Onion

An onion set or seedling wants deep, loose soil that has not grown alliums the year before, and at least six hours of direct sun from the day it goes in the ground. The one detail that decides whether you end up with a fist-sized bulb or a stunted green stalk is day length, not fertilizer: onions read the lengthening days of the growing season as the cue to stop building leaves and start swelling a bulb, and a variety bred for the wrong region will bulb too early, too small, or not at all. Match the type to where you garden before anything else.

Day length decides the bulb

Long-day onions need around fifteen to sixteen hours of daylight to trigger bulbing and are bred for northern summers. Short-day onions bulb on roughly twelve hours of daylight and are grown through the shorter days of a mild winter or early spring further south. Gardeners in between do better with an intermediate type. Plant the wrong category and the top growth stays lush while the base never widens, because the plant is still waiting for a day length it will not get before frost ends the season.

Sets, seed, or transplants

Sets, the small dormant bulbs sold in mesh bags, are the fastest route to a crop and the most forgiving for a first attempt: press them in at half an inch to an inch deep, about six inches apart. Seed gives access to far more varieties and produces bulbs less prone to premature flowering, but it needs the full eighty-five to hundred-and-twenty days of the season, started indoors in most climates. Transplants, young seedlings bought in bunches, split the difference. Space rows twelve to twenty-four inches apart regardless of the starting method, and thin as plants grow if you want larger bulbs rather than a dense clump of small ones.

Soil, spacing and planting depth

Onions want a neutral to slightly alkaline soil, worked to a fine, stone-free tilth so the swelling bulb meets no resistance. Loam or sandy loam with plenty of organic matter drains fastest and warms quickest in spring, which matters because cold, waterlogged soil is what pushes seedlings toward premature flowering later. Set the plant in early spring as soon as the ground can be worked, with the growing tip at or just above the surface. Burying a set too deep slows sprouting and can rot it before the roots take hold.

Watering and feeding

The root system is shallow and inefficient at foraging, so onions depend on the gardener for moisture far more than deep-rooted vegetables. Final bulb size is directly proportional to the water the plant receives, especially in the weeks after the bulb begins to swell at the soil line: let the bed dry out during that stretch and the harvest shrinks accordingly. Water deeply enough to reach the root zone rather than little and often, and ease off once the tops begin to yellow and flop, which signals the plant is shifting into curing mode on its own.

Common mistakes: bolting, splitting and pets

Cold snaps while the seedlings are still pencil-thin are the usual cause of bolting: the plant reads the chill as a false winter and responds the way a biennial should, by sending up a flower stalk instead of building a bulb. A bolted onion is still edible but will not store, because the tough central stem never cures down. Bulbs that split into two or more lobes usually trace back to a growth check, a dry spell followed by heavy rain or a burst of fertilizer, that restarted growth from more than one point. Keep harvested and stored bulbs away from dogs, cats and horses: the ASPCA lists onion as toxic to all three, the culprit being a compound called N-propyl disulfide that damages red blood cells if eaten in quantity.

Pests and diseases

Allium leafminer is the pest to watch for where it has spread, leaving rows of white puncture marks down the leaves before its larvae tunnel into the bulb. White rot is the disease that ends a bed for years once it arrives, since its resting bodies persist in the soil; there is no cure beyond removing infected plants and not replanting alliums in that ground. Downy mildew and neck rot both thrive in wet, poorly ventilated plantings, which is another reason to give each bulb the full recommended spacing rather than crowding the row.

Harvest and curing

Pull or lift bulbs once about two-thirds of the tops have yellowed, toppled over and started to dry on their own; forcing this earlier by bending healthy green tops does not improve storage and can shorten it. Lay the bulbs, tops still attached, somewhere shaded, dry and well ventilated, an open porch or an airy shed rather than direct sun, for one to three weeks until the outer skins papered over and the necks are tight and dry. Only then trim the tops to about an inch and brush off loose soil; cutting green necks invites rot in storage.

Storage

Properly cured bulbs keep for six to seven months in a cold, humid space, close to freezing with around sixty-five to seventy percent humidity, which is why a root cellar or an unheated garage outperforms a kitchen counter. Warm, dry indoor air sprouts bulbs early; damp, poorly ventilated storage rots them. Sort out any bulb with a soft spot or a thick, still-green neck before storing, since one going soft will take its neighbors with it.