Biennial

A biennial is a plant with a two-year life cycle. In its first year it puts all its energy into growing roots and leaves, builds up a store of food, then sits quietly through winter. In its second year it draws on that store to throw up a flower stalk, set seed and die. Compare this with an annual, which races through its whole life in a single season, and a perennial, which lives and crops for many years.

Why we usually harvest in year one

Here is the bit that catches beginners out: a lot of the vegetables we grow are biennials, but we eat them in their first year — long before they would naturally flower. We are after the leaves and roots they build in that first season, not the flowers and seed they would make in the second.

Carrots, parsnips, beetroot, leeks, onions, cabbages and many other brassicas are all biennials. So is parsley, the herb most likely to surprise you when it suddenly runs to seed in its second spring. We lift, cut or pull them in year one because that is when the part we want — a sweet root, a fat leek, a tight cabbage — is at its best. Leave them in the ground and they simply carry on with their natural plan.

How bolting fits in

In their second year, biennials flower on cue. The trigger is usually the cold of a UK winter followed by lengthening spring days — a process called vernalisation. The plant reads the seasons, decides winter is over, and switches from growing to flowering.

The trouble comes when a first-year plant gets the same signal early and flowers ahead of schedule. That premature rush to flower is bolting. A cold snap after sowing, a sudden hot dry spell, or sowing too early in a chilly spring can all fool a young plant into thinking it has already lived through a winter. Once it bolts, the energy goes into the flower stalk instead of the root or leaf, the texture turns woody or stringy, and the flavour often goes bitter. A bolted parsnip or beetroot is past its best for the kitchen.

This is why UK sowing dates matter so much for biennials. Sowing carrots and parsnips after the soil has warmed, rather than into cold March ground, and keeping plants evenly watered both reduce the risk of a false winter signal.

When you might want the second year

Letting a biennial run on isn't always a mistake. If you want to save your own seed, a second year is exactly what you need — leave a few good plants in the ground over winter, let them flower the following summer, and collect the seed once it ripens. Flowering leeks and parsnips also draw bees and hoverflies, so a couple left to bolt on purpose can earn their keep at the edge of a bed.