In pots or trays in a greenhouse at 16-18°C.
When the seedlings are large enough to be handles, prick off into trays and grow on. Gradually harden off and plant out from the end of May onwards.
FLOWER SEED SOWING TECHNIQUE
HALF HARDY ANNUAL
Use seed trays or plastic modules and a good seedling compost, preferably using a peat substitute. For small sowings standard pots can be used.
Firm the compost lightly and moisten before sowing. Very fine seeds, like Begonia (where there are more than 60,000 seeds to a gram) should be mixed with dry horticultural sand to avoid one big clump of seed, and to help see where the seed has been sown.
As a general rule, the depth of sowing depends on the size of the seed. Aim to cover the seed with compost or vermiculite to the same depth as the size of the seed. Very fine seed need not be covered at all.
Sow thinly and cover the tray with a sheet of glass with paper on top. Wipe the glass every day and inspect the tray for signs of germination. Sowings in pots can be enclosed in a polythene bag for the same effect.
As soon as seedlings break the surface, remove the glass and paper or polythene bag. If left under glass they will become leggy and weak. Keep moist with a fine spray.
When the first set of true leaves (not the seedling leaves) have formed they should be pricked off into trays, setting the seedlings about 4-5cm (1 1/2-2") apart. Always handle by the leaves and not the stem and use a fork or knife blade to ease them out of the compost. Keep in the shade for a couple of days after pricking off.
Gradually harden the plants by increasing ventilation and moving the trays into a cold frame and finally outside for 7 days before transplanting into the flowering site.