Credit: Adobe Stock Images
From a historical perspective, crape myrtle (Lagerstroemia indica) is one of the most useful flowering shrubs grown in southern states. A member of the Lythraceae (loosestrife) plant family, it provides abundant summer color with a minimum of maintenance. Crape myrtle is long-lived, withstands droughts after becoming established, and is relatively free of disease and insect problems. Colors range from red and pink to lavender and white. Unfortunately, most of the older cultivars are cold hardy only through zone 7.
Crape myrtle derives its common name from the heavily ruffled petals of its flowers which appear to be made of crepe paper. (Credit: University of Georgia)
During recent years, considerable breeding work has resulted in a number of new crape myrtle cultivars. Many of the newer cultivars have better cold tolerance and disease resistance, improved flower color, better fall leaf color, and more handsome bark than the older types. Plant height ranges from less than 3 feet to more than 20 feet, making it very versatile in the landscape.
In addition to its flowers, crape myrtle has attractive bark which adds interest to the landscape. (Credit: Clemson University)
Although crape myrtle will grow under adverse soil conditions, it grows and flowers better in well-prepared soil. Therefore, good soil preparation prior to planting is well worth the effort involved. Preparation includes digging a hole at least two times wider than the root ball. Set the plant in the hole no deeper than it originally grew in the production container or field. Then backfill with the same soil removed from the hole after breaking apart clods and removing rocks or other debris. Research has shown that organic matter amendments are not necessary when planting in individual holes. Amendments in the hole encourage roots to stay within the hole and not grow outward into the surrounding native soil. Amendments are most beneficial, therefore, when they are incorporated uniformly throughout the soil surrounding the planting hole.
Crape myrtle adds brilliant color to the landscape when very few other trees or shrubs are blooming. (Credit: Adobe Stock Images)
After planting, water thoroughly to settle the soil around the roots, and mulch to conserve moisture, reduce weeds and insulate the roots against extreme cold and heat. Three to five inches of pine straw, pine bark, shredded hardwood mulch or shredded leaves placed over the planting hole is ideal. Mulching an area larger than the planting hole is recommended.
Water crape myrtle plants thoroughly at planting time and once a week, in the absence of rainfall, for the first two months after planting.
Established crape myrtle plants will tolerate drought. However, flowering is enhanced if plants are watered during dry periods that occur during the flowering season.
For strong growth and abundant flowering, crape myrtle requires a full sun exposure. Heavy shade will reduce growth and flowering and will increase disease problems such as powdery mildew on the flower buds and young growth. Large shade trees will also compete with crape myrtle for moisture, causing poor growth and flowering. Lack of sunlight and moisture are the common causes of poor growth and flowering.
Crape myrtle flowers on new growth of the season. Therefore, if you choose to prune, do so in the dormant season (i.e., late winter to early spring) before growth resumes. Avoid pruning in early fall before the first frost, since pruning forces new growth and keeps the plant from going dormant. Severe freezes can kill the plant if it is not fully dormant.
It is a common misconception that crape myrtles require pruning in order to flower. This is not only false but has also resulted in virtually millions of plants being pruned very aggressively, a practice commonly referred to as "crape murder." The most natural and beautiful crape myrtle trees result from limited or no pruning. In addition, aggressive pruning leads to increased suckering (shoots arising from below-ground roots) which is not only undesirable, but it could result in powdery mildew spreading from the suckers to the canopy of the plant. Aphids are also attracted to the succulent growth which results from aggressive pruning. It is far better to plant dwarf, or semi-dwarf varieties which grow to desired mature heights than to continue fighting with a more vigorous, larger cultivars planted in a space too small.
Should pruning become necessary, the ensuing steps should be followed: First, remove suckers from the base of the plant. Second, as the tree grows, remove lower branches from the bottom third of the tree to expose the trunk character. Last, remove crowded or crossing branches from the canopy. The seedheads are an attractive feature and should be left on the tree. As the new growth pushed in the spring, the seedheads fall off. Some gardeners find that objectionable; if desired the seedheads can be removed by heading back to above where a leaf joins the stem, or if no leaves are present, just above a lateral bud. On some cultivars, pruning to remove spent flower blossoms after they fade will stimulate new growth and another blossom flush in late summer. A second bloom is sometimes difficult to force on cultivars that bloom after mid-July.
