How to Grow Petunias

Choose the right type

Petunias fall into a few groups, and the group matters more than the color, especially where summers are humid.

• Grandiflora: the largest flowers, up to 5 inches across, often ruffled. Iowa State notes they flower less heavily than multifloras and hold up poorly in rain. Clemson says most grandifloras struggle in South Carolina’s hot, humid summers because the heavy blossoms rot. Clemson’s exceptions for the South are the weather-tolerant Storm, Ultra and Daddy series.
• Multiflora: smaller flowers in far greater numbers, more compact, and resistant to petal blight. Clemson recommends the Carpet and Primetime series and ‘Heavenly Lavender’.
• Floribunda: between the two in flower size. Clemson recommends the Celebrity, Madness and Double Madness series, which bounce back quickly after rain.
• Milliflora: tiny 1- to 1 1/2-inch flowers on compact plants that rarely need cutting back. Clemson recommends the Fantasy series for pots and baskets.
• Spreading: low plants that run 3 to 4 feet wide. ‘Purple Wave’ was the first, followed by the Wave, Tidal Wave and Opera Supreme series. Tidal Wave grows 16 to 22 inches tall and is sometimes called a hedge petunia.

For South Carolina, Clemson rates multifloras, millifloras and spreading types the best choices. White and lavender petunias are often the most fragrant.

Starting petunias from seed

Most gardeners buy plants, but seed opens up far more varieties. Iowa State and Cornell say to sow 10 to 12 weeks before the last frost. Clemson allows 8 to 10 weeks. In Iowa that means late February or early March.

The seed is dust-fine, at about 250,000 to 300,000 seeds per ounce, so pelleted seed is much easier to handle. Iowa State’s method:

1. Sow on the surface of a moist seed-starting mix and press the seed in. Do not cover it, because it needs light to germinate.
2. Cover the container with clear plastic and keep it at 75 to 80°F in bright light, but out of direct sun, which can overheat it. Seedlings appear in 7 to 10 days.
3. Remove the cover and grow the seedlings under lights 4 to 6 inches above the plants for 12 to 16 hours a day, at 60 to 65°F.
4. Move seedlings to cell packs when they have three true leaves. Let the mix dry slightly between waterings and feed with a dilute fertilizer every two weeks unless the mix already has slow-release fertilizer.
5. Harden the plants off outdoors for a few days before planting.

Planting out

Plant once the danger of frost has passed; the dates for your zone above show when that is. When buying transplants, choose short, compact plants. Clemson notes that young plants not yet in bloom settle in faster than leggy ones in full flower.

Petunias need full sun. In shade they turn spindly and flower poorly. They do best in light, well-drained soil of moderate fertility with a pH of 6.0 to 7.0. Illinois Extension notes they bloom poorly in very heavy soils, so work in compost or build raised beds where drainage is poor.

Space standard types about 12 inches apart, or 12 to 18 inches for a solid groundcover. Iowa State gives spreading types 2 to 2 1/2 feet. At planting, pinch off the top inch of each stem to encourage branching. Grandifloras and multifloras gain the most from this, while millifloras and spreading types do not need it.

Watering and feeding

Once established, petunias in beds need 1 to 2 inches of water every 7 to 10 days. Clemson and UF/IFAS both warn against frequent light sprinklings, which keep roots shallow. Water the soil, not the plants. Wet flowers and leaves invite petal blight.

Petunias bloom heavily and need steady fertilizer to keep going. UF/IFAS recommends a balanced fertilizer monthly or a controlled-release product worked in at planting. Cornell feeds Wave petunias every two weeks until midsummer. Containers and hanging baskets need more frequent watering and regular liquid feeding than beds, because the small volume of mix dries out and runs out of nutrients quickly.

Midsummer cutback

By July many petunias stretch out and flower only at the tips. Cut the stems back to about half their length, leaving some leaves, then fertilize and water well. The University of Georgia notes that petunias can even be cut to within a few inches of the ground in midsummer and look full again within a few weeks. Spreading and milliflora types rarely need this.

Petunias as a winter flower in the South

In Florida, along the Gulf Coast, in south and central Texas and in the low desert, petunias are a cool-season flower because summer heat, humidity and disease finish them. UF/IFAS says October and November are the best months to plant in Florida. Gardeners in north Florida may need to protect plants from frost. In San Antonio, Texas A&M AgriLife reports that fall plantings bloom until late December or a hard freeze, and Laura Bush, Wave and Carpet often survive to bloom again in March and April. University of Arizona’s Phoenix-area chart allows planting from mid-September into early March.

Winter brings a bonus. UF/IFAS Charlotte County notes that pest and disease pressure is much lower in the cool, drier months.

Common problems

• Botrytis and petal blight: flowers and leaves rot in rainy, humid weather. Keep foliage dry, space plants for air flow, and choose resistant types such as multifloras.
• Aphids: they cluster on new growth. Wash them off or treat them early.
• Slugs: they chew leaves and flowers. Use baits.
• Caterpillars: UF/IFAS lists them as an occasional pest in Florida.
• Viruses: these occasionally cause mottling and stunting. Remove affected plants.