How long is my growing season and what does it decide?

Season length is why sweetcorn is easy in Kent and marginal in the Pennines, and why some years an outdoor tomato ripens and some years it does not. It is a number you can establish in five minutes and it silently governs half of what you can grow.

Step by step

1. Get both frost dates first.The frost-free season is simply the interval between them. If you have not established your own dates, this calculation is guesswork.
2. Count the days between them.That is your headline figure. Across the UK it ranges from well over two hundred days on the mild south-western coast to under a hundred and forty in cold upland areas — a difference that changes what is possible, not just what is easy.
3. Discount the shoulders.The first and last few weeks of the frost-free season are cold, dark and slow. A plant makes far less growth per day then than at midsummer. Treat the usable season as shorter than the arithmetic and you will be closer to reality.
4. Compare against days to maturity.Seed packets give days from sowing or from planting out — check which. If the figure is from planting out and your season is short, you also need to be able to raise the plant indoors beforehand.
5. Identify the crops that are marginal for you.In a short season the usual casualties are the long-season heat-lovers: sweetcorn, outdoor tomatoes, aubergines, peppers, melons, winter squash, main-crop maincrop onions from seed, and late-maturing brassicas that need a long autumn.
6. Extend the season deliberately at both ends.Starting under cover buys weeks at the front. Cloches, fleece, a cold frame or a greenhouse buy weeks at the back. Warming soil before sowing buys a fortnight. Choosing short-season varieties buys more than all of it and costs nothing.
7. Recalculate for your own microclimates.The sheltered warm strip against a sunny wall has a longer season than the open middle of the same garden. That is where marginal crops go.

Tips

• The frost-free season and the growing season are not identical. Many hardy plants grow whenever soil temperature allows, which starts before the last frost and continues after the first. The frost-free window is the constraint on tender things specifically.
• Variety choice is the cheapest season extension there is. Seed catalogues list early and short-season selections of most marginal crops, and they routinely mature weeks ahead of standard ones.
• If a crop finishes with weeks to spare every year, you have room for a second sowing or a follow-on crop and are leaving capacity unused.
• Keep the actual dates you got each year. Your own record of frost-free days is far more useful than a published average once you have three or four years of it.

Common mistakes

• Planning to the arithmetic rather than to the usable season — The days either side of the frost dates are cold and dim, and a plant grows a fraction of what it does in July. A crop that needs the full theoretical season will not finish.
• Choosing a long-season variety in a short-season garden and blaming the summer — In a short season the variety is the decision. The same care and the same weather with an early variety produces a crop where a late one produces green fruit in October.

Questions people ask