It would depend totally on what you are sowing....for example, some onions, leeks and parsnips need getting in early, even december. Ive already sown my tomatoes, peppers and chillis and the seeds have germinated. Petunias, begonias and geraniums should be sown in feb, a majority of bedding plants are sown in march, beans, peas, sweetcorn, courgettes, marigolds, stocks are always april sown

Hi, agree with the above, but bear in mind, that if you're growing half hardy annuals, you will have to keep them in a heated greenhouse once they're taken from the propagator, so the earlier you sow them, the longer you will have to heat the greenhouse, which can work out quite expensive, Derek.

it really does depend on what you are growing. I don't start my tomatoes/peppers etc until March. I tend to use it at the time it says on the packet at the temp it states on the seed packet. I find starting plants too early means I cant keep them 'warm' later when they have to come out of the propagator.

Thankyou everyone,it is more the annuals that I was thinking of starting off.

In that case, March is the main month although some smaller, slow growing species such as Lobelia or begonia are February whilst some faster species such as marigolds and socks are april.

Check the back of the seed packet.

Also, later sowings might not require a heat mat....i germinate marigolds just in the normal greenhouse bench

I agree with march too and with the proviso that it depends on the weather!
like Badfish I find marigolds are better germinating in ambient temperature, stockier and stronger than being forced, too early for any plants and a cold snap after they come out of the propagator can be a problem.