A bulbing onion grows leaves until days reach a certain length. Then it sts making leaves and swells a bulb. If your plants are tall and green with straight, pencil-thick necks well into summer, they haven't received that signal. If they made a small bulb early and the tops fell over while the plants were still small, they got the signal too soon. A thick flower stalk from the center is a third problem: . Each of these has a different cause.
A mismatch between variety and latitude is the most common reason. Short-day onions start bulbing at about 10 to 12 hours of daylight and suit gardens south of roughly 35°N, about the latitude of Memis or Albuquerque. Intermediate-day onions respond at about 12 to 14 hours and fit the middle band, roughly 35° to 40°N. Long-day onions need about 14 to 16 hours and do best north of about 40°N, around Philadelphia, Columbus, or Denver. Sources draw these lines a degree or two differently, so treat them as approximate. A long-day onion in the South may never get enough daylight to bulb. A short-day onion in the North tends to bulb early and stay small.
If the variety fits your latitude, look at timing and feeding. Bulb size depends on how many leaves the plant grows before bulbing starts, so onions planted late or set out undersized tend to make small bulbs. Dry soil can also push onions to bulb early and small. Too much nitrogen late in the season delays maturity and leaves thick necks that won't cure well. Bolting usually follows a long cold spell after the plants have grown for a while. Onion sets larger than about an inch across are especially prone to bolting.
Onions that won't bulb this year are still good to eat as green onions, so pull and use them. Don't bend the tops over to force bulbing. It doesn't make a bulb form, and it can let rot into the neck. Bolted onions won't store. Harvest them and use them fresh. If you've been feeding the bed, stop adding nitrogen once bulbs start to swell.
Before ordering next season, find your latitude and buy a matching type. The variety listing or seed catalog usually says short-day, intermediate-day, or long-day. Intermediate-day types are the most widely adaptable if you're near a boundary. Set out about pencil-thick, or sets no larger than an inch, 2 to 4 weeks before your last spring frost. Keep the soil evenly moist, and finish nitrogen feeding before bulbing begins.
- ThripsSilver-streaked or stippled leaves with distorted growth; tiny slender insects visible on flowers and undersides.
- Onion MaggotOnion seedlings and sets collapsing at the base, with white maggots tunneling into bulbs at or below the soil line.
- Leaf MinerPale winding tunnels or blotches inside leaves, with tiny fly larvae visible inside the mine.
- When should I harvest my onions?Harvest onions when about half the tops have fallen over naturally — don't bend them down by hand, and give the rest 1–2 more weeks before pulling.
- Can you give a plant too much nitrogen, and what does it look like?Yes — excess nitrogen produces very dark green, lush leafy growth at the expense of flowers and fruit, and in high concentrations can burn roots and cause wilting.
- How do I know when to harvest garlic?Harvest garlic when roughly the lower third to half of the leaves have turned brown and dried — this corresponds to the bulb having formed its full complement of papery wrapper layers.