Now that the days are starting to get longer, our thoughts turn to summer and growing plants. Whilst it is possible and indeed in some cases desirable to sow early, this is not the case for the majority of seeds unless we take additional steps.
Warmth
For any seed to grow, one of the triggers in warmer weather. So sowing in winter requires the use of bottom heat. A propagator (with or without thermostat) or a heat mat is ideal. Over a radiator is not a suitable environment unless its all you have got. Most radiators sit under windows (most of mine do) and the temperature above the radiator can be too much when it is on and nearer the glass far too cold. The 'goldilocks zone' can be as small as a few cm wide in the middle of that area, hence why we cannot recommend this.
Light
The increasing days will provide this for your seeds, especially those that are sown on the surface and need light to germinate. Avoid South facing windows at all costs, if this is all you have, set the trays and pots back a metre or so, this way you diffuse the light a bit and avoiding the thermal effects of full sun.
Another way with light, is artificial lights. Whether this is a standlight or a hanging light, it must be full spectrum or a specific growlight designed for the purpose. LED versions of these lights may sit just a few centimetres above the seeds, as their main output is light. Sodium or other metal halide lights should be sat well above the tray as most of their output is heat.
We use the LED types on pullies sat above the seed trays so we can adjust the height. Inside a mylar grow tent, little additional heat is required apart from a heat mat to warm the soil. Most of our out of season testing is done this way so that we can check the seeds first before the main growing season.
Our entire set up uses around 300 watts for 12 hours a day and 80 watts for the other 12 hours. With this we can have up to 24 full sized seed trays, although we rarely fill it if we intend to grow the seedlings on for our own use.
Growing On
Don't forget about the space you will need for growing on and where you intend to do this. The temperatures here can and indeed should be lower than those used for germination. The amount of light required will increase too, but never be tempted to place them where they get full sun. Even cacti will scorch and die if subjected to full sun as seedlings.
We sometimes use the grow tent to bring the seedlings on fast. This works well for the stronger growing perennial types, especially Agaves, Aloes and relatives. We also grow chillis (when we have space) under this set up, being able to keep 4 to 8 plants and bring them into fruit as early as June.
