Vegetable · Amaryllidaceae
Onion
Allium cepa
Pick a type keyed to your day length and hold the moisture even, and the bulb swells fat beneath its upright green tops with hardly any help.
Full sun
Every 3–7 days
55–75°F · 13–24°C
90–120 days
Overview
Onions have grown in the kitchen garden for thousands of years. Each plant sends up a spray of hollow, upright green leaves above a bulb that swells at the base, and though the plant is a biennial it is pulled in a single season while that bulb is packed tight. It sits in the same family as garlic, leeks, and chives and carries the same sharp, pungent scent.
Everything hinges on day length. Long-day types hold back the bulb until the long days of a northern summer, short-day types round out their bulbs on the shorter days of a southern winter, and day-neutral types take the middle ground. Plant the wrong type for your latitude and the leaves grow tall and green while the bulb beneath stays thin.
None of this makes onions hard. Full sun, loose and fertile soil, and steady water while the bulb forms are nearly all they ask. Most of the trouble shows up as a flower stalk shooting up from a cold-stressed set or a bulb that softened and rotted after a poor curing.
Care
The short version: onions want full sun on their upright leaves, a loose and fertile bed, and water weekly, about an inch, while the bulb rounds and swells. Give nitrogen while the green tops lengthen, cut it off the moment bulbing starts, and taper the watering as the tops sag and topple.
Light
Onions want full sun, the more the better, because the tall green leaves are what feed the swelling bulb. Skimp on the light and the bulb finishes thin and slight instead of round and full.
Water
Keep the soil evenly moist, around an inch of water a week, since the shallow roots reach only a short way down for a drink. Steady moisture matters most while the bulb is rounding out. Ease off once the tops start to fall so the outer scales can dry to papery skin for storage.
Soil
Plant in loose, fertile, well-drained loam worked full of organic matter, which lets the bulb push outward without resistance. A pH of 6.0 to 6.8 suits them, just on the acidic side of neutral.
Temperature
Onions grow well across cool to mild weather, roughly 55°F to 75°F (13°C to 24°C). The young green leaves take a light frost early on, and that cool start is part of why they go in so early.
Humidity
Ordinary outdoor humidity is fine, so there is no number to chase. What counts more is airflow through the upright leaves, since the damp, crowded foliage that downy mildew favors is the real risk.
Feeding
Feed a nitrogen-based fertilizer through the leafy growth in spring and early summer, side-dressing every few weeks. Stop the moment bulbing begins, because late nitrogen leaves thick, soft necks that rot in storage.
Pruning
Onions need no regular pruning. Snap off any flower stalk the moment it rises from the neck so the plant sends its energy back into the bulb, and pull away yellowed or diseased leaves as you spot them.
Size & spacing
The upright leaves reach 12 to 18 inches (30 to 46 cm) tall on a narrow footprint of just 4 to 6 inches (10 to 15 cm) across. That slim, vertical shape lets a whole row of onions crowd into a small bed.
When to plant
Onions go in early and come out in late summer. Start seed indoors in late winter or press sets into the ground as soon as it can be worked in early spring, then lift the bulbs from midsummer on, once the green tops yellow and flop over the neck.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: 6 to 8 weeks before the last spring frost
- Plant: As soon as soil can be worked in early spring, 2 to 4 weeks before the last frost
- Harvest: Mid to late summer when tops yellow and fall over
Hardiness
USDA hardiness zones: 3–9
Temperature: down to -40°C
Troubleshooting
Most onion trouble shows up in the leaves while the plant grows or in the bulbs after they are stored. Catch the early marks on the foliage and you can save the crop, or at least the bulbs already pulled. Match what you see below, then act while it still helps.
Pale elongated leaf spots furred violet-gray
The long pale spots on the leaves, furred with a velvety violet-gray growth on humid mornings, are downy mildew (Peronospora destructor). As the lesions age they dry to a pale, papery patch, and the worst leaves yellow and fold over. Strip off and destroy the marked foliage, widen the spacing so the leaves dry faster, and rotate onions over several years so the spores never settle in the soil.
Fix:
- Apply a labeled fungicide preventively where disease is common
- Improve air circulation
- Remove and destroy infected foliage
Soft, sunken neck tissue and gray mold in storage
A stored onion that turns soft and water-soaked at the neck, with gray mold on the outside and brown, mushy scales within, has neck rot, a Botrytis infection that surfaces weeks after harvest. Bulbs pulled too green or cured poorly are the usual victims, and a late feeding of nitrogen leaves thick, soft necks that let it in. Throw out the rotted ones, cure the rest in a warm, dry, airy spot, and stop nitrogen once the bulbs begin to swell.
Fix:
- Discard affected bulbs
- Cure remaining bulbs thoroughly in a warm, dry, airy place
A tall flower stalk rises from the neck instead of a bulb
This is bolting, marked by a single upright flower stalk pushing straight up out of the neck. It follows a cold spell after planting, often from oversized sets or transplants set out too early. The bulb left beneath that stalk stays small and stores poorly, so snap the stalk off as soon as it shows and eat those bulbs fresh.
