Begonias can be grown from seed, but if you are planning on doing this, you first need to make sure you can provide the correct growing conditions for the seeds to germinate. They can be sown any time between mid-January and mid-March.
Begonia seed is as fine as dust and, when opening the packet, care should be taken as it is difficult to see and easily discarded if caught in the seam of the packet. A handy tip is to open the packet over the top of a piece of white paper, so any seed that falls can be easily seen.
As the seed is so fine, inexperienced gardeners can find it difficult to handle and sow. A handy tip when sowing any fine or small seed is to open the packet and put a pinch of dry silver sand in with the seed. Shake this together, and then when it comes to sowing the seed you will be able to see the grains of sand fall, making even distribution of the seed far easier.
Before sowing any seeds I first fill some 7cm (3in) square pots with finely sieved soil-less multipurpose compost. I then lightly firm down the soil before watering it with a can fitted with a fine rose until the pots are fully charged with water. I leave them to stand for an hour to drain. As already mentioned, begonia seed is very fine and needs light to germinate, so the seed must be sown evenly over the surface of the compost and must not be covered with any more compost or vermiculite.
Once sown, the pots must be placed in a warm, heated propagator with a clear lid or placed in a polythene bag in an airing cupboard at a minimum temperature of 24-27C (75-80F) to achieve the best results. This temperature and a high level of humidity is essential to achieve good germination. If the temperature is any lower, germination will be erratic or very poor.
After 30 to 60 days germination should have started, all that will be seen will be some small green specks on the surface of the compost. Keep the pots in the enclosed propagator until the specks have grown into small plants that are large enough to handle. If growing in an airing cupboard, move the pots out of the cupboard to a light, but warm position indoors as soon as the green specks are seen. Keep the pot enclosed within the polythene bag until the seedlings are large enough to handle.
Once the seedlings are large enough, transplant them into cell trays or into individual pots and keep the transplanted seedlings in a warm place, preferably on a heated propagator base but without any lid.
Once the plants are larger, they can be moved to a cold frame in late April or early May. Gradually harden them off before planting them in their final positions once all the risk of frost has passed.