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Wednesday - September 30, 2009

From: Houston, TX
Region: Southwest
Topic: Trees
Title: Neighbor's Arizona ash roots in Houston
Answered by: Barbara Medford


There is a huge Arizona Ash tree in my neighbor's yard. Its trunk is about 27 feet away from the foundation of my house and its foliage reaches my roof. I am planning to dig a trench on my side of the fence (about 15 feet from the trunk) and install a root barrier. How deep is the trench supposed to be? What is the best material for root barrier against Arizona Ash? And, how long should the trench be? Thank you.


Fraxinus velutina (velvet ash), also called Arizona ash, is native to far West Texas, but not to the Houston area. We really don't think you need to worry about those roots as much as about the branches touching your roof. 

While it is certainly true that tree roots can grow up to three times the spread of a tree, the problem in foundations is soil subsidence. The tree root does its part on this by looking for water and sucking it up, but if the soil is dry, the foundation is probably going to drop and shrink anyway, and that is much more likely to cause the foundation damage then the tree roots. There is really no way to tell a tree to grow in another direction, and the branches against your roof can definitely cause problems. Insects, not to mention squirrels and raccoons, consider tree branches against a house as an open invitation to come in, have a bite to eat, and spend the winter. Certainly a trained arborist could prune the branches away from the house, but when you prune a plant, where does the new growth appear? Right, it appears in the area you pruned. We think your first step would be to confer with your neighbor on trimming his tree branches back away from your property.

In terms of whether or not the roots will harm your foundation, you can read the recommendations from Iowa State University Extension Service for Sidewalks and Trees which bases the distance trees should be planted near pavement or other concrete structures on the mature height of the tree.  Fraxinus velutina (velvet ash) is a small to medium-sized, deciduous shade tree, usually no taller than 40 ft. in cultivation. Their recommendations are:

1. trees with a mature height of less than 30 feet, 3-4 feet from pavement,
2. trees with a mature height of 30 to 50 feet, 5-6 feet from pavement,
3. trees with a mature height of greater than 50 feet, at least 8 feet from pavement.

With a distance between your foundation and the trunk of the tree of 27 ft., and the expected height of the ash tree of 40 ft., which would only require a 5 to 6 ft. distance, it would appear your foundation is safe, even if your roof is not. If you still wish to consider installing some sort of root barrier between the tree and the foundation, here is more information about root barriers.

From our Native Plant Image Gallery:

Fraxinus velutina



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