Onion

A pungent allium grown from sets or seed, with bulb formation triggered by day length and temperature. Choose short-day, intermediate, or long-day varieties based on your latitude to ensure proper bulbing in your region. Keep beds well weeded since onions have shallow roots and compete poorly against vigorous weeds. Harvest when tops begin to yellow and fall over, then cure bulbs in a warm, dry area for two weeks before storing.

For the widest variety selection and strongest plants, start onions from seed indoors 10-12 weeks before your planned transplant date — this is earlier than most gardeners expect. Sow seeds 6mm (1/4 inch) deep in flats or cell trays filled with sterile seed-starting mix, spacing seeds 1 cm apart. Keep the mix consistently moist at 18-24°C (65-75°F) and cover with a humidity dome until sprouts appear in 7-10 days. Onion seeds are slow and erratic germinators compared to most vegetables, so be patient.

Once seedlings emerge, provide 12-14 hours of strong light daily from grow lights positioned 5-8 cm above the tops. Onion seedlings grow as thin, grass-like shoots that tend to get floppy and fall over — trim the tops to 10 cm tall whenever they exceed 15 cm to encourage thick, sturdy stems and redirect energy to root development. This trimming can be done 2-3 times during the indoor growing period and is one of the most important steps for producing strong transplants. Feed with a dilute liquid fertilizer (half-strength) every 2 weeks from the three-leaf stage onward.

Begin hardening off seedlings 1-2 weeks before transplanting by exposing them to outdoor conditions for increasing periods each day. Onion seedlings are remarkably cold-tolerant and can be transplanted in early spring as soon as the soil is workable — they tolerate light frost down to -3°C (27°F) without damage. Set transplants 10-15 cm apart in rows 30 cm apart, planting just deep enough to anchor the roots with the small bulb base at the soil line. Water in well and apply a thin layer of mulch. Alternatively, plant onion sets (small dry bulbs) directly in the garden 2-4 weeks before the last frost date, pushing them into soil so the pointed tip just shows above the surface. Sets are the easiest method but offer far fewer variety choices and are more prone to bolting than seed-grown transplants.

Onions are a fundamental garden crop that requires understanding of day-length sensitivity for successful bulb formation. Short-day varieties (for latitudes below 35°N) begin bulbing when days reach 10-12 hours, intermediate-day types (35-38°N) at 12-14 hours, and long-day varieties (above 38°N) at 14-16 hours. Planting the wrong type for your latitude results in tiny bulbs or premature bolting. Onions can be started from seed (most variety choices, cheapest), transplants (fast establishment), or sets (easiest but limited selection). For seed starting, sow indoors 8-10 weeks before transplanting outdoors.

Prepare beds by working in 5-8 cm of compost and a balanced fertilizer into loose, well-drained soil. Onions are shallow-rooted and need weed-free conditions — they compete poorly against weeds and yield drops dramatically in weedy beds. Transplant seedlings or sets 10-15 cm apart in rows 30 cm apart, planting just deep enough to anchor the bulb with the top visible above soil.

Water consistently with 2.5 cm per week, especially during bulb enlargement, and keep the root zone weed-free through shallow cultivation or thick mulch. Side-dress with nitrogen fertilizer every 2-3 weeks during active leaf growth — each leaf corresponds to one ring of the bulb, so more healthy leaves means larger bulbs. Stop fertilizing and reduce watering once bulbs begin swelling visibly and tops start to soften. When half the tops have fallen over naturally, bend the remaining tops to initiate the curing process.

Onions' pungent sulfur compounds repel carrot rust fly, making them one of the best companions for carrots — the benefit is mutual since carrots repel onion flies. Lettuce and beets grow well alongside onions because they occupy different soil depths and do not compete for the same resources. Cabbage family crops benefit from onions' pest-deterring scent. Avoid planting near peas and beans — alliums inhibit the nitrogen-fixing Rhizobium bacteria that legumes depend on, significantly reducing their vigor and yield.

