These maps can give only the roughest guide to how long you can expect your vegetable growing season to be, and so what vegetables you can grow, when.   Tender plants can only be sown or planted out after the risk of frost has gone - things like French beans, sweetcorn, the squashes, and tomatoes.   Several factors can affect these dates, particularly

• generally unseasonable weather (and God knows we've had enough of that in the last few years!)
• and the immediate micro-climate for your vegetable beds
  • are they very sheltered?
  • are they very exposed to wind (late wind-chill can be at least as damaging as frost)?
  • are they in a frost pocket, a low lying area into which cold air sinks; or behind a wall which traps sinking cold air?

The maps also help me "translate" seed-packet instructions and gardening books.   Apart from the northerner's perennial grouse, that many are written for the south-east, there's a minor complication with the way we talk of seasons; the "strict usage" followed by the books and seedsmen are just a little different.   I found these dates in a wonderful horticultural book written in the 1930s.

• Autumn starts on 23rd September and lasts 89 days 17¾ hours.
• Winter starts on 21st December and lasts 89 days 1 hour.
• Spring starts on 21st March and lasts 92 days 21 hours.
• Summer starts on 21st June and lasts 93 days 14 hours.

Of course, if you want the real rough and ready, try the "bare-bum test" - the soil hasn't warmed up enough to sow seed, until you can sit on the bare earth with your naked buttocks.   I don't remember where I heard this, but it sounds like a Gardeners Question Time gem!   No I haven't tried it.