Volume 47 Number 1 Spring 2000

Planning to Plant Perennials?
 Flowering Perennials Evaluated for Alabama Landscapes

Perennial border containing (left to right)
Mexican bush sage, thread-leaf coreopsis, Frikart’s aster, and purple loosestrife.

 J. Raymond Kessler, Jr., Jeff Sibley, and James Bannon

Perennials are like old friends; they return year after year to fill a garden with color and fragrance. This may be one reason that flowering perennials continue to gain popularity in Alabama. But when faced with the dazzling array available, many homeowners feel overwhelmed. To help consumers and plant retailers know more about the best perennials to select, AAES researchers evaluated the performance of more than 50 flowering perennials.

During the late summer of 1996 and 1997, AAES researchers conducted a trial of 57 perennial cultivars at the E.V. Smith Research Center in Shorter. Selections included in these trials were based on the plants landscapers and homeowners commonly find available in local nurseries or retail garden centers.

Raised flower beds were tilled and treated with methyl bromide before planting. They were amended with a slow-release fertilizer (18-6-12) before planting then again in the following spring. No additional fertilizer and no fungicides or insecticides were applied during the season.

All bedding plants were grown in full sun. Rainfall was supplemented using overhead sprinkler irrigation to provide an equivalent of one inch of water per week. The only other maintenance performed on any of the plants was “deadheading” of spent flowers, hand weeding, and cutting back in either the fall or spring.

Plants from each entry were evaluated every two weeks from July through October during 1996 and 1997 for flowering, foliage, and overall appearance. Plants were rated primarily on their floral displays, while size, shape, and freedom from insect or disease blemishes also were considered. A 0 to 5 rating scale was used with a rating of 0 indicating undesirable or absence of desired characteristics and 5 indicating superior floral display and extremely showy foliage. Perennials were rated on both flowering and foliage.

The highest average bloom rating for any plant was 4.9 in October 1996 for white sage or Mexican bush sage (Salvia leucantha). This sage was magnificent during its long bloom season (close to three months), with grey pubescent leaves providing a good backdrop for earlier flowering plants. Bumble bees were very attracted to the flowers, which started as lavender buds that opened to white tubular flowers.

The perennial with the best foliage performance in this two-year study was moss verbena (Verbena tenuisecta Purple), a ground cover type perennial with an average rating of 4.0 or above for every month evaluated, with the exception of August. A species of veronica, spiked speedwell (Veronica spicata Red Fox), ranked second in foliage performance. It had several high ratings during the two years, but the high foliage ratings were intermittent. Spiked speedwell has glossy, pointed leaves on many erect stems that grow to a height of about 15 inches. The third best foliage performer was scabious (Scabiosa columbaria Butterfly Blue) with a rating of 4.0 or higher for every month except September. This perennial’s foliage forms a low mound of decorative leaves. A few other outstanding foliage performers were swamp sunflower (Helianthus angustifolius), moonshine yarrow (Achillea × Moonshine), white indigo (Baptisia alba Pendula), thread-leaf coreopsis (Coreopsis verticillata Moonbeam), and obedient plant (Physostegia virginiana Vivid).

Purple loosestrife (Lythrum virgatum Mordens Pink) received the highest overall rating in this study, 4.9, for July (see table). It is an upright perennial with a dense head of wispy, pink- lavender flowers.

Two coreopsis species tied for the second highest overall rating at 4.8: pink coreopsis (Coreopsis rosea) and thread-leaf coreopsis (Coreopsis verticillata Moonbeam). Species of coreopsis produce daisy-like flowers that range in color from lemon-yellow to yellow-orange.

Other selections with an overall rating of 4.0 or higher for several months each year include South American verbena (Verbena bonariensis) and white sage or Mexican bush sage (Salvia leucantha). To highlight the very best overall performers, selections with an overall rating of 4.0 or higher are listed in the table.