Onions require fertile, well-drained, loose soil with a pH of 6.0-7.0. Heavy or compacted clay soil is the enemy of good bulb development — it restricts expansion and holds excess moisture that promotes rot. If your soil is heavy, amend generously with compost, coarse sand, and perlite to improve drainage and create the loose, friable texture onions need. Raised beds 15-20 cm high are ideal for onions in clay soils. Sandy soils drain well but need extra compost to retain moisture and nutrients. Test your soil pH before planting — onions struggle in strongly acidic soil below 5.8.

At planting time, work in a balanced fertilizer (10-10-10) at a rate of 100g per square meter, along with 5-8 cm of compost into the top 20 cm of soil. Nitrogen is the most critical nutrient during the leaf-growing phase because each healthy green leaf corresponds directly to one ring in the mature bulb — more leaves means larger bulbs. Side-dress every 2-3 weeks with ammonium sulfate, blood meal, or fish emulsion until you see bulbs beginning to swell above the soil line. Phosphorus from bone meal supports strong root development, while potassium from wood ash or sulfate of potash promotes bulb firmness and improves storage life.

The critical transition point comes when bulbs begin visibly swelling — typically 3-4 weeks before harvest. Stop all nitrogen fertilization at this stage, as continued nitrogen encourages soft, thick-necked growth that resists curing and rots in storage. Reduce watering gradually as well, allowing the soil surface to dry between irrigations. This mild stress signals the plant to stop producing new leaves and concentrate energy on bulb maturation and the formation of tight, protective outer skins essential for long-term storage.

Ideal Temperature

Hardiness Zone Compatibility

Onions are ready to harvest when the tops begin to yellow, weaken at the neck, and fall over naturally — this typically happens in late summer when the bulbs have finished sizing up. Once about half the tops have fallen, gently bend the remaining upright tops to speed maturation. Do not break or cut the tops — simply push them over at the neck to signal the plant to stop sending energy to the leaves and instead concentrate on sealing the neck. Wait 7-10 days after bending before pulling bulbs to allow the neck tissue to close and dry, which is critical for long-term storage success.

Loosen bulbs carefully with a garden fork inserted 10-15 cm away from the row to avoid puncturing the bulbs. Lift gently and shake off loose soil — never wash onions, as moisture promotes rot. If weather is dry and sunny, lay the pulled onions directly on the soil surface for 1-2 days to begin the curing process, turning them once. If rain threatens, move them immediately to a covered area with good airflow, such as a porch, carport, or greenhouse bench. Handle bulbs carefully throughout — bruised onions develop soft spots in storage and rot quickly.

Cure onions for 2-3 weeks in a warm (24-30°C / 75-85°F), dry, well-ventilated location out of direct sunlight. The curing is complete when the outer skins are papery and rustle when handled, the necks are completely dry and tight with no green tissue visible, and the roots are dry and wiry. Trim roots close to the base and cut the tops to 2-3 cm above the bulb, or leave tops intact for braiding softneck varieties. Inspect each bulb before storing — any with thick, moist necks, soft spots, or damage should be set aside for immediate use rather than long-term storage.

Storage & Preservation

Cured onions store best in a cool (0-4°C / 32-40°F), dry (65-70% humidity), well-ventilated location with good air circulation. Mesh bags, open crates, nylon stockings, or braided strings all work well — the key is preventing moisture buildup around the bulbs. Pungent varieties store far longer than sweet types because their higher sulfur content acts as a natural preservative. Strong-flavored storage onions like Copra and Patterson can last 6-10 months, while mild sweet onions like Walla Walla and Vidalia should be used within 1-2 months. Store onions away from potatoes — potatoes release moisture and ethylene gas that accelerate onion sprouting and rot.

For freezing, peel and dice or slice onions, then spread in a single layer on parchment-lined baking sheets and flash-freeze for 2 hours before transferring to labeled freezer bags. This prevents clumping so you can grab just the amount you need. Frozen diced onions keep for 10-12 months and are perfect for cooked dishes where texture is less important. Caramelize large batches slowly over 45-60 minutes until deep golden-brown, then freeze in ice-cube-tray portions for instant flavor in soups, sauces, and gravies throughout the year.

