Plant types
Tender vs hardy
Also known as: hardy, half-hardy
Tender plants are killed or damaged by frost (like tomatoes and courgettes); hardy plants withstand cold — the line that governs when to plant out.
Whether a plant is tender or hardy comes down to one thing: how it copes with cold, and especially with frost. It is the single most useful idea for a beginner to grasp, because it decides when you can sow, when you can plant out, and which crops you can leave in the ground over winter. Get it wrong and a single cold night can flatten weeks of work; get it right and you simply work with the seasons.
Tender plants are killed or badly damaged by frost, and many sulk even in cold-but-frost-free weather. These are the warmth-lovers: tomatoes, courgettes, pumpkins and other cucurbits, runner and French beans, sweetcorn, peppers, chillies, aubergines and basil. In the UK they cannot go outside until all risk of frost has passed, which is why they are usually started indoors on a windowsill or in a greenhouse and planted out later.
Hardy plants shrug off frost and cold. Broad beans, garlic, onion sets, peas, kale, leeks, parsnips and most winter brassicas all tolerate UK winters, and some — like garlic and overwintering broad beans — actually want a cold spell to crop well. Hardy crops can be sown or planted far earlier in the year, and many can stay in the ground to be picked through winter.
Half-hardy sits in between. These plants take a light frost or a cold snap but not a hard, prolonged freeze — courgettes hardened off in a mild spell, or early lettuce and some salad leaves. In practice you treat half-hardy crops like tender ones in a cold UK spring and give them protection if a frost threatens.
This is where timing comes in. The whole UK growing calendar pivots on the last frost — the average date after which frost is unlikely in your area. It can be late April on the mild south coast but mid-to-late May further north, inland, or at altitude, and a cold spring can push it later still. Tender crops are planted out only after that date; hardy ones can go out well before it. Seed packets follow the same logic, telling you to "sow indoors" or "plant out after the last frost" precisely because the plant is tender.
Even a hardy windowsill-raised seedling needs hardening off before it meets the open garden, and tender plants need it doubly. Keep some horticultural fleece handy in spring: it buys you a few degrees on a borderline night and lets you nudge tender crops out a little sooner. For more on planning around the seasons, see our guide to starting a vegetable garden.
In a UK garden
In the UK this line falls around your local last frost — often late April on the mild south coast but mid-to-late May inland, in the north, or up high — so tender crops wait until then to go out.
Example
Courgettes and runner beans are tender and only go outside once frost has passed, while broad beans and garlic are hardy enough to sit out all winter.