Crapemyrtle: Identify and Manage Problems

Cold-hardy in USDA zones (6)7 to 9 when planted in the ground; hardiness is less reliable if kept in a container for the winter. Performs best in moist, well-drained soil; can also grow well in poor, dry soils, such as next to parking lots, and along the coast. Tolerant of a range of soil acidity (pH). Best grown in full sun, but tolerant of partial shade. Tolerant of salt spray and pruning.

Disease- and insect-resistant plants prevent or help to reduce problems in your landscape. No crapemyrtle cultivars currently have any significant resistance to insects. Some cultivars have been bred for resistance to powdery mildew.

Mature size greatly depends on the cultivar. Cultivars having a more tree-like stature when mature generally range from 10 to 30 feet high and about 15 to 25 feet wide. Shrubby cultivars tend to mature around 5 to 10 feet high and wide, though very dwarf forms could potentially stay as compact as 2 to 4 feet tall and wide. Check plant tags for approximate mature size. Typically, the smaller the mature size, the slower the yearly growth will be; the largest-maturing cultivars grow the fastest, well over 1 or 2 feet per year.

When choosing a cultivar, make sure its mature size will fit in the space available without relying on pruning to shorten the canopy. Even though crapemyrtle usually regrow well from heavy pruning, significant trimming will negatively impact plant health and appearance.

Container-grown crapemyrtle are often root-bound by the time they are sold, having roots tangled or matted together around the edges of the pot. Loosen the roots prior to planting by making several cuts along the outside of the root ball. Spread the roots outward so they can establish into the surrounding soil.

Remove the burlap and any twine or wire cage when planting balled and burlapped (B&B) crapemyrtle. Dig the planting hole just deep enough to accommodate the root ball with the root flare (where roots branch off the trunk base) level with the soil surface. Visit Planting a Tree or Shrub for more details.

Check the soil moisture of new plants regularly, and water deeply if it begins to dry out several inches below the surface.
 

If there is less than 1 inch of rainfall per week, water thoroughly to maintain even soil moisture during dry periods. This is important for plants in their first two years of establishment. Well-established plants should still be watered thoroughly during dry periods. Water the root zone rather than overhead to minimize leaf wetness, since wet leaf surfaces can be easier for disease spores to infect.

Fertilize crapemyrtle only if a laboratory soil test indicates a nutrient deficiency. If planted with an organic soil amendment (such as compost) and mulched with an organic material (bark, wood chips, pine needles, etc.), most plants should not need additional nutrients. Mulch should be no thicker than 2 to 3 inches in depth; keep it several inches away from the base of the trunk/stems.

Prune and shape, if desired, in late winter or early spring (around February or March) before the plant breaks dormancy. Avoid topping branches and trunks, which stresses the tree and makes it more vulnerable to pest problems. (Learn more in the Maryland Grows blog post “Topping Harms the Health of Trees.”)

The widespread practice of topping crapemyrtle is sometimes called “crape murder,” and it is strongly discouraged. These trees and shrubs only need to be pruned to thin a crowded canopy or to remove broken branches. If trees pose a safety issue or are too large to prune, hire a certified arborist or licensed tree expert.

“Rabbit tracks” are elongated chlorotic spots, often with a bronze center, on either side of the midvein of the leaf. They do not appear on the first growth of spring; rather, they can manifest on the second flush of new growth closer to summer. A nutrient deficiency is suspected of being the cause, but it is unknown whether too little sulfur, copper, iron, manganese, zinc, or another element is responsible. Severe cases can cause the leaf edges to become wavy or curl.

Most of Maryland’s planting zones are appropriate for crapemyrtle. However, in winters with extreme drops in temperature, the top growth of crapemyrtles can die back; the roots will stay alive if they are planted in the ground and not in a container. When this happens, branches might not leaf-out in the spring, or the canopy will have scattered dieback

Regrowth from the roots will eventually replace what was lost, but it might take years for a heavily-damaged plant to regain its former size.

For strong growth and abundant blooms, crapemyrtles need full sun (a minimum of 6-8 hours of direct summer sun). Too much shade will result in poor flowering and arching branches reaching for the sun, and weaker branch wood that may flop or droop, especially after heavy rains.

If a crapemyrtle growing in full sun is failing to bloom well, the soil might be deficient in phosphorus. Use a laboratory soil test to confirm nutrient levels. If you recently fertilized a lawn near the crapemyrtle, the plant’s roots could be exposed to too much nitrogen, which can encourage extra leaf growth at the expense of flowering.

Another reason for a lack of blooms in a sunny site is improper pruning. Crapemyrtles generally do not need to be pruned unless a dense canopy needs thinning or there are dead or dying branches. Refer to the pruning care tips above for information about proper pruning. Choose a cultivar that is suited for the growing space at its mature size which won’t require pruning to keep it smaller.

The crapemyrtle aphid, Tinocallis kahawaluokalani, is approximately 1/16 inches (1.5mm) long, pear-shaped, and pale yellow to light green in color. Winged adults have black markings on their body and wings. They are usually found on the underside of the leaves, though overwintering eggs are laid onto twigs.

Aphids are sap-sucking insects that secrete a sticky substance called honeydew as they feed. Honeydew supports sooty mold growth, a black fungal coating on the bark and leaves, or on any surface the honeydew is covering. Sooty mold is often what gardeners notice first if aphids are abundant.

Infestations of crapemyrtle aphids are typically considered a cosmetic issue due to the associated sooty mold. However, if aphid populations are high, a thicker coating of sooty mold on foliage can interfere with photosynthesis, and the stress may cause early leaf drop . This consequence is temporary, and the defoliation is unlikely to affect long-term plant health or vigor; plants typically recover and regrow normally. Crapemyrtle aphids do not feed on other plant species.

