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
People have grown onions in the kitchen garden for thousands of years, and it's easy to see why they've stuck around. It's a biennial, but you harvest it in a single season, since the plant spends its first year pouring growth into the bulb you pull and store. It's cousins with garlic, leeks, and chives, and yes, it's got that same sharp scent.
The one thing every onion really cares about is day length. Long-day types hold out for the long days of a northern summer before they'll start a bulb, short-day types bulb on the shorter days of a southern winter, and day-neutral types sit happily in the middle. Plant the wrong type for your latitude and what do you get? Green tops with barely anything underneath.
Still, none of this makes onions tricky. Give them full sun, loose and fertile soil, and water they can count on while the bulb forms, and they'll ask for next to nothing else. Most of what goes wrong comes down to cold-stressed sets that bolt or bulbs that got cured badly and rotted 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 want full sun, and honestly the more the better, since the leaves are what feed the growing bulb. Go light on the sun and you'll end up trading a fat onion for a skinny one.
Water
Keep the soil evenly moist, around an inch of water a week, because those shallow roots just can't reach far for a drink. Steady moisture counts most while the bulb's filling. Then ease off once the tops start to fall so the bulbs can dry down for storage.
Soil
Plant in loose, fertile, well-drained loam worked full of organic matter, which lets the bulb spread out without a fight. A pH of 6.0 to 6.8 suits them nicely, just a touch on the acidic side of neutral.
Temperature
Onions do well across cool to mild weather, roughly 55°F to 75°F (13°C to 24°C). The leaves can shrug off a light frost early on, and that cool start is part of why they go in so early.
Humidity
Regular outdoor humidity is totally fine, so there's no number to fuss over. What matters way more is airflow, since it's the damp, crowded leaves that downy mildew loves that cause the real trouble.
Feeding
Feed a nitrogen-based fertilizer through the leafy growth in spring and early summer, side-dressing every few weeks. Then stop the second bulbing kicks off, because late nitrogen leaves soft necks that rot in storage.
Pruning
Onions don't need any regular pruning. Just snap off any flower stalk the moment it shows up so the plant sends its energy back to the bulb, and pull away the yellowed or diseased leaves whenever you spot them.
Size & spacing
Plants get to about 12 to 18 inches (30 to 46 cm) tall with a narrow upright spread of just 4 to 6 inches (10 to 15 cm). That slim footprint means you can tuck a whole lot of onions into a small bed.
When to plant
Onions go in early and come out in late summer. Start seed indoors in late winter, or just push sets into the ground as soon as it's workable 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 turns up in the leaves while they're growing, or in the bulbs once they're stored. Catch it early and you can usually save the crop, or at least the harvest you've already got. Find what matches below, then act while it still does some good.
Pale leaf spots with a fuzzy gray-purple coat
If those pale spots wear a velvety violet-gray fuzz in the morning, you've got downy mildew, a fungus that loves cool, damp, humid stretches. Later on the spots go pale and papery, and the worst leaves yellow and flop over. Don't panic though: pull and destroy the sick leaves, give everything more room to breathe, and rotate where you plant onions over a few years so the spores can't settle in.
Fix:
- Apply a labeled fungicide preventively where disease is common
- Improve air circulation
- Remove and destroy infected foliage
Bulbs go soft at the neck once they're stored
When a stored onion turns soft at the neck with gray mold and brown, mushy rings inside, that's neck rot, a Botrytis infection that sneaks up weeks after harvest. It tends to hit bulbs that came out too green or got cured in a hurry, and a late shot of nitrogen doesn't help since it leaves thick, soft necks. Toss the bad ones, cure the rest somewhere warm, dry, and airy, and just stop the nitrogen once the bulbs start to swell.
Fix:
- Discard affected bulbs
- Cure remaining bulbs thoroughly in a warm, dry, airy place
A flower stalk pops up before the bulb is ready
What you're seeing is bolting, and it usually comes from a cold snap after planting, often because the sets were too big or went out a bit too early. Here's the catch: once that stalk appears, the bulb stays small and won't keep well. So break the stalk off the second you notice it, and use those onions fresh instead of trying to store them.
