Every once in a while I hear someone talking about how their crape myrtle hasn't bloomed this year, for several years, or since it was planted. Below is a list of possible reasons why your crape myrtle isn't producing flowers and tips to get them going again.
Crape myrtle type. To start, there are many cultivars (varieties) of crape myrtle, which are in the genus Lagerstroemia. Among the varieties, some, such as those in the 'Early Bird' series, start blooming earlier in the season, in May. Others wait until early to mid summer before they begin to bloom. Some of the more compact varieties, such as 'Pocomoke', often won't start blooming until August. So, maybe it's just too early in the season for your crape myrtle to be blooming?
Sun exposure. Not enough sun can effect flowering. Crape myrtle prefer as much sun as you can give them. Exposure to 8 hours or more of direct sunlight is necessary for optimum bloom. If your tree is receiving less than 8 hours then expect to see less blooms than those out in full blazing sun. If your tree is in shade or mostly shade, expect to see few if any flowers. If your crape myrtle is in too much shade there's nothing you can do but move it to a sunnier location.
Improper pruning. Pruning too heavily or at the wrong time of year can, and often will, seriously reduce or completely shut off flower production. Crape myrtle pruning should be done in late winter or early spring, before new growth begins to emerge. Contrary to popular belief and practice, hacking crape myrtle back to flat-topped trunks or to a fist of "knuckles" does not increase flower production. Instead it causes your tree to use more energy to regenerate the lost branches rather than to flower production. Trees or shrubs pruned too heavily will usually send out long shoots that may or may not bloom and which can break off during heavy rain and/or wind. If you intend on pruning your crape myrtle, make sure to do it properly. Proper pruning can be useful to form a more dense canopy or shrub that will produce twice as many flowers every year. SEE: How To Prune A Crape Myrtle Tree and How To Prune A Dwarf Crape Myrtle
Erratic weather changes. Wild fluctuations in temperatures that occur when your crape myrtle is trying to break from winter dormancy, or soon after new foliage has emerged, can cause serious damage. Some crape myrtle varieties are known to produce early buds which can be damaged or destroyed by a late frost. An exceptionally late frost could set back blooming even further and seriously diminish or prevent flowering altogether. There isn't much you can do when cold weather is the culprit. If your crape myrtle appears stunted after emerging from dormancy, giving it a good feeding with a well-balanced shrub and tree fertilizer should help to promote new growth that might help to produce flower buds.
Soil pH is off. Soil pH is a measurement of the alkalinity or acidity of soil, and is measured on a scale of 1-14, with 7 as the neutral mark. If the soil is too alkaline or acidic this can effect a plants ability to absorb needed nutrients from the soil that are necessary for optimum health. Crape myrtle prefer and will perform and flower best in an acidic soil ranging between 5.0 - 6.5 on the pH scale. If your crape myrtle has not flowered for several years I would suggest testing or have your soil tested for pH. You can test your own soil by purchasing a test kit from your local nursery and garden center or you can buy a soil test kit online here. Your local Extension office may also provide soil testing services.
Improper fertilization. Excessively high amounts of nitrogen (the first number on a package of fertilizer) from fertilizers can cause too much foliage growth, which can reduce or entirely shut off the trees ability to set flowers. Crape myrtle that were planted and are growing in lawn areas, which are regularly fertilized with high-nitrogen lawn fertilizers, are highly susceptible to over-fertilization. To avoid this problem, create large, mulched beds that extend to a point beyond the perimeter of the outer branches. Be especially careful not to spread "weed & feed" type lawn fertilizers over the root systems of any plants or trees. The weed killing chemicals are absorbed by the roots and can seriously damage and even kill ornamental and other types of plants and trees. A soil test can indicate if your soil has a good balance of phosphorus and potassium (the other two numbers on a package of fertilizer) needed to produce flowers. In the absence of a soil test, I recommend feeding crape myrtle in spring with a well-balanced shrub and tree food containing no more than around 10-15% nitrogen, Phosphorus and pottassium (N-P-K). I use a 12-6-6 that contains slow-release nitrogen and a micronutrient package.
Soil moisture extremes. Periods of dry weather in spring and early summer can reduce or delay flowering. In rare occasions, severe, prolonged drought conditions with no supplemental irrigation provided can and will cause a crape myrtle to enter "survival mode," in which case few if any flowers will be produced. Though quite drought tolerant when established, for best flowering, crape myrtle prefer a consistently moist soil. That being said, constantly soggy or wet soil can shut down nutrient absorption and root disease that will diminish flowering. Therefore, plant crape myrtle in a well-drained site and provide occasional good deep soaking during prolonged periods of dry weather.
Pests & Disease. Crape myrtles are tough trees however, like all other plants, are subject to a few pest and disease problems. Heavy outbreaks of any issue can stress a plant delaying or preventing bloom. Some older varieties of crape myrtle are highly susceptible to powdery mildew, which appears as a white, powdery film on the leaves. It's best to avoid planting these older varieties as most of the new varieties available on the market today, such as the Fauriei Hybrids (with native American Indian names), are highly mildew resistant. Another common problem is evidenced by a black sooty mold that appears on leaves, which is actually caused by the honeydew aphid: a small insect that excretes a sticky substance on which the mold grows. Both powdery mildew or black sooty mold caused by honeydew aphids can be controlled using Neem oil. A less common leaf spot disease called 'Cercospora', brought on by warm, wet summers will cause your tree to prematurely shed most of its foliage, weakening its ability to flower. In some areas, the Japanese beatle is a problem. These beatles will munch on the flower buds of crape myrtle. Spray plants with liquid Sevin to quickly eliminate these beatles. Be watchful of these issues and deal with them before they get the better of your tree.
If you rule out all of the possible causes listed above it might be time to contact your local arborist to have a look at your crape myrtle and the environment it's growing in. If you find any other cause for why your crape myrtle has not bloomed please let me know and I'll add it to the list!