Average Overall Rating for Best Performing Perennials in the 1996-97 Trial Garden1
Common Name (Variety) Date Overall
Moonshine yarrow (Achillea × Moonshine) 5/97 4.3
Wormwood (Artemisia × Powis Castle) 7/96 4.0
Frikart’s aster (Aster × frikarkii Monch) 7/96 4.2
Boltonia (Boltonia asteroides Pink Beauty) 7/96 4.1
Boltonia (Boltonia asteroides Snowbank) 8/96 4.1
Pink coreopsis (Coreopsis rosea) 7/96 4.8
Thread-leaf coreopsis (Coreopsis verticillata Moonbeam) 7/96 4.8
  8/96 4.2
Thread-leaf coreopsis (Coreopsis verticillata Zagreb) 7/96 4.4
Gaura (Gaura lindheimeri) 5/97 4.3
Purple loosestrife (Lythrum virgatum Mordens Pink) 7/96 4.9
Purple lythrum (Lythrum salicaria Robert) 7/96 4.5
Black-eyed Susan, Orange coneflower (Rudbeckia fulgida Goldstrum) 8/96 4.0
White sage, Mexican bush sage (Salvia leucantha) 9/96 4.4
  10/96 4.7
  10/97 4.5
Scabious (Scabiosa columbaria Butterfly Blue) 4/97 4.2
South American verbena (Verbena bonariensis) 7/96 4.2
  8/96 4.0
Clump verbena (Verbena canadensis Homestead Purple) 4/97 4.2
Clump verbena (Verbena canadensis Alba) 4/97 4.4
1 Study conducted at the E.V. Smith Research Center of the Alabama Agricultural Experiment Station in Shorter from January, 1996 through December, 1997.

Two varieties of scabious (Scabiosa columbaria Butterfly Blue and Pink Mist) were among the most impressive performers. Once they started blooming, flowers were present on one or more plants even through the winter. Scabious 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.

Several verbena species were also impressive performers. Moss verbena (Verbena tenuisecta Purple) was a favorite, with a moss-like carpet of foliage, which seemed to almost always have a few flowers. Sometimes it was literally covered with dark purple blooms. Another variety of moss verbena (Verbena tenuisecta Alba) also performed well, but seemed to produce fewer flowers. Clump verbena (Verbena canadensis Homestead Purple) was also a ground cover that produced purple flowers in mass. But it had a tendency to flower only along the edges of the spreading plant, leaving the center without blooms. South American verbena (Verbena bonariensis) was also a favorite with a tall, open, airy habit and small purple flowers.

Two other perennials were good performers. Bloody cranesbill (Geranium sanguineum Album) produced delicate flowers on interesting palmate foliage and began to spread slowly, making a nice ground cover. Black-eyed Susan, also called orange coneflower, (Rudbeckia fulgida Goldstrum) had large, golden, ray-like flowers that attracted butterflies.

This study lasted for two years so that researchers could evaluate performance during the second year of growth. Plants in this study generally performed better the first year after planting than the second year. Several selections did not reemerge the second year, though some natural re-seeding occurred. Still other selections never fully recovered from the winter months or were less tolerant to heat stress in the summer.

Even though several plants performed well the first year, they 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 van houtt salvia (Salvia van houttie) with dark maroon flowers that attracted hummingbirds. This plant was very showy, reaching approximately three feet tall and two feet wide, and was completely covered with blooms. Violet boltonia (Boltonia asteroides var. latisquama Nana) also fell into this category of being treated as an annual, as did blanket flower (Gallardia × grandiflora Goblin), and both pink and white boltonia (Boltonia asteroides Pink Beauty and Snow Bank). However, these three varieties of boltonia 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.

Several perennials were slow starters, becoming more established the second year. Gas plant (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, white indigo (Baptisia alba Pendula), had graceful arching limbs with rounded leaflets on pinnately compound leaves that were attractive, with or without flowers.

Some perennials are invasive and others require pruning. White sage (Artemisia ludoviciana Silver King) became so invasive it had to be removed from the trial and would not be appropriate for planting in a mixed border due to its aggressiveness. While the loosestrife species are often considered invasive, very few seedlings in this study germinated or colonized through underground stolons (horizontal branches from the base of a plant that produce new plants from buds at the tip or at nodes along the branch.) Even so, this perennial should not be introduced to native wetland areas.

Swamp sunflower (Helianthus angustifolius) and bog sage (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. Wormwood (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.

For full sun perennial or mixed borders for landscapes in the southeastern United States, plants with high ratings in this study are well qualified. Many of these perennials appear to tolerate a full sun environment, to require little care, and still to perform well. Gardeners and growers should try plants in several locations to determine suitability for a particular area.

Detailed information on the results of this study is available in Circular 323, 1996-1997 Herbaceous Perennial Trial Garden Results, available from the Office of Research Information, Room 2, Comer Hall, Auburn University, AL, 36849.

Kessler is Associate Professor and Sibley is Assistant Professor of Horticulture. Bannon is Director of E. V. Smith Research Center.

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