Fix:
- Remove flower stalks as soon as seen
- Use the affected bulbs fresh rather than for storage
Silver flecks and twisting near the neck
Those silver flecks near the neck? That's usually onion thrips, scraping away at the leaves as they feed. And if they really settle in, the bulb you pull ends up smaller than it should. Hit them with a strong spray of water or a labeled product, keep the plants from drying out, and tidy up any weeds or stray alliums close by that give them somewhere to hide.
Fix:
- Use a strong spray of water or labeled insecticide for heavy infestations
- Encourage natural predators
- Remove heavily infested debris
Toxicity
Onions are perfectly fine on your plate, but they're genuinely dangerous for animals. Every part carries organosulfur compounds that go after the red blood cells of cats, dogs, horses, and livestock, and cats feel it the most of all. Keep raw and cooked onion, scraps, and anything seasoned with onion or garlic powder well away from your pets.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Severe
Cats are the most sensitive of the bunch, so even a little taste is a real problem. Keep an eye out for vomiting, pale gums, fast breathing, and red or brown urine as the red blood cells break down, and it can slide into weakness and collapse from there.
Dogs Severe
A dog that eats onion will often throw up and get the runs first, then go weak and worn out with pale gums and a racing heart. That dark or reddish urine? It's the giveaway that the red blood cells are being destroyed.
Horses Toxic
Horses that graze onion or wild alliums turn weak and dull, their gums fade to pale, and their urine darkens as the anemia creeps in. It's not one bite that does it, though; the damage piles up with repeated grazing.
Rabbits Toxic
Onion just isn't for rabbits, full stop. The same compounds that hurt cats and dogs go after a rabbit's blood too, leaving it weak, pale in the gums, and off its food.
Livestock Toxic
Cattle, sheep, and goats that get into onions or onion-heavy cull piles end up anemic and weak with stained urine. Sheep handle a little better than cattle, but a steady diet of it wears down every one of them.
Birds Toxic
Birds aren't off the hook either, since those organosulfur compounds burst their red blood cells the same way they do in mammals. Keep onion, cooked scraps, and any seasoned food well out of their reach.
People Edible
Good news for you: onions are a kitchen staple and totally safe to eat raw or cooked. The whole toxicity thing is an animal problem, not something the cook needs to worry about.
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 get sorted less by color and more by the day length they need to bulb. Picking the right group for your latitude matters more than pretty much any other call you'll make.
Long-day onions (e.g., 'Walla Walla', 'Yellow Sweet Spanish')
These wait until the days stretch out to about 14 to 16 hours before they start a bulb, which is exactly why they're the go-to for northern gardens.
Short-day onions (e.g., 'Texas Early Grano', 'Granex')
Since they'll bulb on just 10 to 12 hours of daylight, southern growers pop them in during fall or winter and get an early crop out of it.
Intermediate/day-neutral onions (e.g., 'Candy')
They bulb at around 12 to 14 hours of daylight, so they do fine across a lot of latitudes and make a safe bet when you're not sure of your day length.
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
You can grow onions from seed or from sets, and the one you pick really just 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
Starting from seed is the way to go if you want the biggest pick of varieties, but it does need the longest season. Get the seed going indoors 6 to 8 weeks before your last frost, or just sow it right into the ground in early spring, and let the seedlings take several weeks to come up and toughen up 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 hands-down the easiest way to grow onions. Just tuck these little dormant bulbs into the ground in early spring the moment the soil's workable, and they'll root and send up green tops within 1 to 2 weeks. Go for sets smaller than a dime, since it's the big ones that tend to bolt.
Myths
Only raw onions harm pets, so cooked onion or a pinch of onion powder is fine.
Nope, cooking doesn't change a thing here. Cooked onion and the onion or garlic powder tucked into gravy, baby food, and table scraps are every bit as toxic to dogs and cats. The dried powders are actually the worst, because they cram the toxin into a much smaller bite.
Bigger onion sets always grow bigger bulbs.
It's usually the opposite, funnily enough. A set bigger than a dime has already caught enough cold to bolt, and once a plant bolts it throws its energy into a flower stalk instead of the bulb. So the small sets, smaller than a dime, are the ones that hand you the bigger, longer-keeping onions.