"Spreading" Petunias

Robert D. Berghage

Introduction
Vegetatively propagated petunias have become increasingly popular in recent years. These relatively new introductions to the commercial floriculture trade have been selected and promoted for container gardening and hanging basket production; uses which require a spreading or cascading habit. Although there are a number of plant species that have this habit, few have the flower power of the petunia, which can produce large numbers of flowers throughout the summer growing season. This consistent flower production and the variety of flower colors found in the petunia make it a natural for this use. Almost all petunia cultivars will spread to some extent as they grow, often because of branches being dragged down by their own weight. In some cases, this may make them suitable for use as cascading plants; since ultimately they do cascade as the branches fall.

Unfortunately, this type of cascading behavior often leads to plants with open centers as the branches are pulled down exposing the interior or the plant crown. The growing points or tips of the branches in these plants tend to turn up in response to gravity. The result is that the lateral branches look like a hook, as the tips continually turn away from gravity as they grow. The exact mechanism by which plants sense gravity is not yet known, however we do know that some plants lack the ability to respond or respond differently. Their shoots grow without gravitational orientation or respond differently by growing horizontally within the gravitational field.

For the last several years, we have tested field and outdoor container performance of vegetative petunias in The Landisville Penn State Horticulture variety trial. In growing some of the plants, we observed that there seemed to be several different growth habits expressed. Some plants seemed to be growing horizontally. A second group seemed to have predominantly upright growth, while many plants displayed an intermediate growth habit.

Materials and Methods
To examine these tendencies 30 cutting petunia cultivars were grown in 4-inch pots in a greenhouse at the Pennsylvania State University. Cuttings were obtained from a commercial propagator in March, planted in 4-inch standard pots in a commercial medium, and grown in a greenhouse with 60F day and 55F night heating and 70F ventilation setpoints. Plants were fertilized with 20-10-20 N:P:K all-purpose feed at a rate of 100ppm - 150ppm N through the irrigation as needed. Plants were overhead irrigated by hand. A drench of iron chelate was supplied in May. Twenty plants of each cultivar were grown. In May, data were collected on three randomly selected plants from the center of each cultivar block. Data were collected on height, width, and the angles relative to the plant crown and the soil line with which the longest shoot had developed. Two angles were taken to represent the growth of each shoot. The "stem angle" at the base of the shoot (point of attachment to the primary stem), and the "shoot tip angle" of the growing point of the shoot (Figure 1). Analysis of variance was used to examine differences among cultivars for any of the measured growth traits.

Figure 1. Angles measured for the longest and tallest lateral shoot.

Results and Discussion
Height, width, and shoot angles varied significantly among cultivars. Each of these measured characteristics can be related to spreading habit in different ways. Each provides a piece of the puzzle, but only in examining all of them together do you arrive at a more complete picture. Height to width ratio is a relatively easy and common way to evaluate spreading habit. A plant with a height to width ratio less than one "spreads" more than it grows upright. The smaller the height to with ratio the greater the spreading habit. Nearly all the cutting petunias evaluated showed some degree of spreading habit by this measure. Only 2, 'Cascadia Chikara' and 'Sunvale' had height to width ratios greater than 1. The height to width ratios of the other cultivars ranged from 0.14 to 0.86 (Figure 2).

Height to width ratio does not provide an indication of how the plants spread, only that they do. Many cultivars grow upright but as the branches gain weight they eventually become too heavy to support themselves and fall over. This results in "a spreading habit" as measured by height to width ratio, but it may lead to undesirable appearance characteristics, including broken stems, and open centers. This "falling down" type spreading habit would be indicated by a large difference between stem angle and shoot tip angle. Branches of these plants would always be trying to grow up, so the shoot tips would form an upright hook at the end of the stem. A more desirable spreading habit might be a cultivar whose branches grow laterally with a small (or negative) angle for both the stem and the shoot tip. These cultivars would continue to grow out, with both long and short branches developing more or less horizontally. In contrast, a large positive stem angle with an equal or greater shoot tip angle indicates an upright growth habit.

About half (17) of the cultivars examined had negative stem angles, meaning that the branches dropped below horizontal. Of these 7 had positive shoot tip angles suggesting a falling down type spreading habit. Only 5 cultivars had shoot tip angles that were nearly as small as the stem angle, and only 1 'Cascadia Cherie' had a shoot tip angle less than the stem angle (Figure 3).

Taken together this data suggests that all cultivars have some potential to spread, however most also have a tendency to grow upright. Perhaps the easiest way to visualize this is to compare the stem angle and the shoot tip angle for each cultivar (Table 3). The cultivars evaluated fall into 3 groups. Those with strong spreading tendencies have negative shoot tip and negative stem angles. The second group shows more tendency to upright growth but still spreads presumably as the branches are pulled down by gravity. These plants have negative stem angles and increasingly large positive shoot tip angles. The third group has positive stem and positive shoot tip angles suggesting a more upright growth habit. Although each cultivar can be placed into one of these groups it is also clear that the responses are somewhat elastic and continuous. Cultivar responses overlap and follow a scattered path from low to high, indicating the continuous nature of the response and the potential for continued improvement through cultivar selection and plant breeding.

Figure 2. Height to width ratio of petunia cultivars.

Figure 3. "Stem" and "Shoot tip" angles of petunia cultivars.

Figure 4. Base and tip angles of the longest branch on various petunia cultivars.

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Jim Sellmer

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Sales Tax and Snow Removal

James P. Sargent, SE Region Extension Business Agent

Information provided by Rob Berghage and Jim Sellmer
Penn State Department of Horticulture
Ornamental Horticulture Extension Specialist 103 Tyson Building
University Park, PA 16802
(814) 863-2571

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