Somewhere along the line, petunias have gained a wrongful reputation of being difficult to grow from seed. This has more than likely come about by poor sowing techniques or bad compost rather than poor seed performance. Every seed ‘wants’ to grow, some just need a few more specific requirements. It’s wrong to assume that all seeds can be sown the same way.
If you want the earliest summer colour and nice sized, bushy petunia plants to place in the garden after the last frosts in May/June, it’s a good idea to sow petunia seeds 12 weeks ahead of your expected last frost. The first week of June is a safe bet for planting in most of the UK, therefore petunia seeds are best sown in March.
Peat-based composts are still the best option for sowing petunia seeds. Our new incredicompost®
has recently been verified as the best compost for sowing seeds and raising young plants.
The temperature for germination should be between 18-24C (64-75F) at seed level and this can usually be provided in a heated propagator, if this is not available, seal sowing trays in a clear polythene bag in a warm room of the house. A room which becomes cool at night should not be chosen.
It is important to sow thinly and not to cover the seed, even a thin covering of compost can severely reduce germination. The lack of a compost covering necessitates very careful monitoring of the moisture in the compost, for if the surface of the compost dries out the young seedlings will quickly die. This is best achieved by covering the seed pots with polythene or glass and a sheet of newspaper to reflect strong light, so the surface of the compost does not heat up too much, but some light still penetrates.
Transplant seedlings once they have produced two true leaves. After potting on the temperature is important. Temperatures below 10C (50F) 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 15C (59F) basal branching is restricted, the main stem grows more quickly and flowering is hastened. By sowing in early spring and keeping the temperature cool after pricking out, well branched plants should be produced which will flower more effectively when planted out in the garden.
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 they are happy. Although they are not as hardy as their relatives the nicotianas, they are tougher than many people think. They can be planted out as soon as the last frost has passed.