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Planning

Start by selecting a spot that will allow plenty of space for the mature plant. The common crape myrtle can reach a height of 15–25' and a spread of 6–15' at maturity.  With ideal planting conditions this shrub grows at a fast rate, with height increases of more than 24" per year. Full sun is the ideal condition for this shrub, meaning it should get at least 6 hours of direct, unfiltered sunlight each day. Crape myrtle grows in a wide range of soils from slightly alkaline to acidic. It prefers moist, well-drained sites but has some drought tolerance.

Planting

Now dig a hole about three times the size of your pot and the same depth as the root ball. Set the soil you have dug out aside and mix it with compost. Remove the plant from its pot and gently loosen the root ball. Place the plant in the planting hole and replace the soil with the mix and gently pack down the dirt. To avoid planting to deep make sure the plant is at a position with the top most roots at the soil line. 

After planting, water thoroughly to settle the soil around the roots. If desired, construct a water basin at the base of the tree about 36 inches in diameter.

Mulch in the spring & fall. The mulch should be about 4-6 inches deep (acid mulch, pine bark or oak leaves). Keep mulch a few inches away from the trunk of the tree. Do not mulch with mushroom compost.  

Fertilization

The crape myrtle prefers well-draining clay, loam, or sandy soils. The pH level of the soil should be 5.5 to 7.5. Apply a high-nitrogen fertilizer in the spring as soon as the leaves appear and again in two months.

A complete general-purpose garden fertilizer — such as 8-8-8, 10-10-10, 12-4-8 or 16-4-8 — is ideal for crape myrtle. 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 best 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.

Water

The first year is a critical time for your new crape myrtle. It has not had time to establish itself yet and therefore is not as strong as an older shrub. To prevent the plant from dying, it must be watered once a week. Be sure to soak the entire root system deeply, this will take about 45-60 minutes.

For best growth and production, crape myrtle should receive at least one inch of water a week. During dry spells, water is mandatory. If not properly watered during dry spells, flowers may be mitigated.  Keep at least 4 feet around the shrub clear of grass and weeds, for less competition for water. 

Once your tree reaches maturity, it will be naturally drought-resistant.

Pruning

Crape myrtle flowers on new growth of the season, so if you choose to prune, do so in the dormant season, i.e. later winter to early spring before growth resumes. Avoid pruning in early fall before the first frost, because 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 tree. 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 too-small space.

If you choose to prune however, follow the simple steps: First, remove suckers from the base of the plant. Second, as the tree grows, remove lower branches form 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 folks 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.

Crape myrtles in tree form make wonderful accent plants or 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. Then remove side branches from these shoots about halfway up their height. As the plant grows taller, more lower 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. Some landscapers apply a synthetic plant growth regulator, called NAA (naphthalene acetic acid), to suckers after pruning to prevent them from re-sprouting. 

Insect & Disease Control

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. Powdery mildew typically develops in late spring and fall and is associated with warm day and cool night temperatures and 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. 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.

Root rots can be prevented by growing plants in well-drained soil.

 

Color Chart and Variety Information for Crape Myrtles can be found here.