Onion

Onions don't measure time the way most vegetables do. They measure light. At a certain number of daylight hours — the threshold depends entirely on the variety — a bulbing onion stops growing leaves and starts forming a bulb. Plant the wrong type for your latitude and the plant bulbs up when it's the size of a pencil, or it never bulbs at all. This single variable — long-day, short-day, or day-neutral — is the most important choice you'll make when ordering seeds.

Long-day varieties like Walla Walla, Copra, and Red Wethersfield require 14 to 16 hours of daylight to trigger bulbing. In practice, that means they work in gardens at 40 degrees north latitude or higher — roughly the latitude of Columbus, Ohio, or northward. Short-day varieties like Yellow Granex need only 10 to 12 hours of light and are suited to gardens below about 35 degrees north latitude — the Gulf Coast, the Deep South, southern California. Day-neutral types like Ailsa Craig fall in between and can perform reasonably well at mid-latitudes, though they rarely produce the large storage bulbs that the right long-day or short-day variety will.

Start seeds indoors 10 weeks before your last frost date. Onion seedlings are slow to size up, and transplanting undersized plants leads to undersized bulbs. A seedling the diameter of a pencil at transplant time tends to produce a better bulb than a threadlike seedling. Transplant at or just around your last frost date — onion seedlings can tolerate a light frost once established. Space 4 inches apart in rows; tighter spacing produces smaller bulbs.

The failure mode most new growers encounter is bolting. An onion that experiences a prolonged cold spell after transplanting — several days below 50°F when the plant is already a certain size — can interpret the cold as a signal that winter has passed and send up a flower stalk instead of forming a bulb. Once a plant bolts, the bulb underneath begins to deteriorate. Harvest bolted plants immediately and use them fresh. Avoiding very early transplants in cold springs, or using row cover to moderate temperature, reduces this risk.

Stop watering when the tops begin to fall over on their own — that topple is the plant telling you the bulb is done. Let the tops dry in the garden for a few days if the weather is dry, then lift, brush off the soil, and cure in a shaded, ventilated space for three to four weeks. Copra and similar storage types can last six or more months; Walla Walla and fresh-eating types should be used within a few weeks of curing. The ones that last are the ones with the tightest, most papery necks.