Seeds & growth
Vernalisation
The cold period some plants need before they will flower or bulb up; a mistimed cold spell can also trigger premature bolting.
Vernalisation is the cold spell that some plants must go through before they will switch from growing leaves to flowering, setting seed or forming a bulb. In simple terms, a stretch of low temperatures acts as a signal — it tells the plant that winter has passed and spring has arrived, so it's safe to make the next move. Without that chilling, certain crops simply stay in leaf-growing mode and never do what you planted them for.
Why it matters for garlic
Garlic is the classic example. A single clove will only split into a fat, multi-clove bulb if it has spent several weeks below roughly 10°C. That's why most UK gardeners plant garlic in autumn: the cloves sit in the cold ground through winter, get their chilling, and then bulb up properly the following summer. Plant it too late in spring and it often misses the cold it needs, producing one round, undivided "onion" instead of a proper bulb. Many onion sets and shallots respond to cold in a similar way.
Why it can backfire — bolting
The same trigger can work against you. Lots of biennials — plants that naturally grow in year one and flower in year two — read a cold snap as a full winter. If beetroot, onions, carrots or brassicas meet an unexpected cold spell early in the season, they can be tricked into flowering far too soon. This premature flowering is bolting, and it ruins the crop: energy rushes into a seed stalk rather than the root or leaf you wanted.
In a UK garden this is most likely when you sow tender crops too early and a late frost catches young plants, or when seedlings are raised in a cold, unheated greenhouse. It's the main reason seed packets often say "do not sow until the soil has warmed" — they're trying to keep your plants from being vernalised by accident.
The practical takeaway
Think of cold as a switch that some plants are wired to respond to. When you want bulbs or flowers — garlic, autumn-sown broad beans, sweet williams — give the plant its cold by planting at the right time, often before or during winter through overwintering. When you want roots or leaves instead, protect young plants from cold shocks so they don't bolt. Getting the timing right is really just a matter of working with each crop's natural sense of the seasons rather than against it.
In a UK garden
In the UK, gardeners rely on vernalisation when they plant garlic in autumn so it sits through a cold winter and bulbs up properly the following summer.
Example
You plant garlic cloves in October, they sit through the winter cold, and that chilling is what tells each clove to split into a full bulb by June.