Raising petunias
Petunias should be sowed in the beginning of spring at a temperature of 17-21C on a surface of the damp seed or multi-purpose compost. The some hybrid plants demand light for germination so the seed should not be covered, though it demands specific care to guarantee, that the surface does not dry out. Covering of pots or trays with clingfilm is the most simple way to hold a seed constantly damp.
The seed is best sowed as sparsely as possible so that the seedlings have free area to develop two true leaves before transplanting into trays, packs or small pots. Grow on 16C until rosettes of foliage have developed, then reduce the temperature to below 10C if possible as this will help developing basal branching; at higher temperature the plants are can become flabby. Later low temperatures are less significant.
The plants can grow in these containers until planting time in the spring. But be sure that your plants are well hardened well before planting.