Taller cultivars of crape myrtle can be pruned to a tree-like habit of growth. (Credit: Adobe Stock Images)
Crape myrtles in tree form make wonderful accent plants or small specimen trees. Many cultivars develop attractive trunks with exfoliating bark that add interest to the winter garden. To develop a tree shape, select three to five radially-spaced branches slightly leaning to the outside, these will become the main trunks. spaced shoots growing from ground level as the main trunks. Next, remove side branches from these shoots about halfway up their height. As the plant grows taller, lowest branches can be removed each year so the canopy begins 3 to 4 feet above ground level. You may also need to remove suckers (new young succulent spouts that grow from the base) periodically in order to maintain the desired tree shape
A complete general-purpose garden fertilizer (e.g., 8-8-8, 10-10-10, 12-4-8 or 16-4-8) is ideal for crape myrtles. To newly planted small plants (1-gallon size), apply 1 teaspoon of fertilizer monthly from March to August along the perimeter of the planting hole. Larger, established plants will benefit from one broadcast application of fertilizer in spring. Apply 8-8-8 or 10-10-10 at a rate of 1 lb. per 100 sq. ft. or 12-4-8 or 16-4-8 at a rate of ½ lb. per 100 sq. ft. Avoid over-fertilization because it causes excess growth and reduced flowering. The ideal time to fertilize is just before a rain. Otherwise, water in the fertilizer after application with irrigation. It is not necessary to remove mulch when fertilizing.
Propagation of patented cultivars is unlawful. Plant patents expire 20 years after the original filing date with the United States Patent and Trademark Office. Therefore, older cultivars which have been on the market since the early 2000s can be propagated. Crape myrtle is easily propagated from semi-hardwood cuttings taken during the growing season. June, July and August are excellent times for rooting cuttings. Take cuttings from new growth of the season, leaving three to four nodes per cutting and several leaves. Rooting hormone is generally not necessary, and cuttings should root in three to four weeks. Place cuttings in a well-drained rooting medium in a shaded area and keep them moist by enclosing them in a clear plastic bag.
Cuttings also can be placed in prepared outdoor rooting beds. Thoroughly cultivate the soil to a depth of 10 to 12 inches. Add 4 to 5 inches of organic matter such as peat moss, leaf mold or pine bark to the surface and thoroughly mix it into the soil.
Place the cuttings approximately 6 inches apart. Insert them one-half their length into the soil. Mulch with 2 to 3 inches of pine straw, leaf mold or pine bark to conserve moisture. Keep them moist with regular misting.
Several diseases occur on crape myrtle including powdery mildew, Cercospora leaf spot, root rot and sooty mold. Powdery mildew is the most widespread and serious disease in most regions. Powdery mildew typically develops in late spring and fall and is associated with warm days and cool night temperatures along with high humidity. Leaves, young shoots and flowers are heavily coated with a powdery, white mold that can distort new growth. Infected flower buds may not open, and severely infected leaves and buds often drop early.
The other most damaging disease is a leaf spot caused by the fungus Cercospora lythracearum. Spots develop in mid-summer through fall during wet, humid weather. Large, dark brown spots develop on lower leaves and progress upward through the plant. Infected leaves turn yellow around the spots and drop prematurely. Sometimes even one spot will cause a leaf to drop on susceptible cultivars and can cause significant defoliation prior to frost.
Powdery mildew and Cercospora leaf spot can be controlled by applying fungicides when the diseases are first noticed. Consult your local cooperative Extension Office for current fungicide recommendations. The best approach to prevent diseases is to plant disease resistant crape myrtle cultivars. A number of new crape myrtle cultivars from the U.S. National Arboretum and other nurserymen are resistant to powdery mildew and Cercospora leaf spot. They are ideal for gardeners wanting low-maintenance landscapes.
Sooty mold is an unsightly superficial, dark brown or black coating on leaves and stems that can be removed by rubbing. It is the result of a fungus growing on honeydew excretions made by insects such as aphids, which are the most serious insect pest on crape myrtle. Sooty mold usually causes little direct damage, but it can cut vigor by reducing photosynthesis in the leaves. Using crape myrtle aphid-resistant cultivars and insecticide sprays can reduce sooty mold.
Acknowledgement: Adapted from an article by the University of Georgia