Yard and Garden: Your guide to growing petunias from spring to summer

Aaron J. Steil
Iowa State University Extension

Petunias are one of the most common annuals in garden centers each spring. Their colorful blooms grace containers, hanging baskets and garden beds all summer.

In this article, Iowa State University Extension and Outreach horticulturists answer questions about growing petunias in your landscape.

What types of petunias can I plant in my garden?

Pink petunias in a flower bed.

Petunia (Petunia × hybrida) varieties can be divided into four main categories or types based on flower and growth characteristics.

Grandifloras have large single or double flowers up to five inches across on mounding, trailing plants. Common grandiflora petunias include varieties in the Dreams, Ultra, Prism, Limbo, Daddy, Supercascade and Storm Series.

Overall, grandifloras have large flowers, but not as many flowers as the other types. The blooms don’t hold up as well during rainy weather or in hot conditions, either. Plants may become unkempt and straggly by late summer.

While Multiflora petunias produce smaller, single or double flowers (1-2 inches across), they are more abundant than grandifloras. Multifloras also have a mounding, trailing habit, but are generally more compact and resistant to wet weather than the grandifloras. They work well in mass plantings, flower beds and containers.

More modern cultivars of multiflora petunias are sometimes referred to as floribundas. Multiflora petunias include varieties in the Celebrity, Carpet, Mirage, Madness and Hurrah Series.

Milliflora petunias are compact, miniature plants that produce abundant 1- to 1½-inch-diameter flowers. They are good choices for containers and edging flower beds.

Milliflora petunias include varieties in the Fantasy and Picobella Series.

Spreading or groundcover petunias are vigorous, low-growing plants that spread to cover large areas, sometimes up to 3 or 4 feet in diameter. Spreading petunias possess excellent heat and drought tolerance and require little maintenance. They are great choices for containers and baskets and as annual groundcovers.

Some varieties in this group are referred to as hedgiflora types. Spreading petunias include those in the Wave, Shock Wave, Easy Wave, Tidal Wave, Ramblin’, Triology and Avalanche Series.

A few popular series of petunias don’t fit neatly into these four groups. They possess some of the best characteristics from all the groups. They tend to have large flowers with a mounding, trailing but compact habit, and excellent heat and rain tolerance. These include petunias in the Surfinia, Supertunia and Suntunia series.

Million bells (Calibrachoa) is a closely related species to petunia with abundant small petunia-like flowers and a compact trailing habit that is perfect for containers and hanging baskets. Popular million bells varieties include those in the Superbells, Cruze and Million Bells series.

What is a good planting site for petunias?

Petunias perform best in sunny locations. The planting site should receive at least six hours of direct sun per day.

They also require a moderately fertile, well-drained soil. Crown or root rots may be problems in wet, poorly drained sites. Poorly drained soils can often be improved by incorporating organic matter (compost, peat or composted manure) into the soil.

Many varieties of petunias can be planted in window boxes and hanging baskets.

Many cultivars of petunia are also well-suited for containers, window boxes and hanging baskets.

What should I look for when purchasing petunia plants?

When buying petunias, select compact, stocky plants. Tall, spindly petunias will need to be pinched to encourage the plants to spread and take longer to develop into attractive plants.

If buying plants from a greenhouse, harden the petunias outdoors for a few days before planting in the garden. Initially place the plants in a shady, protected location and then gradually expose them to longer periods of direct sun. Bring plants indoors if freezing temperatures are predicted.

When can petunias be planted outdoors?

Petunias should be planted in the garden after the danger of frost is past. It’s usually safe to begin planting petunias in late April in southern Iowa and mid-May in northern parts of the state.

Plant petunias about 12 inches apart. The spreading types should be planted 2 to 2½ feet apart. Pinch back grandifloras and multifloras to encourage branching. Pinching is especially helpful for tall, leggy plants. Millifloras and spreading petunias usually don’t require pinching.

How should I care for petunias during the growing season?

During dry weather, a deep watering once every seven to 10 days should be sufficient for petunias in beds and borders. Plants in containers will need to be watered more frequently. Check containers often and water when the soil surface becomes dry. Petunias growing in containers should be fertilized every two to four weeks with a dilute fertilizer solution.

To encourage additional blooms and improve plant appearance, remove the spent flowers on grandiflora petunias. Milliflora, spreading and some multiflora petunias are self-cleaning and don’t require deadheading.

Leggy petunias can be rejuvenated by cutting them back to about half their length in mid-summer. After pruning, fertilize and water the plants to promote new growth.