Vegetable · Amaryllidaceae
Onion
Allium cepa
Match the variety to your day length, keep the water steady, and onions all but grow themselves.
Full sun
Every 3–7 days
55–75°F · 13–24°C
90–120 days
Overview
Gardeners have leaned on onions for thousands of years, and they will happily reward you too. Though the plant is a biennial that you harvest in a single season, it spends that first year pouring its growth into the swelling bulb you get to pull and store. It keeps good company with garlic, leeks, and chives, sharing their bright, sharp scent.
The one thing an onion truly cares about is day length, and once you know that, the rest falls into place. Long-day types hold off until the long days of a northern summer before starting a bulb, short-day types bulb on the shorter days of a southern winter, and day-neutral types rest easily in between. Match the type to your latitude and you will avoid the disappointment of leafy tops with little underneath.
None of this should feel daunting. Offer full sun, loose and fertile soil, and water they can rely on while the bulb forms, and onions ask for very little more. Most stumbles come down to cold-stressed sets that bolt or bulbs that were cured in a rush and softened in storage, and both are easy to avoid once you know to watch for them.
Care
The short version: give them full sun, loose and fertile soil, and about an inch of water a week while the bulbs fill out. Feed nitrogen through leaf growth and stop once bulbing begins, then gently ease off the water as the tops fall over.
Light
Give onions full sun and all of it you can, because those leaves are what feed the growing bulb. Go easy on the light and a fat, satisfying onion quietly becomes a thin one, so a bright, open spot really pays off.
Water
Keep the soil evenly moist, around an inch of water a week, since those shallow roots simply cannot reach far for a drink. Steady moisture matters most while the bulb is filling, and there is no need to fuss beyond that. Ease off once the tops begin to fall so the bulbs can dry down nicely for storage.
Soil
Plant into loose, fertile, well-drained loam worked full of organic matter, which lets the bulb swell without any struggle. A pH of 6.0 to 6.8, just on the acidic side of neutral, keeps them content.
Temperature
Onions are comfortable across cool to mild weather, roughly 55°F to 75°F (13°C to 24°C). The leaves shrug off a light early frost, and that cool beginning is part of why you can plant so early with confidence.
Humidity
Ordinary outdoor humidity suits them, so there is no number to chase and nothing to fret over. What really helps is good airflow, since it is the damp, crowded leaves downy mildew loves that pose the real risk.
Feeding
Offer a nitrogen-based fertilizer through the leafy growth of spring and early summer, side-dressing every few weeks. Stop the moment bulbing begins, because late nitrogen leaves soft necks that can rot in storage, and stopping in time spares you the trouble.
Pruning
Onions ask for no regular pruning, which is one less thing to remember. Snap off any flower stalk as soon as it appears so the plant sends its energy back into the bulb, and gently pull away yellowed or diseased leaves whenever you notice them.
Size & spacing
Plants reach 12 to 18 inches (30 to 46 cm) tall with a slim, upright spread of just 4 to 6 inches (10 to 15 cm). That tidy footprint means you can fit a generous row of onions into even a small bed.
When to plant
Onions like an early start and a late finish, and the timing is kinder than it sounds. Sow seed indoors in late winter or slip sets into the ground the moment it can be worked in early spring, then lift your bulbs from midsummer on, once the tops yellow and flop over.
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 troubles turn up either in the leaves as the plant grows or in the bulbs once they are put away, and nearly all of them are recoverable. Spot the early signs and you can often rescue the crop, or at the very least the harvest already in hand. Find what matches below, then act while your effort still counts.
Pale leaf spots wearing a soft gray-purple fuzz
Those pale patches with a velvety violet-gray fuzz in the morning point to downy mildew, a fungus that moves in during cool, damp, humid spells. In time the spots go pale and papery, and the worst leaves yellow and flop over, but you can still turn things around. Pick off and destroy the affected leaves, give the plants more room to breathe, and rotate where you grow onions over several years so the spores never settle in.
Fix:
- Apply a labeled fungicide preventively where disease is common
- Improve air circulation
- Remove and destroy infected foliage
Bulbs turn soft at the neck and spoil in the bin
When a stored onion goes soft at the neck with gray mold and brown, mushy inner rings, you are looking at neck rot, a Botrytis infection that quietly shows itself weeks after harvest. Bulbs lifted a bit green or cured in a hurry are the ones most at risk, and a late feeding of nitrogen leaves thick, soft necks that let it in. Set aside the spoiled ones without worry, cure the rest in a warm, dry, airy spot, and simply stop the nitrogen once the bulbs begin to swell.
