Using seed is an easy and cheap way to grow your own flowers and veg. If you are growing vegetables from seed, you can choose varieties you like and get a head start on the growing season.
To germinate and grow well, seeds need a controlled environment with as much light as possible. Light is as important as warmth and being in a frost free environment, which is why sowing early in the year is more difficult when light levels are low. If you sow in February/March, you will need somewhere frost free and warm for the seeds to grow on. An unheated greenhouse is likely to be too cold for the early seeds started in February.
It is a balance; sow too early and the seedlings may struggle and be leggy, so too late with some crops such as sweetcorn and tomatoes may just crop as Autumn arrives, especially if we have a bad summer or your garden is in an exposed area.
Video advise on how to sow and germinate seeds.