A useful, topical post for the season from Stephen Tyrer (Apologies it was ready to go onto the website during February and is only just appearing now).
I have been growing vegetables in allotments in Newcastle for 40 years. I keep a diary of when I sow seeds and when I plant them out so I have an idea of the best time to enable my plants to thrive.
It is best to start as early as possible. If you have a greenhouse or a warm patio at your home you can start seeds off at the beginning of February. Alternatively, you can use an electric propagator where you can set the temperature for what is best for your growing seedlings.
On the whole I find it best to sow seeds in rectangular containers that have 6 to 8 spaces. They can be grown in compost that you either buy yourself or use from your own compost heap. John Innes No. 1 compost can be used for potting young seedlings once they have germinated. I only repot seedlings if there is bad weather outside or if they are too crowded in their present situation.
Seeds can be sown as early as the first week in February even when there are frosts outside. Seeds that can be sold at this time include Brussel sprouts, cabbage, kohl rabi, kale, leeks, beetroot and Swiss chard but there are many others. If you are able to cover these with glass so much the better but be careful to remove the protective container if the sun gets too hot.
It is useful to add vermiculite to the compost in which you are growing your seeds. Vermiculite is a mineral which undergoes significant expansion when heated and it helps to aerate the soil while simultaneously retaining water and nutrients.
The seeds sown at this time should be ready to plant out in a cold frame outside in the last two weeks of March. I find that brassicas can even tolerate temperatures as cold as 0°C if adequately protected from the cold.
I do not normally directly sow other seeds until mid-March or later. I usually plant out onion sets in the second week in March and one week later I put in my second early potatoes. This is supposed to be too early but I rarely have problems with frost as I heap up soil over my potatoes on cold nights. Main crop potatoes should normally be planted in April.
It is important to look at your seedlings every second day and they may also need watering every 48 hours if the weather is quite warm.
I would suggest that you keep a diary so you are aware of your successes and failures with your judgement of the reasons for problems that have arisen.