COLUMBIA, Mo. –Petunias and calibrachoas are colorful garden favorites, but they have a reputation for not tolerating heat very well. However, recent years have seen the development of new varieties that are better equipped to withstand summer in the Show-Me State.

Kerry Meyer, a graduate of the University of Missouri’s horticulture program and representative of the Proven Winners plant company, introduced new varieties of petunias and calibrachoas at the recent 17th annual Greenhouse Growers’ School at the MU Bradford Research Center.

Calibrachoas (million bells) look like small petunias, but they aren’t sticky like most petunias, and they perk up after rains and stay compact even when stressed, Meyer said.

“Calibrachoas come in every color of the rainbow except for true blue,” she said.

One of the new calibrachoa is the heat- and drought-resistant Superbells Lemon Slice. Just 7-10 inches tall, it has a unique pinwheel pattern of yellow and white bicolor flowers and trailing branches that are perfect as a “spiller” cascading from hanging baskets or containers, Meyer said.

Calibrachoas do best in containers because they don’t like to get their feet wet, a cause of root rot. They don’t require deadheading and are a magnet for hummingbirds.

The petunia is easy to grow and relatively heat-tolerant, says David Trinklein, MU Extension horticulture specialist and Meyer’s former professor. Like calibrachoas, petunias are members of the Solanaceae (nightshade) family, which also includes such diverse plants as tomatoes, tobacco, peppers and potatoes.

Petunias are classified into one of two flowering types: grandiflora and multiflora. Grandiflora petunias have large, heavily ruffled, individually ornate flowers growing up to 6 inches in diameter. Multiflora types have smaller, less ornate flowers, but more of them. Grandiflora types have a reputation of being showier, Trinklein said, but the multiflora are more tolerant of heat and rain and have more abundant blooms.

Although grandiflora and multiflora petunias still enjoy a great deal of popularity, the gardening world is awash with new petunias, many of which are vegetatively propagated.

Vegetatively propagated petunias usually are heavier feeders than common seed-propagated types and must be fertilized accordingly, Trinklein said.

A recent favorite is the seed-propagated “Wave” series of petunia. Its spectacular garden performance makes it a great choice for beds where a brilliant ground cover is desired, he said. Unlike older petunias cultivars, they continue to flower freely all summer without the need to trim them back.

In the garden, petunias prefer full sun and perform well in a wide range of soil types. Best growth occurs in a well-drained, porous soil of medium fertility. A pH of 6.0-6.5 along with high levels of phosphorus and potassium is ideal. Soil porosity is important for the development of a vigorous root system and to help prevent root rot.

Before planting petunias, apply maintenance levels of fertilizer to the bed. Use 1-2 pounds per 100 square feet of a complete fertilizer such as 5-10-5, depending upon initial soil fertility. The fertilizer should be well-incorporated into the soil before planting.

Later in the growing season, an additional top-dressing of ammonium nitrate at the rate of about 1 pound per 100 square feet is desirable. You might have to repeat this procedure for the more vigorous and heavy-feeding vegetatively propagated petunias. Plant vigor is a good indicator of fertilizer need.

Once established, petunias need little care. Deadheading is not essential but does help increase flower production. If plants become “leggy” and stop blooming, cut them back to a few inches from their base to rejuvenate. At this time, apply a top-dressing of fertilizer at the rates given above.

Petunias are relatively pest-free, with root rots and Botrytis being the major troublesome diseases. Both are more problematic during wet weather or from overwatering.

For more information on petunias and other bedding plants, go to extension.missouri.edu/topics/flowers-and-houseplants.