Petunias are primarily plants for sunshine and to ensure that you get the best out of them sunshine really is essential. It's true that they will put up a reasonable show in half shade but give them plenty of sun and the difference is dramatic.
For flowering outside in summer, petunias are sown in the warmth in spring. To produce bushy plants to set out in late spring or early summer, sow in early spring. Choose a peat based seed sowing compost such as John Innes 'Seed and Cutting' and if it is one with no or very little drainage material you can add some extra sharp sand, perlite or vermiculite.
The temperature for germination should be between 18-24 Celsius and this can usually be provided in a heated propagator or, if this is not available, keep your germinating seed in a warm, bright room in the house, but out of direct sunlight. A room which becomes cool at night should be avoided.
It is important to sow thinly and not to cover the seed. Some varieties will need light to help with germination and even a thin covering of compost can severely disrupt this. The lack of a compost covering necessitates very careful monitoring of the moisture in the compost. If the surface of the compost does dry out the young seedlings will quickly die. Maintaining suitable moisture levels in the compost is best achieved by covering the seed pots with polythene or glass.
If sown thinly the seedlings can be left in the seed pot until two true leaves can be seen and the seedlings will suffer less of a check if moved at this stage. To ensure that seedlings have enough space in the early stages it may pay to use a seed tray when you might otherwise use a pot. Petunias can be prone to fungal diseases at this stage onwards so make sure that adequate ventilation is provided. To you may wish to periodically spray with a water soluble fungicide such as cheshunt compound or benlate.
After pricking out the temperature is important. Temperatures below 10 Celsius will discourage growth of the main central shoot and encourage the development of side shoots from low down on the plant. Unfortunately this also delays the appearance of the first flowers.
At temperatures above 15 Celsius basal branching is restricted, the main stem grows more quickly and flowering is hastened. Although garden centres like to sell plants in flower, this does not matter to the home gardener so by sowing in early spring and keeping the temperature after pricking out cool, well branched plants should be produced which will flower more effectively when planted out. The length of daylight also influences flowering time and basal branching but the control of day length using artificial lighting and blackouts is beyond the scope of the amateur.
When the rosettes of foliage cover the compost the trays can be moved from the greenhouse to frames and grown cool. As long as the plants are frost free then they are happy. Although they are not as hardy as their relatives the nicotianas, they are tougher than many people think and can be planted out as soon as the last frost has passed.
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