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Missouri Environment & Garden


David Trinklein
University of Missouri
Plant Science & Technology
(573) 882-9631

Petunia: Better Than Ever

David Trinklein
University of Missouri
(573) 882-9631

Published: March 19, 2012

Few other flowers are as synonymous with gardening as is petunia. A long-time favorite, petunia had the reputation of being available in a plethora of colors, relatively easy to grow, but not very heat tolerant. As the floricultural world searched for new species to adorn our gardens (and increase bedding plant sales) improvement of petunia as an ornamental plant took second place to more "exotic" species. Recent work, however, has resulted in petunias the likes of which gardeners of yesteryear might not recognize because of improvements made in vigor, heat tolerance and overall garden performance.

Petunia is a member of the Solanaceae (or nightshade) family. This is a family that contains a few very sinister members such as deadly nightshade, angel's trumpet and tobacco. In contrast, potato, tomato, pepper, and egg plant are all very valuable food crops which also belong to the nightshade family. The common garden petunia carries the scientific name of Petunia x hybrida, indicating it is hybrid in origin. In fact, common garden petunia is a result of crosses between P. axillaris, P. inflata, and P. violaceae, all are wild species of petunia native to the more temperate regions of South America. The latter is significant in that it explains the reason why petunia, often classified as a semi-hardy annual, does not fare well in the heat and humidity of a typical Missouri summer. This especially is true of the "older" cultivars that typically would go through a "summer swoon" at which time they were trimmed back severely to prepare them for renewed growth and bloom in the cooler months of fall.

Traditionally, petunias are classified into one of two flowering types: grandiflora and multiflora. Grandiflora types are characterized as having large, heavily-ruffled, individually-ornate flowers; in certain cultivars, flowers can approach five to six inches in diameter. Multiflora types have smaller, less ornate flowers, but more of them. Grandiflora types have the reputation of being "more showy"; multiflora types are thought to be more weather-tolerant (heat and rain) with more abundant blooms. Along the way, plant breeders developed double cultivars of each of these two types as well as a myriad of different flower colors and color patterns. A "breakthrough" in petunia breeding came with the introduction of the 'Madness' series in the 1970's. This series had flowers that approached the grandiflora types in size and ornate appearance yet had the weather tolerance and bloom number associated with multiflora types. The term "floribunda" was coined to describe this series to set it apart from other petunias on the market. The success of the 'Madness' series was on factor which led to a renewed interest in petunia's use in warmer regions of the United States and renewed attention given to petunia by plant breeders.

Although grandiflora, multiflora and floribunda petunias still enjoy a great deal of popularity, the gardening world is awash with new petunias, many of which are vegetatively propagated. The 'Supertunia®' series was probably the first of the vegetatively-propagated petunias to hit the market and is part of the Proven Winners® collection of plants. They are very vigorous in growth habit and exhibit improved weather (heat) tolerance and abundance of blooms. If must be noted, however, vegetatively-propagated petunias usually are heavier feeders than common, seed-propagated types and must be fertilized accordingly. The 'Surfinia®' series of petunia is another Proven Winners® introduction which some consider to be an improvement over the Supertunias in growth habit and ease of culture. Other companies have followed suit and, today, names like 'Cascadia™', 'Sanguna™' and 'Suncatcher™'are commonplace in the bedding plant world. Most are either trademarked or plant patented by their developers.

In addition to the above, is the outstanding seed-propagated 'Wave®' series of petunia. Their spectacular garden performance makes them great choices for beds where a brilliant ground cover is desired. Unlike "regular" petunias, they continue to flower freely all summer without the need to trim them back. 'Tidal Wave®', 'Easy Wavy®', and 'Shock Wave®' are relatively new additions to the series; each has its own unique growth habit but all display superb garden performance. A final (relatively) new seed propagated series is the 'Morn' series which is a miniature (or "milliform") petunia. The series is characterized by compact plants with profusely-born small flowers displaying excellent weather tolerance.

In the garden, petunias prefer full sun and perform well in a wide range of soil types. Best growth probably occurs in a well-drained, porous soil of medium fertility. A pH of between 6.0 and 6.5 along with high levels of phosphorous and potassium is ideal. Soil porosity is important to facilitate the development of a vigorous root system and to help prevent root rots. To test for porosity, dig a hole in your flower bed about 10 to12 inches deep and fill it with water. The next day, fill the hole again and time to see how long it takes for the water to drain. If the water drains within 8 to 10 hours, the porosity of your soil should be adequate for good growth. If the soil takes longer to drain, the incorporation of organic matter is recommended to help establish greater porosity.

Before planting petunias, maintenance levels of fertilizer should be applied to the bed. About two pounds per 100 square feet of a complete fertilizer such as 5-10-5 should be applied to the area to be planted. Make sure the fertilizer is well-incorporated into the soil before planting. An additional top-dressing during the summer of nitrogen in the form of ammonium nitrate at the rate of about one pound per hundred square feet is desirable. Water in lightly after applying if rain is not forecast. This procedure might have to be repeated for the more vigorous and heavy-feeding vegetatively-propagated petunias. Plant vigor is a good barometer to measure fertilizer need.

Once established, petunias need little care during the growing season. Dead-heading is not essential but does help to increase the attractive nature of the plants as well as to give increased flower production in certain cultivars. During hot, dry weather, supplemental irrigation to provide about one and one-half inches of water per week is suggested. If plants become excessively "leggy" and stop blooming, cutting them back to a few inches from their base can help rejuvenate the plants. At this time, a top-dressing of fertilizer should be applied at the rates given above. Petunias are considered to be relatively pest free with root rot diseases and Botrytis being the major troublesome diseases. Both are more problematic during wet weather or from over-watering.

If you have not tried petunias in your garden lately, you might be in for a pleasant surprise. Thanks to the efforts of plant breeders many new and improved forms of petunia are available to the gardening public.

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REVISED: March 23, 2012