Petunias (Petunia spp.) are one of the most popular garden flowers for both borders and containers. They are prolific bloomers; you can find them in just about every color but true blue; and they have wide, trumpet-shaped flowers and branching foliage that is hairy and somewhat sticky. Within the petunia genus, there is great variety and most are sold as hybrids: single and double blooms; ruffled and smooth petals; striped, veined, and solid colors; mounding and cascading growth habits; and even some with fragrance.
Petunias are fast-growing plants. When started as seeds, they’re germinated and ready to be planted outdoors—after the last frost date—in about 12 weeks, reaching full size by late spring.
How to grow petunias
Cultivation
Petunias prefer to be grown in full sun, although during hot, sunny summers they will tolerate light shade. They grow best in a fertile, moist but well-drained soil. Dig in plenty of organic matter – such as garden compost, well-rotted manure or other soil improver – especially in very well-drained sandy soils to hold moisture.
Petunia varieties
Bedding varieties are either grandifloras with larger flowers or multifloras with smaller flowers that hold up better in the rain.
Spreading or trailing petunias include Surfinia, Wave, Tumbelina, Supertunia and Cascadia series.
Sowing petunias
The annual or bedding petunias can be grown from seed sown indoors with warmth in spring in cell trays, seed trays or small pots at a temperature of 18-24°C (65-75°F).
When large enough to handle, prick out plants individually into cell trays or small pots and grow on at a temperature of 50-65°F (10-15°C) in good light.
Gradually acclimatise the plants to outdoor conditions for 7 to 10 days before planting outdoors when all risk of frost has passed.
If you don’t have the facilities to grow them from seed, young petunia seedlings are available from garden centres and mail order suppliers in late winter/early spring.
Planting petunias
Petunias are planted out in May/June after the danger of frost has passed.
Dig a good sized planting hole, big enough to easily accommodate the rootball. Add a layer of organic matter – such as compost or planting compost – to the base of the hole and fork it in.
Place the rootball in the planting hole and adjust the planting depth so that it is planted at the same depth as it was originally growing (except hardy fuchsias) and the top of the roots are level with the soil surface. Mix in more organic matter with the excavated soil and fill in the planting hole. Apply a granular general feed over the soil and water in well. Applying a 5-7.5cm (2-3in) deep mulch of well-rotted garden compost or similar over the soil will help maintain soil moisture and keep down weeds.If you have a warm greenhouse, you can plant up containers and baskets in spring and grow on the plants, ready to place outside in late May/early June. This way you’ll have flowers earlier in the summer.
How to care for petunias
Keep the soil moist by watering regularly during prolonged dry periods in summer. Plants in containers will need regular, possibly daily watering – the aim being to keep the compost evenly moist. But don’t overwater, as too much water will cause the plants to become leggy with few flowers.
Feed regularly throughout summer with a liquid plant food to ensure a continuous supply of flowers. A high potash liquid plant food will encourage more, better blooms over a long flowering period until the first autumn frosts.
Removing faded flowers and any developing seed pods will prolong the display. Straggly plants can be cut back quite hard and then fed with a liquid plant food to produce fresh new growth and a profusion of flowers.
In the autumn, once damaged by frosts, bedding petunias are best dug up and composted.
Common Problems With Petunias
Petunias are easy-going plants that bloom often, but they occasionally have issues you can keep under control.
Wilted Flowers or Leaves
There are a number of reasons for wilted petunia flowers or leaves, but most of the reasons come down to water: too much, or too little. Check the soil and if it’s not damp, water your petunias. If moist, ease up on your watering routine.
Leggy Stems
Petunias often develop leggy stems, but it’s easy enough to remedy: deadhead flowers regularly by pinching back. If this doesn’t help your petunia fill out, you can prune its stems back to 2 to 3 inches long, and as it regrows it will be less leggy.