The short answer
The last frost is the hinge. Nothing tender goes outside until it has passed, which is roughly mid-May in much of Britain and later in the north and at altitude.
- February to March: sow indoors on a windowsill: tomatoes, chillies, aubergines.
- March to April: direct sow hardy things: broad beans, peas, early salad, carrots.
- After the last frost, usually mid-May: plant out anything tender: tomatoes, courgettes, beans, squash.
- June to July: successional sowing of salad, and late beans.
- August to September: overwintering onions and garlic, winter salad.
- October to November: bare root fruit bushes and trees.
What will ruin it
A single late frost kills tender plants outright. Harden plants off over a week or two, putting them out by day and in at night, and watch the forecast rather than the calendar.
Why it works
Britain's growing season is short and the last frost date varies by several weeks across the country. Planting a fortnight early gains nothing, because cold soil stalls growth anyway, and risks losing everything.
Tipking is general household information, not professional advice. Test anything on a hidden patch first. Full disclaimer.