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
The onion has held its place in the kitchen garden for thousands of years, an anchor of the everyday table. It grows as a biennial that you gather in a single season, spending its first year building the bulb that moves from bed to basket to the shelf where you keep it. It shares a family, and a bright, keen scent, with garlic, leeks, and chives.
Everything about growing an onion turns on day length, the quiet rhythm that decides your harvest. Long-day varieties wait for the long light of a northern summer before they set a bulb, short-day varieties bulb through the shorter days of a southern winter, and day-neutral varieties keep an easy balance between. Choose against your latitude and the bed gives you handsome tops with little to lift beneath them.
For all that, the onion asks very little of the space it keeps. Full sun, loose and fertile soil, and dependable water while the bulb forms carry it through the season with room to spare. Most of the season's regrets trace to cold-stressed sets that bolt or bulbs cured in haste that soften on the shelf, and both are simple enough to design out.
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 through leaf growth and stop once bulbing begins, then ease off the water as the tops fall over.
Light
Onions ask for full sun, and the more of it the bed catches the better, since the leaves are what feed the swelling bulb. Stint on the light and a generous onion quietly gives way to a slight one.
Water
Keep the soil evenly moist at about an inch of water a week, for the shallow roots cannot reach far in search of a drink. Steadiness matters most while the bulb is filling. Ease off once the tops begin to lean so the bulbs can dry down toward the shelf.
Soil
Give them loose, fertile, well-drained loam worked full of organic matter, a bed that lets the bulb open without resistance. A pH of 6.0 to 6.8, just to the acidic side of neutral, keeps them content.
Temperature
Onions live happily across cool to mild weather, roughly 55°F to 75°F (13°C to 24°C). The young leaves take a light frost in stride, and that cool opening is part of why they go in so early in the calendar.
Humidity
Ordinary outdoor humidity serves them, so there is no figure to chase. What earns your attention is airflow, since the damp, crowded leaves that downy mildew favors are the season's real risk.
Feeding
Offer a nitrogen-based fertilizer through the leafy weeks of spring and early summer, side-dressing every few weeks. Set it aside the moment bulbing begins, since late nitrogen leaves the soft necks that soften on the shelf.
Pruning
Onions call for no routine pruning. Snap off any flower stalk as soon as it rises so the plant turns its energy back to the bulb, and draw away yellowed or diseased leaves as they appear.
Size & spacing
Plants stand 12 to 18 inches (30 to 46 cm) tall on a slim upright spread of only 4 to 6 inches (10 to 15 cm). That trim footprint lets a whole row of onions settle into a modest bed.
When to plant
The onion keeps an early-in, late-out calendar that suits a gardener's spring routine. Start seed indoors in late winter or slip 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 lean 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 announce themselves in the leaves through the season or in the bulbs once they reach the shelf. Read the early signs and you can save the crop, or at the least the harvest already in the basket. Find the match below, then act while the moment is still yours.
Pale leaf spots dressed in a soft gray-purple bloom
Those pale spots wearing a velvety violet-gray bloom on a humid morning belong to downy mildew, a fungus at home in cool, damp weather. The marks fade to pale and papery in time, and the most affected leaves yellow and give way. Lift and destroy the touched foliage, open the bed so air moves freely through it, and move the onion patch to fresh ground over several seasons so the trouble never puts down roots.
Fix:
- Apply a labeled fungicide preventively where disease is common
- Improve air circulation
- Remove and destroy infected foliage
Stored bulbs soften at the neck and are lost
An onion that softens at the neck in the pantry, gray with mold and brown within, has met neck rot, a Botrytis infection that reveals itself weeks after the harvest is in. Bulbs lifted a shade too green or cured in haste are the ones it claims, and a late feeding of nitrogen leaves the thick, soft necks that invite it. Let the spoiled ones go, cure the keepers in a warm, dry, airy corner, and set the nitrogen aside once the bulbs begin to swell.
Fix:
- Discard affected bulbs
- Cure remaining bulbs thoroughly in a warm, dry, airy place
A flower stalk rises before the bulb has come into its own
This is bolting, the season's small disappointment, and it follows a cold spell after planting, usually the result of oversized sets or transplants set out ahead of their time. Once the stalk appears the bulb stays modest and loses its place in the pantry, since it will not keep. Snap the stalk away the moment it shows and fold those onions into the week's cooking while they are fresh.
