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Monday - November 24, 2008

From: Vacaville, CA
Region: California
Topic: Trees
Title: Root suckers growing from base of oak in California
Answered by: Barbara Medford

QUESTION:

I planted four trees labeled by the nursery when purchased as "Louisiana Oaks" approximately 20 yrs ago in my front yard. All trees are growing well but one of the four has a progressing mass of root suckers growing about its base requiring regular trimming. The other three trees have never had a sucker. Is there a way to stop this growth without harming the Mother tree? Was the root ball planted too shallow initially? Can increasing the soil depth around the tree by 4-6 inches stop the sprouts and encourage more root growth? If this could work, I have thought of putting a 6 ft diameter ring of retaining wall stones to create a ring bench under the tree and could double as a flower planter for perennials.

ANSWER:

Our first problem in identifying your tree was in finding which species of Quercus could be called a "Louisiana" oak. Nurseries can (and do) give a plant just about any name they want to, usually it make it more sellable. "Louisiana oak" conjures up an image of lush, tree-lined avenues up to old plantations right out of Gone With The Wind. So, calling an oak a Lousiana oak probably made it more attractive to you 20 years ago. As the trees have obviously done well, there is no problem with your selection. The problem for us is not knowing what actual tree it is, in hopes of discovering what is causing the suckering. There are 71 members of the Genus Quercus in our Native Plant Database, none of them having a common name with "Louisiana" in it, although many of them are native to Louisiana. Quercus agrifolia (California live oak) is native only to California. Again, we don't know if your oaks are live oaks (that is, they shed only part of their leaves, in the spring, which are quickly replaced by new leaves) or just plain old deciduous oaks. So, we'll just shoot for some general information and hope that helps. 

First, to answer your questions. No, there really is no way to stop the suckering without harming the parent tree. Sometimes a tree will begin to sucker because it has sustained some kind of damage, such as shearing off the top of roots or over-pruning. The suckers are one of the trees' ways to guarantee survival of the species. Left untrimmed, they could eventually develop into a small grove of trees, which you surely don't want. Applying any kind of herbicide to the suckers will cause the toxic substance to go into the tree itself, and can cause substantial damage or death. 

Was the root ball planted too shallow initially? After twenty years, who knows?  If the tree has only recently developed this suckering habit, it's hard to attribute that back to its planting all that time ago. Even so, what are you going to do about it now? 

We most definitely do NOT recommend putting more soil around the tree roots or putting down retaining walls on the roots. The addition of that much soil over the roots is going to stress the tree. You might try about 2" of shredded bark mulch, which will still permit the tree roots to "breathe," but also shade out the emerging suckers. The majority of tree roots are in the upper 6 to 12" of the soil. And they spread out far beyond the dripline of the tree itself. The roots are responsible for obtaining nutrients and water from the soil, and any kind of barrier over those roots is going to cause trouble. In the first place, those roots are pretty strong, and capable of cracking sidewalks, foundations and ring benches, still in their quest for water and nutrients. In the second place, oaks do have some allelopathic ability, which means they secrete substances to discourage plants from growing beneath them (and trying to share the above-mentioned water and nutrients), as well as creating too much shade for most flowering plants to prosper. 

So, the best we can tell you is keep nipping off those sprouts, or mowing them, mulch lightly to discourage them but not discourage the tree, and don't try to plant any flowering perennials beneath the trees. 

 

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