Jeff L. Sibley, J. Raymond Kessler, Jr., Bridget K. Behe , Darby M. Quinn, and James S. Bannon

Variety trials are generally ranked by growers as being one of the most valuable projects at public institutions and research and demonstration stations. While herbaceous perennials continue to gain popularity, many field trials continue to lump annuals and perennials together for evaluation. The objective of this study was to evaluate the overall performance of full-sun perennials in a USDA Hardiness Zone 8, AHS Heat Zone 8 environment in south-central Alabama (26 miles east of Montgomery, Alabama). Results from the study will assist horticultural professionals and consumers in similar climates with selection of flowering perennials for landscape use. Selections included in these trials were based on the plants landscapers and homeowners might find available in local nurseries or retail garden centers.

METHODS
In the spring of 1996, 57 species/cultivars of herbaceous perennials were planted for evaluation at the E.V. Smith Research Center (EVSRC) located in Shorter, Alabama. Raised beds of Norfolk-Orangeburg loamy sand soil (fine, loamy, siliceous, thermic Typic Kandiudults) were tilled and fumigated with methyl bromide 2 weeks before planting. No other fungicides or insecticides were applied during the trial period. A commercially available slow-release fertilizer (18-6-12) was pre-plant incorporated into the beds as per soil test recommendations and then side-dressed again in the following spring (1997). No additional fertilizer was applied during the growing seasons. Six beds, each 6 feet × 80 feet, were prepared for planting on April 11, 1996. Three plants per entry were grown in three separate beds (a total of nine plants per entry) in full sun. Evaluations began July 3, 1996. Rainfall was supplemented by overhead sprinkler irrigation to provide an equivalent of 1 inch of water per week. Minimum deadheading of spent flowers, weeding by hand, and cutting back in either the fall or spring was the only other maintenance performed during the trial.

Plants were evaluated every 2 weeks from July 3, 1996 through October 1997. Plants were rated by the same individual using a 0-5 scale in three categories: flowering, foliage, and overall appearance. Plants were rated primarily on their floral displays, while size, shape, and freedom from insect or disease blemishes were also considered. The ratings were as follows: 0— absence of a desired characteristic; 1—a minimal amount of the characteristic, but not impressive, could be considered negative in the “foliage” and “overall” categories; 2—a small amount of the characteristic, not very impressive in the “foliage” and “overall” categories; 3—sufficient display to be attractive in the landscape; 4—above average display and quite beneficial in the landscape; and 5—superior display and extremely showy in the landscape. Ratings were made in whole number units.

CLIMATIC CONDITIONS
Average daily air temperatures in the winter (December through February) of 1996 and 1997 were 1.5 and 2.6^oF warmer than normal (daily air temperatures averaged over the years from 1961 through 1990), respectively (Table 1). Spring (March through May) of 1996 was cooler than normal by 0.6^F, while spring of 1997 was warmer than normal by 1.0^oF. Average summer (June through September) temperatures in 1996 and 1997 were only 0.2 and 0.4^oF warmer than normal, respectively. Fall (October through November) of 1996 and 1997 was warmer than normal by 0.7 and 2.1^oF, respectively. Table 1 presents data on average daily sunlight, average rainfall, and average chilling hours (< 45^oF) during 1996 and 1997.

RESULTS
BLOOM. The highest average bloom rating for any plant in 1996 was 4.9 for Salvia leucantha in October; this plant also performed well in September (4.5) and November (3.3) (Table 2). Salvia leucantha ratings for 1997 were 3.3 in September and 4.5 in October (Table 2). The second highest rating in 1996 was received by Lythrum virgatum ‘Mordens Pink’, also with a rating of 4.9 in July 1996; however, performance and bloom duration was not as long as Salvia leucantha. In 1996, Coreopsis rosea had a peak rating of 4.7 in July while Coreopsis verticillata ‘Moonbeam’ rated a 4.7 in July and kept on blooming with a rating of 4.1 in August. Aster × frikartii ‘Monch’ had a rating of 4.6 in July and a 4.0 in August 1996, also extending its effectiveness in the landscape. Verbena canadensis ‘Alba’ had peak rating in April 1997 of 4.4, while Coreopsis verticillata ‘Zagreb’ peaked in July 1996 with a rating of 4.2 and kept performing well into August with a 3.2. Another good performer was Boltonia asteroides ‘Pink Beauty’ which rated 3.6 in July and 4.2 in August 1996. Two of the longest blooming perennials evaluated were Verbena bonariensis with average ratings of 4.2, 4.0, and 3.4 for the months of July, August, and September 1996, respectively. In 1997, Verbena bonariensis started to show excellent color in May, June, and July with a ratings of 3.9, 3.7, and 3.2, respectively. Lythrum salicaria ‘Robert’ also had a long showing with a rating of 4.4 in July and a 3.9 in August 1996 and a 1997 rating of 3.4 in June and July and 3.9 in August. For a listing of selections with flower ratings of 3.0 or higher, see Table 2.

FOLIAGE. The perennial with the best foliage performance in this 2-year study was Verbena tenuisecta (Table 2). This ground cover type perennial received an average rating of 4.0 or above for every month evaluated in 1996, with the exception of August. From May through October of 1997, Verbena tenuisecta did not fall below a rating of 4.0. This indicates a very healthy, disease- and heat-resistant perennial. Veronica spicata ‘Red Fox’ had a shorter duration of high foliage ratings than Verbena tenuisecta but ranked second in foliage performance due to several high ratings during the 2 years. The third best foliage performer was Scabiosa columbaria ‘Butterfly Blue’. This exceptional perennial kept a rating of 4.0 or higher for every month evaluated in 1996 except September and a rating of 4.0 or higher from March through June 1997. A few other outstanding foliage performers were Helianthus angustifolius, Achillea × ‘Moonshine’, Baptisia alba ‘Pendula’, Coreopsis verticillata ‘Moonbeam’, and Physostegia virginiana ‘Vivid’. For a listing of selections with foliage ratings of 4.0 or higher, see Table 2.

OVERALL. The highest overall rating for the perennials in this study was Lythrum virgatum ‘Mordens Pink’ with an overall rating of 4.9 for July 1996 (Table 3). While the second highest overall rating was a tie between Coreopsis rosea and Coreopsis verticillata ‘Moonbeam’ at 4.8, ‘Moonbeam’ had another excellent rating of 4.2 for August 1996. Another selection with an overall rating of 4.0 or higher for 2 months was Verbena bonariensis, rating 4.2 and 4.0 for July and August 1996, respectively. The only selection to have a rating of 4.0 or higher for 2 months in 1996 and then 1 month in 1997, was Salvia leucantha. We believe any plant with an overall rating of 2.5 or higher could be considered a very good performing plant and would be a worthwhile addition to a full sun garden located in USDA Hardiness Zone 8 and AHS Heat Zone 8. However, to highlight only the very best overall performers, selections with an overall rating of 4.0 or higher are listed in Table 3.

Helianthus angustifolius and Salvia uliginosa responded well to pruning once or twice in the middle of the growing season to maintain a manageable height and prevent plants from falling over. Artemisia × ‘Powis Castle’ responded better to early spring pruning than fall or winter pruning, due to a tendency to die from winter injury if pruned too early. Dictamnus albus ‘Purpureus’, known for its slow development, gradually increased in size each year and should not be dismissed until given a few years to mature. Another slow starter, Baptisia alba ‘Pendula’ has graceful arching limbs with rounded leaflets on pinnately compound leaves that are attractive, with or without flowers.

Among the most impressive performers were Scabiosa columbaria ‘Butterfly Blue’ and ‘Pink Mist’. Once they started blooming, flowers were present on one or more plants even through the winter. Scabiosa had a tendency to re-seed in the immediate vicinity, not aggressively, but enough to provide additional plants for planting or to share with friends. Another favorite was Verbena tenuisecta with a moss-like carpet of foliage, which seems to almost always have a few flowers and sometimes was literally covered with dark purple blooms. Verbena tenuisecta ‘Alba’ also performed well, but seemed to produce fewer flowers. Verbena canadensis ‘Homestead Purple’ is another ground cover that produces purple flowers in mass but has a tendency to flower only along the edges of the spreading plant, leaving the center without blooms. Verbena bonariensis was also a favorite with a tall, open, airy habit and small purple flowers. Geranium sanguineum ‘Album’ produces delicate flowers on interesting palmate foliage and began to spread slowly, making a nice ground cover. Rudbeckia fulgida ‘Goldsturm’ was also a very good performer, with large, golden color ray flowers that attracted butterflies. Salvia leucantha was magnificent during its long bloom season (close to 3 months), with grey pubescent leaves providing a good backdrop for earlier flowering plants. Bumble bees were very attracted to the flowers.

See photos 1-12 for several of these high-ranking perennials.

Several plants performed well the first year, but did not over-winter. These could be treated as annuals and still be valuable additions to the landscape if replaced every year. One such selection was Salvia van houttie with dark maroon flowers that attracted hummingbirds. This plant was very showy, reaching approximately 3 feet tall and 2 feet wide and completely covered with blooms. Once the flowers fell, the dark maroon calyx remained, extending the “effective” period for several weeks. Boltonia asteroides var. latisquama ‘Nana’ also fell into this category of being treated as an annual, as did Boltonia asteroides ‘Pink Beauty’ and ‘Snow Bank’, along with Gallardia × grandiflora ‘Goblin’. However, these three did produce seedlings to replace the mother plant. In general, Coreopsis selections (‘Moonbeam’, ‘Zagreb’, and rosea) were disappointing, performing beautifully the first year, but not blooming well in the second year.

Artemisia ludoviciana ‘Silver King’ became so invasive, it had to be removed from the trial. Therefore, we would not recommend planting it in a mixed border due to its aggressiveness. While Lythrum species are often considered invasive, we had very few seedlings germinate and no colonizing through underground stolons. Caution should still be used when planting this perennial, and it should not be introduced to native wetland areas.
Plants in this study generally performed better the first year after planting than the second year. Several selections did not re-emerge the second year, though some natural re-seeding occurred. Still other selections never fully recovered from the winter months or succumbed to stress in the summer.

Plants that maintained an attractive foliage display while out of bloom and had highly rated bloom displays during the bloom season are worth incorporating into a full-sun perennial or mixed border in the southeastern United States. Gardeners and growers are encouraged to try plants in several locations to determine suitability for a particular area. However, plants with high ratings in this study are a good place to start, many of which appear to be tolerant of a full-sun environment, require little care, and still perform well.

Performance of perennials in the landscape may vary from year to year as climatic conditions affect performance. Comparison of results from several display sites will increase the reliability of findings because weather, soil type, exposure, cultural practices, and other variables can greatly affect plant performance. Horticulturists are urged to visit several trial gardens to gain a better understanding of variety performance throughout a region or market area.