Fix:
- Remove flower stalks as soon as seen
- Use the affected bulbs fresh rather than for storage
A silvery sheen and a twist near the neck
A faint silver sheen where the leaves gather at the neck marks the arrival of onion thrips, quietly rasping the foliage as they feed. Their steady work costs the bulb its final size, so the harvest that reaches the counter comes up smaller than the season promised. A firm rinse of water or a labeled spray sets them back, and a well-watered bed kept clear of nearby weeds and stray alliums leaves them nowhere to settle.
Fix:
- Use a strong spray of water or labeled insecticide for heavy infestations
- Encourage natural predators
- Remove heavily infested debris
Toxicity
Onions are a pleasure on the plate yet genuinely dangerous to animals, a line worth drawing clearly in any home. Every part carries organosulfur compounds that attack the red blood cells of cats, dogs, horses, and livestock, and the cat is 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
Of every animal in the home the cat is the most sensitive, and even a small taste is dangerous. As the red blood cells break down, look for vomiting, pale gums, quick breathing, and red or brown urine, with weakness and collapse close behind.
Dogs Severe
A dog that has eaten onion tends to vomit and develop diarrhea first, then grow weak and weary with pale gums and a racing heart. Dark or reddish urine is the plain sign that the red blood cells are being destroyed.
Horses Toxic
Horses left to graze onion or wild alliums turn weak and dull, their gums pale, and their urine darkens as anemia takes hold. The harm gathers over repeated grazing rather than a single mouthful.
Rabbits Toxic
Onion belongs nowhere near a rabbit. The compounds that trouble cats and dogs wound a rabbit's blood in the same way, bringing weakness, pale gums, and a fading appetite.
Livestock Toxic
Cattle, sheep, and goats that reach onions or onion-heavy cull piles show anemia, weakness, and stained urine. Sheep bear small amounts more easily than cattle, yet a steady diet tells on all of them.
Birds Toxic
Birds are no less at risk, since the organosulfur compounds rupture their red blood cells as surely as they do in mammals. Keep onion, cooked scraps, and seasoned food well beyond their reach.
People Edible
For the household cook, onions are a cupboard essential and safe to enjoy raw or cooked. The danger is the animals' alone and never the cook's.
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 themselves less by color than by the day length each needs to bulb. Choosing the group that suits your latitude shapes the harvest more than any other decision at the bed.
Long-day onions (e.g., 'Walla Walla', 'Yellow Sweet Spanish')
They begin their bulbs only as daylight lengthens toward 14 to 16 hours, which makes them the natural companion to a northern garden's long summer evenings.
Short-day onions (e.g., 'Texas Early Grano', 'Granex')
Content to bulb on a mere 10 to 12 hours of daylight, they let southern gardeners plant through fall or winter and set an early crop on the table.
Intermediate/day-neutral onions (e.g., 'Candy')
Bulbing at roughly 12 to 14 hours of daylight, they settle easily across a wide sweep of latitudes and make the graceful choice when the local day length is a question mark.
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 come to the garden from seed or from sets, and the choice is a graceful trade between the widest selection and the simplest 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 the route for the gardener who wants the fullest choice of varieties on the table, though it asks for the longest season in return. Start it indoors 6 to 8 weeks before your last frost, or sow it straight into the ground in early spring, and allow the seedlings several weeks to rise and firm up before they take their place 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 effortless way into an onion crop. Settle these small dormant bulbs into the ground in early spring the moment the soil can be worked, and green tops rise within 1 to 2 weeks. Choose sets smaller than a dime, since the larger ones are the ones inclined to bolt.
Myths
Only raw onions harm pets, so cooked onion or a pinch of onion powder is fine.
The stove offers no protection at all. Cooked onion and the onion or garlic powder folded into gravy, baby food, and table scraps stay just as toxic to dogs and cats. The dried powders are the most dangerous of them, since they gather the toxin into a far smaller bite.
Bigger onion sets always grow bigger bulbs.
The truth runs the other way. A set larger than a dime has already tasted enough cold to bolt, and a bolting plant spends itself on a flower stalk rather than the bulb. The small sets, smaller than a dime, are the ones that reward you with larger, longer-keeping onions.