Onions bolting: why they flower and what to do with the bulb

An onion that sends up a stiff flower stalk in its first year has been triggered by cold, and the bulb will not finish. Pull it and eat it fresh. What you can still control is the next planting: small sets, thin transplants and a start date after the cold has passed.

If you are still deciding what to plant, growing bulb onions in containers covers day length, pots and curing. For a plant that has already flowered, read on.

What bolting is

Onions are biennials. Lincoln University Cooperative Extension describes the natural pattern: they grow into bulbs before the hot summer, go dormant, then sprout and flower the next spring. Flowering needs a stretch of cold first, called vernalization. A plant that gets that cold in its first spring behaves as if a winter has already gone by, and starts a flower stalk instead of fattening a bulb.

Two details from the same Lincoln University source:

• Onions need about five leaves before they can register the cold. Seedstalk formation may begin before the bulb matures if plants are held below 50°F for 3-4 weeks, and lower temperatures above freezing count for more.
• Bolting in onions is not induced by day length, as is sometimes believed. Bulb formation does need long days, but day length is not what sends the flower stalk.

Clemson Cooperative Extension puts the outcome plainly: a prolonged cold period after good growth gives small bulbs with large necks, which are hard to cure and generally unusable.

Match what you see to the cause

What happened	Why it matters	Source
Cool weeks after the plants had leaves	The cold effect builds up over time, so a late cold snap on a leafy plant is the classic trigger	Lincoln University Cooperative Extension
Sets were large when planted	Larger plants at the time of cold bolt at a higher rate	Clemson Cooperative Extension
Planted very early	Early-planted sets and transplants often bolt	Utah State University Extension
Sets were kept near freezing before you bought them	Cold storage can vernalize sets before they reach you	Lincoln University Cooperative Extension

Set size is the part you can pick

Extension services split sets into two piles, and each draws the line in a slightly different place.

• University of Illinois Extension: sets larger than a dime are best used for green onions; sets smaller than a dime produce the best bulbs and usually do not bolt.
• Iowa State University Extension: the line is a nickel. The larger sets often bolt and do not produce good-sized bulbs.
• Utah State University Extension: larger sets or transplants, over 5/8 inch in diameter, have a greater tendency to bolt.
• Clemson Cooperative Extension: choose sets of an inch or less, and transplants about 6 inches high and about half the thickness of a lead pencil.

The cut-offs differ, but the direction does not: pick the small end of the bag for bulbs and plant the big ones close together for scallions.

What to do with the onion that has bolted

Lincoln University Cooperative Extension says that removing seedstalks may help with bulb formation but the effect is limited, and that if sets bolt shortly after planting, bulbs are next to impossible. Its advice is to pull them and use them as green onions, the sooner the better. University of Illinois Extension agrees on the use: remove plants that have formed flower stalks and use them immediately, since they do not make good bulbs for dry storage.

So treat a bolted onion as a fresh ingredient, not a crop for the shelf. If you want to keep onions going into winter, set aside the ones that did not flower, cure them and store them by the steps in storing onions, garlic and potatoes.

Avoiding a repeat

1. Sort the sets. Plant the small ones for bulbs, as above.
2. Do not rush the start. Utah State says sets and transplants often bolt if planted too early, and that later plantings are less likely to produce seed stalks. Clemson says planting too early can increase seed stem production. Use your local dates in first and last frost dates for a balcony to see how much cold is still likely.
3. Skip heavy fall feeding. Clemson advises avoiding high fertilizer rates applied in the fall.
4. Watch the cold forecast once leaves are up. A pot can be moved to shelter, but the cold effect is cumulative, so a plant that has already had weeks of it will not be rescued.

Garlic has its own flower stalk, the scape, and its own reasons for small heads. Small garlic bulbs explains that separately.

Sources

• Why Onions Bolt, Failing to Bulb? (Lincoln University Cooperative Extension, MU Integrated Pest Management)
• Onion (University of Illinois Extension)
• What is the proper way to plant onion sets? (Iowa State University Extension)
• Onion, Leek, Shallot and Garlic (Clemson Cooperative Extension)
• How to Grow Onions in Your Garden (Utah State University Extension)