The crape myrtle is often planted as an ornamental landscape tree. Its abundant flowers and spreading canopy make it a boon to any home garden. But, as crape myrtle trees age and grow, they can develop a number of problems. Some decline and develop persistent infestations and diseases. Others grow too abundantly and send up shoots all over the yard. Whatever your problem with your crape myrtle tree, its easy to remedy with a number of herbicides.

Oil-Soluble Herbicide

Oil-soluble herbicides are ideal for killing crape myrtles with trunks smaller than 6 inches in diameter. The oil in the herbicide allows the chemicals to penetrate beneath the waxy protective layer on the crape myrtle's bark. From there it travels down to the crape myrtle's roots to kill them from the inside out. Horticulturalists recommend using Pathfinder II, Remedy or any other herbicide pre-mixed with an oil carrier.

Systemic Herbicide

Systemic herbicides provide effective control of crape myrtle trees by killing their roots. The herbicide is designed to travel through the crape myrtle's vascular system to disable it. Penn State horticulturalists recommend herbicides that contain 2,4-D, glyphosate, Imazapyr or dicamba. Unlike oil-based herbicides, systemic herbicides cannot travel through the crape myrtle's bark. It must be directly introduced to the interior of the plant through a wound or cut.

When to Apply Herbicide

The best time to control crape myrtle is during the growing season between spring and early fall. When crape myrtle is actively growing, nutrients and water travel through its vascular system. These are the channels through which both oil-based and systemic herbicides travel. When the trees are dormant, during colder weather, the herbicide has a harder time traveling through the tree's system. It may take more than one season to kill your crape myrtle. If it does, wait until warm weather comes around again to re-treat.

Application Methods

Oil-based herbicides are the easiest to apply. Simply spray them over the bottom 12 to 18 inches of each of the crape myrtle's trunks. Follow the manufacturer's instructions to the letter and coat the bark to just before the point of run-off.

Systemic herbicides must be introduced directly to the interior of the crape myrtle. The best way to do that is to cut the crape myrtle's trunks down. Apply the herbicide directly to the cut surface with a paint brush as soon as you make the cut. If you can do it within 30 seconds, you'll have the best success of killing the crepe myrtle the first time. Systemic herbicides may also be used to kill any sprouts that the crape myrtle puts up as it dies. Paint or spray it onto the sprouts as soon as you spot them.