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 grown onions for thousands of years, and this crop still pulls its weight in any plot. It runs as a biennial that you harvest in a single season, driving its whole first year of growth into the bulb you get to pull and store. It runs in the same family as garlic, leeks, and chives, and it packs the same bold, sharp scent.
The one thing every onion answers to is day length, and getting it right unlocks the whole crop. Long-day types hold off for the long days of a northern summer before they launch a bulb, short-day types bulb on the shorter days of a southern winter, and day-neutral types settle right between them. Miss your latitude and the bed hands you lush green tops with next to nothing underneath.
None of this makes onions a chore. Hand them full sun, loose and fertile soil, and water they can bank on while the bulb forms, and they barely ask for anything more. Nearly everything that trips a crop up traces back to cold-stressed sets that bolt or bulbs cured poorly that rot in the bin.
Care
The short version: full sun, loose and fertile soil, and about an inch of water a week while the bulbs fill out. Feed nitrogen during leaf growth and stop once bulbing begins, then ease off the water as the tops fall over.
Light
Onions crave full sun, and the more they soak up the better, since those leaves are the engine feeding the growing bulb. Shortchange the light and a fat onion shrinks to a thin one.
Water
Keep the soil evenly moist, around an inch of water a week, because the shallow roots just can't chase a drink very far. Steady moisture pays off most while the bulb fills. Back off once the tops start to fall so the bulbs dry down for storage.
Soil
Plant into loose, fertile, well-drained loam loaded with organic matter, so the bulb can spread out without a struggle. A pH of 6.0 to 6.8 hits the sweet spot, just on the acidic side of neutral.
Temperature
Onions power through cool to mild weather, roughly 55°F to 75°F (13°C to 24°C). The leaves shrug off a light frost early on, and that cool jump-start is exactly why they go in so soon.
Humidity
Everyday outdoor humidity does the job, so skip chasing a number. What really moves the needle is airflow, since the damp, crowded leaves downy mildew loves are the real threat.
Feeding
Push a nitrogen-based fertilizer through the leafy stretch in spring and early summer, side-dressing every few weeks. Then cut it the second bulbing kicks in, because late nitrogen leaves soft necks that rot in storage.
Pruning
Onions skip routine pruning altogether. Snap off any flower stalk the instant it shoots up so the plant pours its energy back into the bulb, and pull the yellowed or diseased leaves as you spot them.
Size & spacing
Plants shoot up 12 to 18 inches (30 to 46 cm) tall on a slim upright spread of just 4 to 6 inches (10 to 15 cm). That tight footprint lets you pack a whole crowd of onions into one small bed.
When to plant
Onions go in early and come out in late summer, so the season starts fast. Fire seed up indoors in late winter or drive sets into the ground the moment it can be worked in early spring, then lift the 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 trouble surfaces in the leaves while the crop grows or in the bulbs once they hit storage. Spot the early signs and you can rescue the crop, or at least the harvest already in hand. Match what you see below, then move on it while it still counts.
Pale leaf spots dressed in gray-purple fuzz
Pale spots wearing velvety violet-gray fuzz at dawn mean downy mildew has moved in, a fungus that thrives when the weather turns cool, damp, and humid. The spots soon fade to pale and papery, and the hardest-hit leaves yellow and flop. Strip off and destroy the infected foliage, throw the bed wide open for airflow, and keep onions on the move with a multi-year rotation so the spores never settle in.
Fix:
- Apply a labeled fungicide preventively where disease is common
- Improve air circulation
- Remove and destroy infected foliage
Necks turn soft and bulbs rot in storage
A stored onion that goes soft at the neck, gray with mold and brown and mushy inside, has caught neck rot, a Botrytis infection that surfaces weeks after harvest. It pounces on bulbs pulled too green or cured in a rush, and a late hit of nitrogen only feeds the problem by leaving thick, soft necks. Pitch the rotten ones, cure the keepers in a warm, dry, airy spot, and cut the nitrogen the moment the bulbs start to swell.
Fix:
- Discard affected bulbs
- Cure remaining bulbs thoroughly in a warm, dry, airy place
A flower stalk races up before the bulb fills
That shooting stalk is bolting, and it kicks off when a cold spell hits after planting, usually from oversized sets or transplants that went out too soon. The moment that stalk climbs, the bulb quits sizing up and loses its keeping power. Snap the stalk off the instant you catch it, then bring those onions straight to the kitchen and cook them fresh.
Fix:
- Remove flower stalks as soon as seen
- Use the affected bulbs fresh rather than for storage
Silver flecks and a twist right at the neck
Silver flecks near the neck give away onion thrips, tiny feeders scraping the foliage as they go. Let them run and their feeding shrinks the bulb waiting at harvest, so jump on them early. Blast them off with a hard jet of water or a labeled spray, keep the plants watered so they never stress, and rip out the nearby weeds and stray alliums that hand them a hideout.
Fix:
- Use a strong spray of water or labeled insecticide for heavy infestations
- Encourage natural predators
- Remove heavily infested debris
Toxicity
Onions are a joy on your plate but flat-out dangerous to animals. Every part packs organosulfur compounds that go straight for the red blood cells of cats, dogs, horses, and livestock, and cats take the hardest hit 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 rank as the most sensitive of the lot, so even a small taste turns dangerous fast. As the red blood cells break down, watch for vomiting, pale gums, quick breathing, and red or brown urine, with weakness and collapse following if it goes unchecked.
Dogs Severe
A dog that swallows onion usually vomits and gets the runs first, then slides into weakness and fatigue with pale gums and a pounding heart. Dark or reddish urine flags the moment the red blood cells start breaking down.
Horses Toxic
Horses grazing onion or wild alliums turn weak and dull, their gums pale off, and their urine darkens as anemia takes hold. The damage stacks up over repeated grazing rather than striking from one bite.
Rabbits Toxic
Keep onion off the menu for rabbits entirely. The very compounds that hammer cats and dogs poison a rabbit's blood too, dragging in weakness, pale gums, and a lost appetite.
Livestock Toxic
Cattle, sheep, and goats that raid onions or onion-heavy cull piles turn anemic and weak with stained urine. Sheep shrug off small amounts better than cattle, but a steady diet wears down every one of them.
Birds Toxic
Birds land squarely in the danger zone too, since the organosulfur compounds burst their red blood cells the same way they do in mammals. Keep onion, cooked scraps, and seasoned food well out of reach.
People Edible
Great news for the cook: onions are a kitchen powerhouse and totally safe for people to eat raw or cooked. The toxicity trips up animals only, never the person at the stove.
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 sort out less by color and more by the day length they need to bulb. Locking in the right group for your latitude drives the harvest more than any other call you make.
Long-day onions (e.g., 'Walla Walla', 'Yellow Sweet Spanish')
These fire up their bulbs once the days stretch toward 14 to 16 hours, which makes them the star pick for northern gardens.
Short-day onions (e.g., 'Texas Early Grano', 'Granex')
They bulb on just 10 to 12 hours of daylight, so southern growers plant them in fall or winter and pull an early crop.
Intermediate/day-neutral onions (e.g., 'Candy')
Bulbing at around 12 to 14 hours of daylight, these thrive across a wide band of latitudes and make the safe bet whenever 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 launch from seed or from sets, and the pick comes down to a trade between choice and ease.
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 throws open the biggest lineup of varieties, and it just asks you to commit to the longest season. Fire the seed up indoors 6 to 8 weeks before your last frost, or sow it right into the ground in early spring, then let the seedlings spend several weeks pushing up and toughening before they head outdoors.
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 fastest, easiest way to get onions going. Drive these little dormant bulbs into the ground in early spring the moment the soil crumbles loose, and they root and shoot up green tops within 1 to 2 weeks. Reach for sets smaller than a dime, since the big ones are the ones that bolt on you.
Myths
Only raw onions harm pets, so cooked onion or a pinch of onion powder is fine.
Cooking changes absolutely nothing here. Cooked onion and the onion or garlic powder buried in gravy, baby food, and table scraps hit dogs and cats just as hard. The dried powders top the danger list, since they cram the toxin into a far smaller bite.
Bigger onion sets always grow bigger bulbs.
It flips the other way, more often than not. A set bigger than a dime has already soaked up enough cold to bolt, and a bolting plant throws everything into a flower stalk instead of the bulb. Grab the small sets, smaller than a dime, and they hand you the bigger, longer-keeping onions.