As Tony rightly states most seeds have about three or more years life perhaps more. Save for parsnip which you should always use fresh each year and my tip and this is from Sarah Raven is to grow them initially as with carrots and beetroot in plastic guttering and then just slip them into the trench in the garden.
This year in the Vendee have leased a huge plot and I thought it would not be possible to fill it. However looking at the stuff already in and under cover my eyes are bigger than my belly and I will run out of space. Hence envy and Tony's three plots.
I have forced true Jersey Royal seed pots under cover and perhaps within a month we will have the first picking.
If you have a lot of seeds ... try germinating a few in a propagator (or seeds tray covered in clingfilm to keep the warm in). Keep in the house and see if they germinate and grow into tiny seedlings. If they do, then you can safely sow them in prepared earth with no risk of wasting the space.
Also depends where the seeds have been kept. If in a cold and dry place, then you'll have a lot more chance of success than if they were found on a dusty windowsill. I keep all of mine in a tin box in the fridge.
More good suggestions...thanks. While we were in a Brico shop yesterday I noticed plants for sale, tomato, peppers and many others but surely it is a bit early to put plants ? I know last year I did not plant toms until a lot later.
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