Linda Benedict | 5/3/2005 12:57:55 AM
Allen Owings, Drew Bates, Gordon Holcomb and Edward Bush
The LSU AgCenter has long been actively involved in evaluating ornamental plants, providing recommendations for county agents and green industry professionals (landscape contractors, retail garden centers) to use when working with home gardeners. One group of plants that has generated considerable interest recently is the crape myrtle. In Louisiana and across the southeastern United States, crape myrtles are the most popular and widely used summer flowering tree for residential and commercial landscapes. Most people are familiar with the traditional upright-growing crape myrtles (10-30 feet). These include Natchez (white), Tuscarora (coral pink) and Watermelon Red (dark pink). However, miniature and dwarf crape myrtles are available for landscape use, too.
Miniature and dwarf crape myrtles were evaluated in LSU AgCenter studies over the 1999 and 2000 growing seasons. Industry standards generally classify miniature crape myrtles as having a mature height of 3 feet or less and dwarf crape myrtles as being 3 to 6 feet tall at maturity. The primary objective in these trials was to evaluate the trees for growth habit, flowering performance, winter damage and disease susceptibility. Two-gallon containers of Pixie White, Delta Blush, Baton Rouge (also known as Beverly), Mardi Gras, New Orleans (also known as Passion), Lafayette, Pink Blush, Purple Velvet, Orlando, World’s Fair, Bicolor, Sacramento, Cordon Bleu (also known as Louisa), Houston, Chickasaw and Pocomoke were planted in raised beds 4 feet apart on October 30, 1998, at the LSU AgCenter’s Burden Center in Baton Rouge.
The raised beds were an Olivier silt loam soil amended with aged pine bark and composted rice hulls. The beds were located in full sun and plants received drip irrigation as needed throughout the growing season. Plants were mulched annually with 2 inches of baled pine straw.
Crape myrtles were fertilized annually in April with StaGreen Nursery Special 12-6-6 at the rate of 1 pound of nitrogen per 1000 square feet of bed area and in early July with StaGreen Nursery Special 12-6-6 at the rate of 0.5 pound of nitrogen per 1000 square feet of bed area. Plants were not pruned (other than to remove winter-damaged stems) or dead-headed (flower removal) during the evaluation period, and no fungicides or insecticides were applied. Weeds were controlled with mulch, spot applications of Roundup and pre-emergent applications of Surflan.
Data collected over the 1999-2000 evaluation period included plant height, date of first flower, visual quality ratings and susceptibility to Cercospora leaf spot and powdery mildew. Leaf spot and powdery mildew are the two primary diseases affecting crape myrtles in Louisiana. Visual quality ratings were conducted monthly during the growing season, and the criteria included a combination of growth habit, visual aesthetics, flowering and pest presence. Plant height was measured from ground level to the top of tallest shoot each July. Disease evaluations were conducted when diseases were the most prevalent (normally mid-spring through early summer).
Results from 1999 and 2000 evaluations indicated that the most ideal growth habit and foliage characteristics (weeping/cascading, compactness, red foliage) were found for New Orleans, Sacramento, World’s Fair and Houston. These plants also had some of the higher visual quality ratings. Pixie White achieved a desirable visual quality rating also.
Flowering began as early as April 29 or as late as early June. Cordon Bleu was the first to flower in 1999 and 2000. Other early flowering varieties were Pixie White, Lafayette and Purple Velvet. Chickasaw and Pocomoke, new releases from the U.S. National Arboretum in the last five years, were among the later flowering varieties. Flowering generally terminated by mid to late August. Typically, flowering time for dwarf and miniature crape myrtles is equal to, and in some cases earlier and longer than, traditional intermediate and tall crape myrtles.
Powdery mildew was not seen in 1999 but was observed in early May 2000. Slightly susceptible varieties were Pixie White and Bicolor. Baton Rouge and Purple Velvet were moderately susceptible to powdery mildew. Approximately 1 percent to 10 percent of foliage on Cordon Bleu, Delta Blush and Pixie White had Cercospora leaf spot in June 2000. In rainy years, leaf spot can cause significant defoliation when not prevented or controlled.
A concern green industry professionals have about miniature and dwarf crape myrtles is their cold hardiness. In our studies, we observed dieback caused by winter damage on a number of varieties. Major dieback (40 percent to 50 percent of stem growth) occurred on Baton Rouge, Orlando, Mardi Gras, World’s Fair, Houston and Lafayette. Chickasaw had 40 percent dieback in the winter of 1999-2000, and Pocomoke had 15 percent.
Miniature and dwarf crape myrtles have the potential for increased use in Louisiana landscapes. Wholesale nurseries have limited inventory of these plants now but have been increasing production in the last two to three years. Plants are generally available at retail garden centers during the late spring and early summer. Based on this study, Chickasaw, Pocomoke, Sacramento, Houston, New Orleans and World’s Fair have the most positive landscape attributes for Louisiana.
Allen Owings, Specialist, Department of Horticulture, LSU AgCenter; Drew Bates, Associate Professor, Burden Center, Baton Rouge, La.; Gordon Holcomb, Professor, Department of Plant Pathology and Crop Physiology, LSU AgCenter, Baton Rouge, La.; and Edward Bush, Assistant Professor, Department of Horticulture, LSU AgCenter, Baton Rouge, La.