Hardiness

Hardiness is simply a measure of how much cold a plant can take. A hardy plant shrugs off frost and keeps going through a UK winter outdoors. A tender one is killed or badly damaged by even a light frost, so it needs a greenhouse, a windowsill or warm summer weather to survive. Knowing a plant's hardiness tells you when it's safe to put it outside, and whether it will last the winter.

Gardeners and seed packets usually sort plants into three rough bands:

• Hardy — survives frost and cold outdoors all year. Examples: kale, leeks, parsnips, broad beans and garlic. Many actually taste sweeter after a frost.
• Half-hardy — copes with cool weather but not a hard frost. It can go outside once frosts are over, around mid-to-late May in most of the UK. Examples: courgettes, French beans and many bedding flowers.
• Tender — needs warmth and no frost at all. Examples: tomatoes, peppers, chillies, aubergines and basil. These are grown under cover or planted out only after the last frost.

The RHS uses a more detailed rating system (H1 to H7), where H1 plants need heated glass and H7 plants survive the very coldest UK winters. You'll sometimes see these codes on plant labels, but for everyday growing the hardy / half-hardy / tender split is all you really need.

Hardiness matters most in spring. Tender crops sown indoors must not go outside too early, however warm a sunny April day feels. A clear night can still bring frost well into May, and one cold night will undo weeks of growth. This is why we wait for the last frost date for your area before planting tomatoes and beans out, and why tender seedlings are gradually toughened up first.

Hardiness also shapes what you can grow over winter. Hardy crops like kale, leeks and winter cabbage stand in the ground through the cold months, giving you fresh food when little else is growing. Half-hardy and tender plants, by contrast, finish in autumn and either die back or need lifting and storing somewhere frost-free.

It's worth remembering that hardiness is not fixed. A plant in a sheltered, sunny, free-draining spot will take more cold than the same plant in an exposed, soggy corner, because cold wet roots do as much damage as the frost itself. A cloche or a layer of fleece can nudge a borderline plant through a cold snap. And a young seedling is always more vulnerable than an established plant, so even hardy crops appreciate a little shelter early on. When a seed packet or label gives a hardiness rating, treat it as a sensible guide rather than a hard rule.