A cole crop set out too early can bolt or "button": a long spell of cold on a young plant vernalizes it, and instead of a full head it runs to a premature flower stalk or forms a small, misshapen head.
Promising · suboptimal if ignored → advise
How it works
Cabbage, broccoli, cauliflower and their kin are biennials, so cold is their signal that a winter has passed and it is time to flower. A young plant that accumulates enough cold - a stretch of weather in the 40s F - is vernalized and switches from building leaves and a head to running up a flower stalk and setting seed. Set out before the weather has settled, a small transplant crosses that cold threshold before it has grown the leafy bulk a head needs, so it buttons (a tiny premature head) or bolts (runs to seed) rather than heading properly.
What to do about it
Aim for cool-but-moderate weather - roughly the 50s and 60s F - through the plant's first weeks rather than rushing an early set-out; a stout transplant with several true leaves shrugs off a cold snap that would vernalize a tiny one, and a row cover holds the cold off if a spell threatens. This is the opposite failure from over-tender growth: harden the plant, but do not set it out into a cold that will check it.
How much it matters: No threshold or figure is invented - the sources give a range (settled weather in the 50s-60s F for the first weeks), not a single number. The risk is highest for a small transplant set out into a cold spell and lowest for a well-grown one set out into settled weather, so the rule advises rather than warns.
The evidence
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
UMN Extension, 'Growing cool-season crops - non-pest issues', extension.umn.edu/planting-and-growing-guides/non-pest-issues-cool-season-crops (read 2026-08-21): 'Bolting refers to premature flowering. It occurs at any point after a plant has shifted into its reproductive stage'; 'Since bolting is partially triggered by the accumulation of cold, these strategies can reduce exposure to cold temperatures for young plants'; 'the ideal situation for cool-season vegetables is cool but moderate temperatures (50s and 60s) for the first month of growth'; 'when spring weather is quite cool, plants can experience vernalization early, and the result is buttoning up, or the formation of small, often misshapen heads.'
UMD Extension, 'Flower Stalks Form or Bolting of Vegetables', extension.umd.edu/resource/flower-stalks-form-or-bolting-vegetables (read 2026-08-21): 'Biennial vegetable crops complete their life cycle in 2 years. But many will produce flower stalks in response to cool or hot temperatures and long days'; 'Onion-biennial seed stalks form if weather is cool early in its growth cycle.'
Where this rule stops
Scoped to the cole crops (brassica_oleracea, brassica_rapa), where the buttoning/bolting-from-early-cold outcome is documented. The same biennial vernalization drives early bolting in beet, carrot and celery, and in onion - but onion is already handled by another rule here (latitude and photoperiod), so it is left out here. This is the COLD counterpart to the corpus's existing heat/daylength bolt model (the species `bolt_risk` field and another rule here): same symptom, opposite trigger. It gates nothing - the app has no forward-weather feed - and surfaces on the cole-crop species pages.
Plants this fires on
Change history
Every change to this rule is one commit. This is what the project's version history records — the date and the reason — not a summary of it.
- verified: maintainer read the sources 2026-08-21 (invariant 4)
- cole crops bolt/button if set out too early (Promising advise, unverified)
Cite this page
Milpa Gardens. "A cole crop set out too early can bolt or "button": a long spell of cold on a young plant vernalizes it, and instead of a full head it runs to a premature flower stalk or forms a small, misshapen head." Evidence grade: Promising. https://milpa.garden/rules/cole-crop-set-out-too-early-can/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.