Onions dehydrate exceptionally well — slice into thin rings or dice, then dry at 57°C (135°F) for 6-10 hours until brittle. Dried onion flakes last 1-2 years in airtight containers and can be ground into homemade onion powder with far more flavor than commercial versions. Pickled pearl onions and cocktail onions in spiced vinegar brine are a classic preservation method, lasting 6-12 months in the refrigerator. Onion jam or relish made with balsamic vinegar and brown sugar is a gourmet condiment worth preserving in small jars for gift giving.

Troubleshooting Common Problems

Bolting — the premature formation of a flower stalk — is the most frustrating problem for onion growers and is almost always triggered by cold stress on young plants. Onion seedlings or sets exposed to prolonged temperatures below 10°C (50°F) for 10 or more days interpret this as a winter signal and switch from vegetative growth to reproductive mode. To prevent bolting, avoid transplanting too early into cold soil, do not expose sets to freezing temperatures before planting, and choose bolt-resistant varieties for your region. If a flower stalk does appear, snip it at the base immediately — the bulb is still edible but will have a tough central core and will not store well, so use it first.

Small bulbs are one of the most common disappointments and can result from several causes: planting the wrong day-length variety for your latitude (the single most common mistake), overcrowding without adequate thinning, insufficient nitrogen during the leaf-growth phase, or weed competition during the critical bulb-sizing period. Onions are shallow-rooted and compete very poorly with weeds — even modest weed pressure can reduce bulb size by 50% or more. Keep beds meticulously clean through shallow cultivation or thick mulch throughout the growing season.

Thick necks that refuse to dry during curing indicate over-fertilization with nitrogen late in the season or excessive watering during the final weeks before harvest. These thick-necked bulbs will not store and should be used promptly. Split or doubled bulbs occur when growth is interrupted by drought and then restarted by heavy rain or irrigation — the plant restarts from multiple growing points rather than resuming the original bulb. Maintain consistent, even moisture throughout the growing season to prevent splitting. Green sprouts emerging from stored onions indicate exposure to light, warmth, or moisture — store in complete darkness at 0-4°C for longest dormancy.

Growing Tips

1. Choosing the correct day-length type for your latitude is the single most important decision for onion success. Short-day varieties (below 35°N), intermediate-day (35-38°N), and long-day (above 38°N) — planting the wrong type guarantees tiny bulbs or no bulbing at all.
2. Start seeds indoors 10-12 weeks before transplanting for the widest variety selection and strongest plants. Trim seedling tops to 10 cm whenever they get floppy — this is the secret to producing thick, sturdy transplants that establish quickly.
3. Onions are extremely poor competitors against weeds due to their shallow root system and narrow upright leaves that cast almost no shade. Keep beds immaculately weed-free through the entire growing season — even moderate weed pressure reduces yields by 50% or more.
4. Each green leaf on the plant corresponds to exactly one ring in the mature bulb. Maximize leaf production through consistent nitrogen feeding every 2-3 weeks during the vegetative phase — more leaves means bigger onions.
5. Stop all nitrogen fertilization and reduce watering once bulbs begin swelling above the soil. Continued nitrogen at this stage produces thick, soft necks that resist curing and rot in storage.
6. Water consistently with 2.5 cm per week during active growth. Onions have shallow roots concentrated in the top 15 cm of soil and dry out quickly. Drip irrigation or soaker hoses are ideal — avoid overhead watering which promotes foliar disease.
7. For the best storage onions, let tops fall over naturally before harvesting. Cure thoroughly for 2-3 weeks in a warm, dry, ventilated area. Only store bulbs with completely dry, papery necks.
8. Rotate onion beds on a 3-4 year cycle to prevent buildup of soil-borne diseases like white rot, pink root, and Fusarium basal rot. Never plant alliums (onion, garlic, leek, shallot) in the same spot in consecutive years.