and 3–6 years for pink root). Use well-drained soil and space plants at least 30 cm (12 inches) apart to promote air circulation and quick foliage drying. Choose disease-resistant varieties and certified clean seed. Avoid bruising bulbs during harvest, which creates entry points for pathogens.

How do I treat or manage onion diseases?

Treat seeds with an appropriate fungicide before planting to reduce rot in mature bulbs. For botrytis, apply organic fungicide sprays at early symptom appearance. For fusarium and pink root, rely on long crop rotations, resistant varieties, and soil solarization or fumigation. Remove and destroy infected plant material—never compost it. Store harvested onions below 15°C (59°F) to limit post-harvest black mold development.

Disorders & troubleshooting

What physiological disorders affect onion (such as blossom-end rot, cracking, or sunscald)?

Common physiological problems include poor or small bulb development (from crowding, nitrogen imbalance, wrong variety for day length, or too short a growing season), soft or rotting bulbs (from waterlogged soil, overwatering late in the season, or premature harvest in wet conditions), and yellowing foliage (from nutrient deficiencies or overwatering). Bolted onions—those that form flower stalks—also become unmarketable for storage.

What causes these disorders and how do I prevent them?

Small bulbs result from crowding (thin to 4–6 inches for full-sized bulbs), excess nitrogen late in the season causing lush tops but tiny bulbs, or an insufficient growing season. Soft, rotten bulbs result from overwatering or waterlogged soil. Bolting is more likely with early plantings—later transplant dates reduce the risk. Prevent problems by thinning early, switching from high-nitrogen to potassium-rich feeds as tops swell, choosing the right variety for your day-length zone, and maintaining steady moisture of about 1 inch per week.

Why are my onion leaves yellowing, curling, or spotted?

Older leaves yellowing first indicate nitrogen deficiency. New growth yellowing or stunted suggests iron or sulfur deficiency. Yellowing after heavy rain or overwatering is caused by waterlogged soil leaching nitrogen. Silvery streaks and white speckled spots on leaves are a sign of thrips feeding. Curving, yellow, or necrotic leaves with wilting tips point to fusarium basal plate rot.

Why is my onion not growing or producing well?

Poor growth most commonly results from crowding (thin to at least 4–6 inches for full-sized bulbs), using the wrong variety for your day-length zone (causing bulbing too early with too few leaf rings), low soil fertility or nitrogen deficiency, inconsistent watering, or hot summer temperatures triggering premature bulb formation before the plant has grown enough leaves—since every leaf becomes a ring inside the bulb.

Harvest

How long does onion take to mature?

Onions typically mature 100–120 days after planting, though the range spans 90–150 days depending on cultivar and conditions. Storage types grown from seed may take at least 120 days to reach a harvestable stage.

How do I know when onion is ready to harvest?

Harvest bulb onions when the tops naturally fall over and begin to brown and papery skins have formed on the bulb. A practical rule is to start harvesting when about half the leaves have died back. Do not wait until every plant’s foliage has fallen—those that matured earliest can become over-mature. For green onions, harvest as soon as tops reach 6 inches tall.

How do I harvest onion correctly?

Use a garden fork to loosen soil around plants without skewering the bulbs, then lift them by hand or trowel. Shake off excess soil and do not wash the bulbs. Leave the tops intact—cutting tops before the neck is fully cured provides an entry point for bacteria and fungi that cause bulb rot. Harvest in the early morning when temperatures are cooler.

Storage, preservation & seed saving

How should I store fresh onion, and how long does it keep?

Cure onions first by spreading them in a well-ventilated, dry location at temperatures below 85°F for 2–4 weeks until the neck is fully dry and outer skin is slightly crisp. Then store in a cool (32–40°F / 0–4°C), dark, dry, and well-ventilated place—such as a cellar, pantry, or garage—in wire baskets, crates, or mesh bags; avoid plastic bags that restrict airflow. Most properly cured bulb onions keep 2–4 months; storage varieties like Copra can last 10–12 months and Stuttgarter 6–8 months. Sweet onions have high moisture content and keep only a few weeks; refrigerate them wrapped in paper towels.

Uses & nutrition

What is onion commonly used for in cooking?

Onions are a staple in a vast range of savory dishes—soups (such as French onion soup), stews, burger toppings, baked chicken, lasagna, and pizza. They can be eaten raw, caramelized, or grilled, and add flavor and texture to nearly any meal without overwhelming other ingredients.

Season extension & regional growing

Can I grow a fall or second crop of onion?

Yes. Onion seeds can be planted in mid to late August for overwintering; these crops should be mulched late in the year to minimize winter damage and typically mature in June or July of the following year. Special overwintering varieties are required. In California and other mild-winter regions, sowing or setting plants in fall through late winter is common practice to give the crop a long, cool growing runway before spring bulbing.

How can I extend the onion growing season?

Starting onions in containers indoors allows earlier planting than direct outdoor seeding. Overwintering special varieties (sown mid to late August, mulched in late fall) extends production into the following June or July. In mild-winter regions, fall planting gives the crop the long cool period it needs before day length triggers spring bulbing.

How do I grow onion in a particularly hot or cold region?

In hot regions, plant early—most plantings before April 15 yield the best results—to give onions a long cool period before summer heat triggers premature bulbing, which causes reduced final bulb size. In cold regions, choose long-day varieties suited to higher latitudes, start seeds indoors 10–12 weeks before the last frost, and mulch overwintered crops to protect against winter damage.