What to do now
Mildest step first.
- If indoor seedlings are flopping but green and firm at the base, trim the tops to about 4 inches with scissors.
- Lower the light to 4 to 6 inches above the seedlings and keep them around 55 to 65°F.
- Pull out any seedlings with a brown, pinched stem at the soil line, and let the mix dry slightly between waterings.
- In the garden, dig up a collapsed onion and look for small white maggots in the base; if you find them, cover the next planting with row cover.
Thin and floppy is normal
Onion seedlings look nothing like tomato seedlings. K-State's Johnson County office notes that they tend to be spindly, often with the seed coat stuck to a leaf tip for several weeks, and Wisconsin's grower guide describes the bent "knee stage" that newly sprouted onions go through. A seedling that leans or lies over but is green and firm at the base is usually fine. Iowa State notes that low light and temperatures above 65°F make the growth more spindly, so a warm room and a distant light make the flopping worse.
Give them a haircut
Trimming is the usual fix. Iowa State suggests clipping seedlings back to 4 inches with scissors once they reach 5 inches, and notes this is often done two or more times before planting. Penn State says trimming the tops to 4 inches produces a stout, sturdy transplant, and K-State suggests trimming when plants reach 4 to 5 inches. Wisconsin clips the tops to 5 inches at transplanting, so the plant loses less water while its roots settle in.
Light, warmth and timing
Indoors, onions want bright light and cool air. Iowa State suggests a light no more than 4 to 6 inches above the seedlings and 55 to 65°F for best growth. Wisconsin suggests at least 10 hours of light a day, with 60 to 65°F by day and 55 to 60°F at night, and K-State moving seedlings somewhere cooler once they are 1 to 2 inches tall. Iowa State suggests thinning crowded seedlings within 2 to 3 weeks of sprouting, and K-State starting a soluble fertilizer once they are 2 to 3 inches tall. Sources differ on when to sow: Utah State and Maryland say about 8 weeks before planting out, Iowa State 8 to 10 weeks, and Penn State and Wisconsin 10 to 12.
Collapsed at the soil line
Seedlings that suddenly collapse are a different problem. Penn State describes damping-off as a stem rot at or slightly below the soil line; affected seedlings may discolor, wilt, collapse and die. Utah State notes that the lower stem turns brown or black before the seedling falls over. Those plants rarely recover. To protect the rest, Penn State suggests sterile mix and clean trays, not overwatering, and not sowing so thickly that air can't move between the seedlings. Wisconsin suggests keeping the mix moist but not wet.
Transplant size and depth
Onion transplants are meant to be slim. Clemson suggests plants about 6 inches tall and about half the thickness of a pencil at transplanting, while Wisconsin's grower guide aims for 1/4 inch thick. Plant them shallow. Iowa State and Clemson suggest 1 to 1 1/2 inches deep, and Texas A&M advises against setting transplants more than 1 inch deep.
Collapsing in the garden
Young onions that wilt and die in the garden, often several next to each other, are often onion maggots. Utah State notes that the maggots feed on the roots and bulbs of young onions, causing wilting and death, and that one larva typically kills several neighboring seedlings. Iowa State notes the damage is worse in cool, wet springs, and Purdue that seeded onions are more susceptible than transplants. Iowa State and Utah State suggest over the bed at planting to keep the flies from laying eggs. Utah State also advises against planting in soil with a lot of undecomposed , and suggests clearing out culls and debris after harvest. Cutworms can also cut young onions off overnight, and thrips leave pale, speckled or silvery streaks on the leaves; Iowa State notes thrips are especially hard on young seedlings.
Plants that stay thin
Onions that stay thin usually have competition. They have shallow roots and no leafy canopy, so they compete poorly with weeds, as Iowa State, Wisconsin and UNH all note. Keep the bed weeded, but hoe shallowly; Texas A&M warns that deep hoeing cuts onion roots. UNH notes that crowded onions make smaller bulbs, and Utah State suggests 3 to 4 inches between plants, thinning before they start to bulb. Shallow roots also dry out quickly, so steady water matters. Iowa State suggests with fertilizer 4 to 5 weeks after planting, though UNH cautions that too much nitrogen can delay bulbing. If plants look healthy but never form bulbs, the variety may not match your day length.
- OnionA day-length sensitive allium — plant the wrong variety for your latitude and you tend to get a handful of scallions instead of bulbs.
- LeekA long-season allium that rewards hilling with a foot of white shank.
- ScallionThe fast crop for impatient gardeners — harvestable in two months and amenable to continuous succession.
- ShallotA bulb-clustering allium with a milder, sweeter flavor than its onion cousin.
- Damping OffSeedlings collapse at soil level with a pinched, rotted stem — happens quickly in wet, cool conditions.
- Onion MaggotOnion seedlings and sets collapsing at the base, with white maggots tunneling into bulbs at or below the soil line.
- ThripsSilver-streaked or stippled leaves with distorted growth; tiny slender insects visible on flowers and undersides.
- Why are my seedlings tall, pale, and floppy?Seedlings stretch toward inadequate light — the usual fix is moving the light closer, to about 2–6 inches above the leaves, and running it 14–16 hours a day.
- Why are my onions not bulbing?Onions start bulbing based on day length. If the variety doesn't match your latitude, it may never bulb or may bulb tiny. Check whether yours are short-day, intermediate-day, or long-day.
- Why are my onion tops falling over?If bulbs are full-sized and the necks are softening late in the season, it's maturity: harvest once half to two-thirds have fallen. Tops collapsing early, with yellow tips or rotten bases, point to thrips, onion maggot or rot.