From sowing to flowering will take around 18-20 weeks, depending on the sowing date and other important factors such as light, temperature etc. For earliest flowers sow from mid to late December onwards to produce flowers in late April, early May. An early January sowing should produce flowers in mid-late May. An autumn sowing can also be made for overwintering.
Use a moist, free draining seed compost. Sow carefully in a half seed tray or small pot spacing the seeds out so that each has maximum space and cover with a thin layer of compost 1/6in (1mm). Water gently after sowing with a fine rose or mist spray and cover the seed container with a sheet of glass or cover with a polythene bag and seal with an elastic band. Turn the glass daily and remove once the seedlings have germinated. Ensure that the compost stays moist and not wet.
It is essential that a steady warm soil temperature is maintained 70-75F (21-24C) is best and at this temperature the seedlings can emerge in 3-21 days.
One important factor in successful germination of geraniums is constant moisture in the early stages. If the seed starts to germinate and the delicate young root comes in contact with dry compost it will probably not survive.
Once the plants have germinated the temperature can be reduced somewhat but the growing on temperature will influence the date of flowering.
Two moves should be made; first prick out into boxes as soon as the seedlings can be handled, which may be as early as 7-10 days after sowing; second 5-6 weeks after sowing, pot up into 3-4in (7.5cm) pots. In each case use a light, well drained potting compost and water in gently.
After pricking out, maintain an air temperature of 65F (18C) at night and 70F (21C) during the day. Keep at these levels until 1-2 weeks after potting (to encourage rooting) after which it should be gradually reduced until at approximately 6-8 weeks after potting the night temperature is 55-60F (13-15C). During the day, the temperature can be allowed to rise to 70-75F (21-24C). Lower growing temperatures are quite acceptable but the plants will take longer to develop. To get flowers in early July the temperature can be reduced to 65F (18C) after germination (or pricking out if applicable) and two months after sowing reduced again to 45F (7C). It is impossible to be entirely definite about this timing because varieties vary in their natural rate of maturity and light levels also play a significant part.
Always give the plants adequate space to give good air circulation and to ensure healthy growth. Ensure that there is a space between each plant and that leaves are not touching. (You should aim for three to four 4 inch pots per square foot or five to six 3 inch pots per square foot).
Geraniums need regular feeding whilst in growth, or small hard plants with yellowed leaves will result. Commence regular feeding with liquid fertiliser 3-4 weeks after transplanting. Likewise care should be taken with watering, they will suffer if kept over wet but too little water will slow down the growth and delay flowering.
Recent research at a government research station has revealed another successful approach. Now that heating equipment is more sophisticated and insulation and other heat saving techniques have improved so much, there is an argument for sowing seed in autumn and growing through the winter very cool.
Seed is sown in October, when ambient temperatures are still fairly high so fuel costs are reasonable, and then grown right through the winter and spring at 45F (7C). This system saves a little on fuel compared with January sowing but produces earlier flowering and better plants. Flowering time is more comparable with plants grown from January sowing but kept at higher temperatures.
Whichever method is chosen, regular feeding is necessary as is attention to disease control, especially on overwintered plants. Fumigation with a fungicide is preferable to spraying. Spring sown plants are best watered using a capillary system but autumn sown plants should be watered by hand to avoid over wet compost in the winter months when growth is slow. Of course, if you are overwintering young seed raised plants, you can utilise the warmth to keep fuchsias, regal pelargoniums and other tender perennials through the winter and so make the best of your insulation and fuel.