Management

• Biological: Beneficial organisms (natural enemies) will help to suppress aphid populations. They include predatory and parasitic insects as well as insect-infecting fungi. For example, the adults and juveniles of both ladybugs and green lacewings are common aphid predators. Learn to identify these helpers so you don’t harm them while trying to control the aphids.
• Mechanical: Aphids are easy to knock off of foliage by spraying the leaf undersides with a strong jet of plain water from a garden hose. While this won’t eliminate a population, it will reduce their numbers and feeding injury so natural enemies can control the rest.
• Chemical: If aphid populations persist, insecticidal soap or horticultural oil can be used to smother them. Either product needs to directly contact the aphids to work. Hiring a certified pesticide applicator may be necessary in order to thoroughly spray the canopy of tall crapemyrtles. Plant-absorbed systemic insecticides might work well and be easier to apply, but several of the active ingredient options (like imidacloprid and dinotefuran) can only legally be applied in Maryland by a professional pesticide applicator. To reduce the risk of exposure to pollinators, do not apply systemic insecticides when a plant is flowering or about to bloom. As an added precaution, flowers or flower buds on a treated plant can also be pruned off for the season to shield pollinators.

Crapemyrtle bark scale, Acanthococcus lagerstroemiae, is a non-native type of soft scale insect. Scale can be difficult to manage since their waxy body covering shields them from some predators and certain pesticides. This is a common pest of crapemyrtle, and infestations will cover the bark, typically beginning in furrows or branch forks. As sap-sucking insects, they secrete a sticky substance called honeydew, which can support sooty mold growth and attract wasps, ants, and flies.

Management

Several natural predators help to suppress this scale. Prune out any branches that might be dying or dead due to high scale populations; this will also physically remove most of the scale. If the infestation is not too heavy, use a soft brush to remove the white scale covers from the branches, which will expose the insects to predators and dry them out.

Refer to our general scale management recommendations for both chemical-based and pesticide-free options. Avoiding pesticides will support a higher population of natural predators to help manage the infestation. As a last resort, if pesticides are warranted, a combination of dormant oil applications and the use of systemic or growth-regulating insecticides is the most effective approach. For large populations, scale suppression may require more than one year of intervention, and professional pesticide applicators will be needed to apply certain treatments. Mature trees should be evaluated by a certified arborist. If hand removal efforts or chemical controls are not obtaining good results, remove and replace the infested plant.

Avoid over-pruning crapemyrtles, because the injury and stress can make them more susceptible to scale.

Ambrosia beetles are tiny insects that bore into the wood of trees. They target trees that are stressed by environmental conditions or heavy pruning. As the beetles tunnel, they push a mixture of excrement (frass) and sawdust backward out of the entrance holes. The mixture sticks together, and is called a “frass toothpick.”

Management

The best management for ambrosia beetles is prevention: healthy trees are able to withstand a small population of ambrosia beetle attacks. Once the beetles are inside the tree, insecticidal sprays will not kill them, and the damage has already been done that treatment cannot reverse. Heavily-infested trees are not salvageable and should be removed.

To prevent stress on crapemyrtles, keep the trees adequately watered in times of high heat and drought. Make sure that trees are not mulched too deeply at the base of the trunk. Avoid pruning beyond removing broken branches and performing minor trimming to reduce canopy crowding to improve air circulation.

Powdery mildew is a widespread disease on crapemyrtles in Maryland. The fungus (Erysiphe lagerstroemiae) creates a grayish-white coating on the leaves, young stems, and flower buds. Heavy infection can cause plant stunting when extensive fungal growth reduces sunlight reaching the leaves. Fortunately, infections are usually not fatal to the plant and tend to be more of an aesthetic issue. High humidity during periods of warm days and cool nights provides optimum conditions for powdery mildew development. Infections may be more severe on plants grown in shady environments without good airflow.

Management

Discourage powdery mildew by selecting mildew-resistant crapemyrtle cultivars and by growing them in a location with full sun in summer. If growth becomes very dense, periodically thin the canopy to increase airflow around the branches and to remove suckers from the trunk base.

Powdery mildew is rarely severe enough to warrant chemical control. Fungicides are intended to prevent infection and won’t cure existing disease, nor will they always be completely effective. They must be applied at regular intervals through the growing season, as long as conditions exist that promote infection, to maintain plant protection.

Due to the cost and hassle of making several applications per year, and the potential harm to other organisms, fungicide use for this minor disease is not usually recommended.  If mildew infections become severe and intolerable, replace the plant with a more resistant cultivar or another species. If a fungicide is deemed necessary, use a registered product and follow all label instructions. Horticultural oil is a low-toxicity option that is sometimes labeled for mildew suppression.

Over one hundred crapemyrtle cultivars exist, and their resistance to mildew varies, though none are completely immune. One group of resistant varieties were developed by the U.S. National Arboretum and are widely available.

Leaf spots on crapemyrtle are commonly caused by the fungus Cercospora lythracearum. This disease typically develops during periods of warm, moist weather. Small yellow spots (⅛ to ¼ inch) appear on the upper leaf surface, with greyish fungal growth on the underside. In years with heavy infection due to favorable weather, susceptible cultivars might drop leaves (defoliate) in summer or early autumn. Despite this, infections typically do not cause long-term plant health problems and are more of an aesthetic issue.

Management

Select disease-resistant varieties when possible, and plant them in areas with good air circulation (not too close to a building wall or solid fence, for example), a full sun exposure, and not crowded by neighboring plants. By the time symptoms appear, it is typically too late in the season to treat with a fungicide, since existing infections cannot be cured. Rake up any shed leaves and dispose of them (do not compost) to limit how many fungal spores overwinter.