Why Are My Onions So Small?

Day length has to match your latitude

An onion is signaled to stop making leaves and start filling a bulb by day length, and that trigger point differs by variety. University of Illinois Extension states that onions begin bulb formation when the day length is of the proper duration and that different varieties require different day lengths, which is why onions grown in the South are not adaptable to the North and vice versa. Long-day varieties are bred for best performance in the North, short-day varieties perform best in southern locations, and Illinois warns that long-day types may not get the bulbing signal in the Deep South and should be avoided there.

The size consequence is direct. If the day length triggers bulbing before the plant has built much top growth, as a short-day type does at northern latitudes, the resulting bulb is small because leaf area sets the ceiling on bulb size. Clemson Extension similarly notes short-day types are suitable throughout South Carolina while long-day onions cannot be successfully grown in the South. Matching the type to your region is the single most important step for larger onions.

Spacing, timing, and other factors

Even the right variety stays small if it is crowded. University of Illinois Extension reports that spacing transplants 4 to 5 inches apart in the row produces large bulbs, whereas closer spacing significantly decreases bulb size, so thin or space generously if big onions are the goal, and expect smaller ones from tightly grown rows. Illinois also lists late planting of unsuitable varieties as a cause of small bulbs or no bulbing.

A couple of other things are worth ruling out. Onions that sent up a flower stalk have bolted, which is a separate disorder that also yields poor, non-storing bulbs, so a flowered plant is a different diagnosis. And steady growth matters, since anything that checks the plant, poor fertility, drought, or weed competition, limits top growth and therefore bulb size. If the variety, spacing, and timing all look right and bulbs are still small, treat the cause as uncertain and ask your Extension office to help you look at your specific conditions.

Sources

Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.

• University of Illinois Extension - Onion — Onions begin bulb formation when day length is the proper duration and different varieties need different day lengths; long-day types are bred for the North and short-day types perform best in the South, and long-day types may not get the bulbing signal in the Deep South; unsuitable varieties and late plantings cause small bulbs or no bulbing; spacing 4 to 5 inches apart produces large bulbs while closer spacing significantly decreases bulb size.
• Clemson Cooperative Extension HGIC - Onion, Leek, Shallot & Garlic — Short-day onion varieties set bulbs with about 12 hours of daylight and are suitable throughout South Carolina, while long-day onions cannot be successfully grown in the South; bulb onions should be spaced about 3 inches apart in the row.

Keep going