Yard and Garden: Petunias

Petunias are easy to grow, bloom reliably all summer and are available in a wide range of colors, flower forms and growth habits. Iowa State University Extension and Outreach horticulture specialists answer questions about selecting and growing petunias. To have additional questions answered, contact the Hortline at 515-294-3108 or hortline@iastate.edu.

When should I sow petunia seeds indoors? 

Petunia seeds should be sown indoors 10 to 12 weeks before the last average frost date. In Iowa, late February or early March is an appropriate sowing date for petunias. 

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. 

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. 

What would be 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 well-rotted manure) into the soil. 

How do I choose among the different types of petunias? 

Petunia varieties can be divided into four main categories or types based on flower and growth characteristics. The four types are grandiflora, multiflora, milliflora and spreading or groundcover. Choose the type that best meets your needs.  
Grandifloras have large single or double flowers. Single grandifloras produce blooms that are up to five inches across. Some single varieties have ruffled or fringed petals. Double grandifloras produce double, fringed flowers. Generally, grandifloras (single and double) don’t flower as heavily as the other types. Also, the flowers don’t hold up as well during rainy weather. Plants may become unkempt and straggly by late summer. Popular grandiflora petunias include varieties in the Dreams, Ultra and Storm series.

Multiflora petunias produce smaller flowers than the grandifloras, but in greater quantity. Both single and double flowering varieties are available. Multifloras are generally more compact and resistant to wet weather than the grandifloras. Single multifloras are excellent in mass plantings in flower beds. Multiflora petunias include varieties in the Celebrity, Carpetand Hurrah series.

Milliflora petunias are compact, miniature plants that produce large numbers of 1- to 1½-inch-diameter flowers. The compact size and abundant flowers of the millifloras make them good choices for edging flower beds and containers. Milliflora petunias include varieties in the Fantasy and Picobella series.

Spreading or groundcover petunias are vigorous, low-growing plants that spread like groundcovers. By the end of summer, a single plant may cover an area three to four feet in diameter. Spreading petunias possess excellent heat and drought tolerance and require little maintenance. They are excellent choices for containers (hanging baskets, window boxes, etc.) and as annual groundcovers. Spreading petunias include those in the Wave, Shock Wave, Easy Wave and Avalanche series.

To learn more about horticulture through training and volunteer work, contact a county ISU Extension and Outreach office for information about the Iowa Master Gardener program, or visit www.mastergardener.iastate.edu/.