Fix:
- Remove flower stalks as soon as seen
- Use the affected bulbs fresh rather than for storage
Silvery flecks and twisted leaves at the neck
The pale silvery flecks and slight twisting of the leaves come from onion thrips, insects only about 1 mm long and pale yellow to brown, hiding down in the leaf axils and the neck. They rasp the green surface and leave the silver stippling where the leaves meet. Heavy feeding shrinks the bulb, so wash them off with a strong spray of water or a labeled product, hold the moisture steady, and clear the weeds and stray alliums nearby.
Fix:
- Use a strong spray of water or labeled insecticide for heavy infestations
- Encourage natural predators
- Remove heavily infested debris
Toxicity
Onions are safe on your plate but genuinely dangerous to animals. Every part, from the papery bulb to the green leaves, holds organosulfur compounds that attack the red blood cells of cats, dogs, horses, and livestock, and cats are the most sensitive of all. Keep raw and cooked onion, scraps, and anything seasoned with onion or garlic powder well away from pets.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Severe
Cats are the most sensitive of all, and even a small taste is dangerous. As the red blood cells break down, look for vomiting, gums gone pale, fast breathing, and urine stained red or brown, with weakness and collapse following.
Dogs Severe
A dog that eats onion vomits and has diarrhea at first, then grows weak and tired with pale gums and a racing heart. Dark or reddish urine is the visible sign that the red blood cells are breaking apart.
Horses Toxic
Horses grazing onion or wild alliums grow weak and dull, their gums fade to pale, and their urine darkens as anemia sets in. The damage deepens with repeated grazing rather than a single bite.
Rabbits Toxic
Onion is never safe for a rabbit. The same compounds that harm cats and dogs poison a rabbit's blood, bringing weakness, pale gums, and a fading appetite.
Livestock Toxic
Cattle, sheep, and goats that reach onions or onion-rich cull piles show anemia, weakness, and urine stained dark. Sheep bear small amounts better than cattle, but a steady diet harms them all.
Birds Toxic
Birds are vulnerable as well, since the organosulfur compounds rupture their red blood cells just as they do in mammals. Keep onion, cooked scraps, and seasoned food well out of reach.
People Edible
Onions are a kitchen staple and safe for people to eat raw or cooked. The toxicity is a danger for animals, not for the cook.
Keep all parts of the plant, scraps, and any foods seasoned with onion or garlic powder away from pets and livestock. Cats are especially sensitive.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
United Kingdom
Animal PoisonLine: 01202 509000 (fee applies)
Australia
Animal Poisons Helpline: 1300 869 738
Varieties
Onions are sorted less by the color of the skin than by the day length they need to bulb. Picking the right group for your latitude matters more than any other choice you make.
Long-day onions (e.g., 'Walla Walla', 'Yellow Sweet Spanish')
These begin swelling their bulbs only once the days lengthen to about 14 to 16 hours, which makes them the fit for northern gardens.
Short-day onions (e.g., 'Texas Early Grano', 'Granex')
These start bulbing on just 10 to 12 hours of daylight, so southern growers plant them in fall or winter for an early crop.
Intermediate/day-neutral onions (e.g., 'Candy')
These bulb at around 12 to 14 hours of daylight and do well across a wide band of latitudes when the day length is unknown.
Companions
Onions earn their spot beside crops their strong scent helps protect, and they sit poorly next to the legumes whose growth alliums are said to check.
Plant near
Carrots: Onions are traditionally interplanted with carrots, as the strong scent of alliums is said to help deter carrot pests.
Keep apart
Beans: Alliums such as onions are traditionally said to inhibit the growth of beans.
Peas: Alliums such as onions are traditionally said to inhibit the growth of peas.
How to propagate
An onion begins two ways: seed sown fresh, or a small dormant set that is already a bulb in miniature. Seed brings the broadest pick of varieties, while sets give the quicker, simpler start.
SeedModerate
Start indoors 6-8 weeks before the last frost, or sow directly in early spring · Several weeks to germinate and establish seedlings before transplanting
Seed opens the widest choice of varieties and asks for the longest season. Start it indoors 6 to 8 weeks before your last frost, or sow it straight into the ground in early spring, and give the thin, grassy seedlings several weeks to rise and firm up before they go out.
Bulbs & cormsEasy
Plant sets in early spring as soon as soil can be worked · About 1-2 weeks to root and begin top growth
Sets are the easiest way to grow onions. Press these small dormant bulbs into the ground in early spring as soon as the soil can be worked, and they root and push up green tops within 1 to 2 weeks. Choose sets smaller than a dime, since the larger ones bolt.
Myths
Pets are only in danger from raw onion; once it has been cooked, or dried down to powder, the harm disappears.
Cooking changes nothing. Cooked onion and the onion or garlic powder hiding in gravy, baby food, and table scraps are just as toxic to dogs and cats. The dried powders are the worst of all, because they pack the toxin into a far smaller bite.
The larger the set going into the ground, the larger the onion coming out of it.
It runs the other way. A set larger than a dime has already felt enough cold to bolt, and a bolted plant puts its energy into a flower stalk instead of the bulb. Small sets, smaller than a dime, give the bigger, better-keeping onions.