Grow your own stunning petunias from seed this year
Image: Petunia ‘Mirage White’ F1 Hybrid from Thompson & Morgan

There’s a real sense of satisfaction in growing your own show-stopping display of petunias from seed. And the good news is that sowing petunia seeds isn’t difficult at all. Follow the advice from T&M’s petunia expert, Kris Collins, and produce a bumper supply of strong and healthy seedlings. Here’s our quick and easy guide to germinating petunia seeds.

Are petunias difficult to grow from seed?

Petunia x hybrida ‘Sparklers’ is the first star shaped petunia available from seed
Image: Petunia x hybrida ‘Sparklers’ from Thompson & Morgan

Somewhere along the line, petunias have earned the incorrect reputation of being difficult to grow from seed, but they’re not. This misinformation is likely to have come about by incorrect sowing techniques or poor quality compost, rather than seed performance.

All seeds ‘want’ to grow, however every plant requires different conditions for germination to occur – usually temperature and moisture related. Petunias are no more difficult to grow from seed than any other plant.

When’s the best time to sow petunia seeds?

Make sure you sow seeds at the right time to avoid frost damage
Image: Petunia x hybrida ‘Super Cascade Improved’ F1 Hybrid from Thompson & Morgan

If you want early summer colour and good-sized, bushy petunia plants that are ready to plant out after the last frosts in May/June, then you should 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 from March.

If you run out of time, or your seeds don’t germinate for any reason, you can always order some petunia plug plants which are another cost-effective way to fill your baskets and containers.

How to sow petunia seeds

Plenty of petunia varieties can be successfully grown from seed
Image: Petunia grandiflora ‘Double Cascade Burgundy Plum Vein’ F1 Hybrid from Thompson & Morgan

It’s important to sow your seed onto the right compost. Our new vegan-friendly reduced peat incredicompost® is the best medium to use for sowing seeds and raising young plants. The temperature for germination should be between 18-24C (64-75F), which is easily provided in a heated propagator. If you don’t have one, seal your seed trays in a clear polythene bag and move them to a warm room of the house. But don’t choose somewhere that gets cool at night, like a conservatory. Aim for a steady temperature.

Sow your seed thinly over the top of the compost. It’s important not to cover the seed with any compost at all, as this can reduce germination. Carefully monitor the moisture levels, as the exposed seeds can dry out. Cover the seed pots with plastic and a sheet of newspaper which will reflect strong light and prevent moisture evaporating.

You can transplant your seedlings once they’ve produced two true leaves. Use a good peat-free potting-on compost mix, like our Incredipeatfree, when you transfer your seedlings into modular trays.

How can I make my petunia plants bushier?

Mix different petunia varieties together in their final position for a great mix of colour
Image: Petunia ‘Surfinia’ collection from Thompson & Morgan

After potting on, correctly controlling the temperature is the main way to regulate the size and shape of your mature plants. 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 though, 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 speeds up.

For fuller, bushier plants that eventually produce the most flowers, sow your seeds in early spring and keep the temperature cool after pricking out. This will encourage well-branched plants that flower more profusely when planted out in the garden.

When can I plant out my petunia plants?

Make your petunias centre stage to show off their fantastic summer colour
Image: Petunia hybrida ‘Purple Tower’ F1 hybrid from Thompson & Morgan

When the rosettes of foliage cover the compost, the trays can be moved from the greenhouse to outdoor cold frames and grown on. As long as the plants are kept frost-free, they’re happy. Use a special petunia fertiliser when you place them in their final position; this will provide food for the plant throughout the season. Although they’re not as hardy as their relatives the nicotianas (tobacco plants), petunias are tougher than many people think. They can be planted out as soon as the last frost has passed.

We hope you have success growing petunias from seed and that they provide a glorious display throughout the summer months. Whether you plant up beds, containers, or hanging baskets – keep us in the loop through our social channels! If you’re looking for more help and advice with growing petunias, visit our petunias hub page for a wealth of online resources.

Kris Collins

Kris Collins works as Thompson & Morgan’s quality control manager, making sure customers new and old are kept up to date on the latest plant developments and company news via a wide range of media sources. He trained in London’s Royal Parks and has spent more than a decade writing for UK gardening publications before joining the team at Thompson & Morgan.