Gardening glossary
Vernalisation
Vernalisation (US spelling: vernalization) is the biological clock that stops some plants from flowering until they have experienced winter. Without it, a winter-wheat plant sown in autumn would try to flower in the dark days of December and fail; an onion would never bulb up; a biennial like foxglove or parsley would never produce seed in its second year.
The temperature window. Research on winter cereals shows the response is most active between roughly 3 and 10 °C, with an effective optimum near 5 °C. Temperatures below about -1 °C halt vernalisation, and temperatures above roughly 15 °C are ineffective or can even partially undo it (a "warm winter" can leave winter wheat under-vernalised).
The duration. It is cumulative — what matters is total time spent in the effective window, not consecutive days. Most winter cereals need 6–8 weeks below about 9 °C. Strong-winter wheat cultivars want more than 4 weeks below 5 °C; weak winter types are satisfied with under 2 weeks. Garlic typically needs 4–8 weeks below 10 °C to form proper bulbs.
Why this matters in the home garden:
- **Garlic** planted in spring rarely splits into proper cloves because it has not been vernalised. Autumn planting fixes this — the cold winter soil does the work. - **Onions and shallots** sown too early can vernalise prematurely and "bolt" to seed in their first year instead of producing a useful bulb. - **Brassicas** held in cool conditions for too long (e.g. seedlings on a chilly windowsill) can vernalise and bolt as soon as warmth returns. Aim for steady temperatures above 12 °C when raising brassica transplants. - **Spring bulbs** need a cold dormancy to bloom. Forced tulips and hyacinths sold for indoor display have been artificially vernalised in cold storage at a commercial scale. - **Cherry, apple, and other temperate fruit trees** rely on related (but distinct) [chill hour](/glossary/chill-hours) accumulation to break bud dormancy in spring.
Mild winters in the UK can leave both crops and fruit trees under-vernalised. If autumn-planted garlic produces strange single-bulb "rounds" rather than separated cloves, insufficient vernalisation is the most likely culprit.