Fix:
- Discard affected bulbs
- Cure remaining bulbs thoroughly in a warm, dry, airy place
A flower stalk shows up before the bulb has filled
Take heart, because this is just bolting, and it is not a sign you did anything wrong. A chilly stretch after planting, usually from sets that were on the large side or transplants set out a touch early, is what tips a plant into flowering. Once that stalk appears the bulb will stay on the small side, so snap it off right away and enjoy those onions fresh at the table instead of storing them.
Fix:
- Remove flower stalks as soon as seen
- Use the affected bulbs fresh rather than for storage
Silver flecks and twisting where the leaves meet the neck
If you spot silvery flecks and a little twisting near the neck, gentle onion thrips are usually the reason, scraping at the leaves as they feed. Left alone their feeding can shrink the bulb you eventually lift, but you have plenty of time to help. A firm blast of water or a labeled spray sets them back, and keeping the plants watered while you tidy away nearby weeds and stray alliums takes away their hiding spots.
Fix:
- Use a strong spray of water or labeled insecticide for heavy infestations
- Encourage natural predators
- Remove heavily infested debris
Toxicity
Onions are perfectly safe on your own plate but genuinely dangerous to animals, so a little caution goes a long way. Every part carries organosulfur compounds that attack the red blood cells of cats, dogs, horses, and livestock, and cats feel it most keenly. Keep raw and cooked onion, scraps, and anything seasoned with onion or garlic powder well out of a pet's reach.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Severe
Cats take this the hardest of any pet, so even a small nibble deserves real care. Keep an eye out for vomiting, pale gums, quick breathing, and reddish or brown urine as the red blood cells give way, which can slide into weakness and collapse.
Dogs Severe
A dog that swallows onion often starts with vomiting and loose stools, then grows tired and weak with pale gums and a fast heartbeat. Dark or reddish urine is the clear signal that the red blood cells are breaking down, so reach out for help.
Horses Toxic
Horses that graze onion or wild alliums slowly turn weak and dull, their gums fade to pale, and their urine darkens as anemia takes hold. The harm gathers with repeated grazing rather than a single mouthful.
Rabbits Toxic
Please keep onion away from rabbits entirely. The very compounds that wound a cat or dog also poison a rabbit's blood, leaving it weak, pale in the gums, and off its food.
Livestock Toxic
Cattle, sheep, and goats that reach onions or onion-heavy cull piles turn anemic and weak with stained urine. Sheep cope with small amounts more easily than cattle, yet a steady diet troubles every one of them.
Birds Toxic
Birds are at risk here too, since the organosulfur compounds burst their red blood cells much as they do in mammals. Keep onion, cooked scraps, and anything seasoned safely out of their reach.
People Edible
Good news at the table: onions are a kitchen mainstay and safe for people to enjoy raw or cooked. The danger sits with the animals in your home, not with 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 grouped less by their color than by the day length they need to form a bulb. Choosing the right group for your latitude matters more than any other decision you will make, and it is an easy one to get right.
Long-day onions (e.g., 'Walla Walla', 'Yellow Sweet Spanish')
Once daylight lengthens to about 14 to 16 hours these begin their bulbs, which makes them a comforting choice for northern gardens.
Short-day onions (e.g., 'Texas Early Grano', 'Granex')
Happy to bulb on only 10 to 12 hours of daylight, they let southern growers plant in fall or winter and enjoy an early crop.
Intermediate/day-neutral onions (e.g., 'Candy')
Bulbing at around 12 to 14 hours of daylight, these forgive a lot and do well across many latitudes, so they are a reassuring pick when your day length is a mystery.
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
Onions grow either from seed or from sets, and choosing between them is simply a friendly trade between the widest choice and the easiest 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 is a wonderful path when you want the fullest range of varieties, though it does ask for a little patience and the longest season. Sow it indoors 6 to 8 weeks before your last frost, or drop it straight into the ground in early spring, and let the young seedlings take several weeks to rise and grow sturdy before they head outside.
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 friendliest way to start onions, and they are hard to get wrong. Ease these small dormant bulbs into the ground in early spring the moment the soil can be worked, and they settle in and send up green tops within 1 to 2 weeks. Reach for sets smaller than a dime, since the larger ones are the ones prone to bolting.
Myths
Only raw onions harm pets, so cooked onion or a pinch of onion powder is fine.
Cooking sadly makes no difference at all. The cooked onion and the onion or garlic powder tucked into gravy, baby food, and table scraps stay every bit as toxic to dogs and cats. The dried powders are the most dangerous of the lot, since they concentrate the toxin into a much smaller bite.
Bigger onion sets always grow bigger bulbs.
It tends to go the opposite way, so you can relax about buying the tiny ones. Sets bigger than a dime have already met enough cold to bolt, and a bolting plant spends itself on a flower stalk rather than the bulb. Small sets, smaller than a dime, are the ones that reward you with bigger, longer-keeping onions.