Installation of 4” drainage pipe near large crepe myrtle

Asked August 8, 2020, 11:38 AM EDT

We have a water problem in our yard and it’s been recommended that we install a drainage pipe which will run along one side of a large (25? year old, 30’? high) crepe myrtle. The pipe will be situated 5-6 feet from the trunk of the tree. We are thinking some roots will be cut when the trench for the pipe is dug. I am not sure how deep the trench will be. I’m guessing between 8-12 inches. Will such a trench damage the tree? Any advice to keep the tree from suffering damage?

Montgomery County Maryland

3 Responses

Yes, your tree will sustain root loss. Most tree roots are in the top 12-18" of soil.
How much root loss will be determined by how far out the crape myrtle canopy goes (edge of leaves) from the trunk. Most of the roots are in that space (plus maybe a foot farther). Though some roots can extend out far past that.

Make a circle on paper, one inch for each foot of diameter. Put a dot in the middle to represent the trunk, and then draw a line across where circle where the pipe will go. You'll see how much root loss there will be. May 1/4th or so of the circle of roots?

The main thing is to water, water, water after the pipe goes in. The tree will be functioning on much less root, trying to keep the same amount of top growth supplied with water. Be very alert for fall droughts! They are killers. Water from spring through fall for at least 2 years if we don't get 1" of water a week. Get a rain gauge so you know how much rain is actually falling on your property (thunderstorms are so spotty!) Any straight-sided container can be a rain gauge.

You may get some dieback as the tree adjusts to its new, reduced root system. Let it do that and then prune out the dead branches.


By watering after the pipe goes in, what is recommended? With new trees we’ve had planted, the schedule for deciduous trees has been deep watering 3x a week for1 hour for the first 2 or 3 weeks, then 2x a week for 1 hour until leaves drop in the Fall. The 1 hour time was based on the size of the trunk of the tree planted. Think around a 2 3/4 inch trunk.

Should we deep water the crape Myrtle? How many times per week and for how long? Until leaves drop in Fall?

When should a rain gauge factor into how much we water once we begin deep watering the tree after the pipe goes in?

With new trees, the nursery instructions are to deep water regardless if it rains


As mentioned above, you're aiming for about 1" of rainfall a week, supplementing when necessary. Plants change in size and everyone's hose is different, so there is no formula we can give you. You don't need to be exact.

Get an idea of how long your hose takes to fill a bucket (just counting aloud as it fills works just fine) . Then estimate how many buckets you think it needs. Or leave the hose running very slowly at the base of the tree (again, you can see how long that takes to fill a bucket) and move it every 10-30 minutes, depending. Just do your best.

This page has good tips and may be more helpful for you: