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Tuesday - March 22, 2011

From: Monahans, TX
Region: Southwest
Topic: Compost and Mulch, Pruning, Trees
Title: Existing live oak taking over in Monahans TX
Answered by: Barbara Medford

QUESTION:

I have just purchased a home with a huge Live Oak tree in the front yard. The previous owners have over the years allowed the sucker roots to grow unchecked. The tree is shading most of the lawn (dirt..) and making it unsuitable to grow anything else. Acorns and debris from the tree have been allowed to accumulate. The lawn is not huge. I'm wondering if it is going to be worth my efforts to try to get anything to grow there. The last owner gave up, I'm think. I understand that these trees will excrete substances into the soil and make it difficult to grown any other. Is there anything I can do to this area to make it suitable for other plants?

ANSWER:

Whether you realize it or not, you asked us several questions there. What we are going to do is list the questions we think you asked and give you some help on as much as we can.

1. How do I care for my Live Oak? Given your location, we are going to bet that what you have is Quercus fusiformis (Escarpment live oak). There are no oaks specifically native to your area, but that oak probably has the best chance of surviving there, since its normal habitat is the Edwards Plateau. The first thing to do to care for your oak is to avoid Oak Wilt. You may be in luck there, as we can find no indication that Oak Wilt has moved that far west in Texas, but you don't want to find out we're wrong the hard way. Please see this Texas Oak Wilt Information Partnership (in which the Lady Bird Johnson Wildflower Center is a partner) for more complete information. Live Oaks are the most vulnerable to Oak Wilt, with Red Oaks a close second. It is spread by common roots between diseased and non-diseased trees and by the nitidulid beetle. This little bug is very fond of the oak sap that will appear if there is an open wound in the tree, such as from pruning or damage from a lawnmower or a weedeater. If that bug has visited an infected tree and fed on the sap, he will have the spores of the fungus that causes Oak Wilt on his body, which can then infect the new tree. We bring all this up because you should not prune nor otherwise damage a Live Oak when the beetle is active, which is now until about June 30. In fact, it is better to prune only during the coldest part of the winter.

2. What do I do about the tree? We pointed out the dangers of Oak Wilt so you wouldn't rush out and whack all the offending branches off. No doubt it will help if some of those low branches shading everything around can be pruned, not to mention all those suckers that have been allowed to grow, but we strongly recommend that you (a) wait for December and (b) get a trained, licensed arborist to do the trimming.

3. What can I do in the meantime? That mess needs to be cleaned up before you can make any decision about what to plant and when. The acorns, twig debris, leaves and other trash need to be cleared away and disposed of. If you have room for a compost pile, that would be a good place for the leaves. When the suckers have been cleared away (remember - December) you can start evaluating if anything will grow there, which brings us to...

4. Is anything going to be able to grow under that oak? We don't blame you for wanting something colorful, but that is going to be a problem. Most colorfully blooming herbaceous plants and shrubs need a good quantity of sunlight to fuel their blooms. We consider full sun to be 6 or more hours of sun a day, part shade 2 to 6 hours of sun, and shade less than 2 hours of sun a day.There are other factors besides heavy shade that might be causing problems in getting plants to stay alive, including the fact that oak roots tend to be in the upper 12 inches of the soil, and form a mat that would be discouraging to other plant roots. In addition, there is the question of allelopathy. From the University of California Cooperative Extension article Landscape Notes by James Downer, Farm Advisor, we have extracted this paragraph about the allelopathy of oaks:

"Various studies have demonstrated that oaks can have allelopathic affects on surrounding plants. Allelopathy is the production of plant inhibiting chemicals by one plant to regulate the growth of others in its vicinity. One important group of chemicals produced by oaks is tannins. They are produced in leaves and litter and also directly by root systems in soil. Tannins are inhibitory to many organisms. Salicylic acid and other organic acids are also produced by oaks and are toxic to other plants. Allelopathy is species specific for the oak in question and the species that is inhibited."

In other words, it depends on which plant and which oak, and we don't have lists of plants that will grow under specific species of oak.

We realize that what we have recommended is a lot of time and work and probably expense, just to get started. You did ask.