As good as they are, oak trees still bring out lots of questions, and spring is a prime time for some of the most interesting. I’ve assembled a sampling.
1. “What are these odd growths on the leaves and twigs of my oak trees? Do they do any damage? How can I stop them?”
These are insect galls, and there are many types. The adult female, often a small wasplike insect, stings the leaf or twig tissue as she deposits her eggs. The leaf develops the contorted growth in response to the sting, and the larvae develop within the gall. Eventually they emerge, fly away, mate and begin the process over again.
Woody oak galls on live oaks are among the most common. They will develop a little later in the season. They look like wooden marbles on the small twigs. Wooly oak galls will look like creamy white, fuzzy growths the size of English peas on the backs of live oak leaves. You’ll see pocket vein galls on Shumard red oaks as well as oak blister galls and several others.
There is nothing much you can do to prevent or control galls. The female insect is only there for a short while, so your odds of hitting her with a spray are almost zero. And she’s not feeding on the plant, so that door is shut as well. But they do little harm, so there’s really no call to action.
2. “My oak tree has several roots that have worked their way to the top of the soil. I don’t know if there has also been some erosion. Can I put topsoil on top of them?”
Do not add topsoil whatever you do! There has been no erosion. The fact is that 90 percent of any tree’s root system, oaks included, will be in the top foot of soil. That’s where rainfall hits, and that’s where organic matter decays to release its nutrients back into the soil. Add that all up and it means that that’s where the trees’ roots need to be in order to compete with the dense roots of grasses and shrubs.
As a tree’s branches grow in diameter, so will its roots. It’s almost a mirrored effect. The roots grow thicker and thicker until they extend up above grade. Look at old-growth live oaks along the Gulf Coast and you’ll see surface roots an amazing 18 to 30 inches above the surrounding grades! Don’t obsess about a few roots showing around your trees. Remove any that are hazardous. That’s best done in October, after the worst of our hot weather has passed. Learn to tolerate the rest, or conceal them with a tall ground cover.
3. “I have tons of little live oaks beneath my old tree. How can I get rid of them?”
Check first to see if you have seedling trees or root sprouts. Dig a few of them up carefully using a sharpshooter spade. If there are acorns attached to each one of them, you have seedling trees, and mowing alone will get rid of almost all of them after two or three run-throughs.
If, on the other hand, you find that the little trees are tethered to a large umbilical root of the mother tree, they are root sprouts that will keep coming back up. Unfortunately, that’s a genetic blemish of 10 to 15 percent of our live oaks, and about all we can do is dig them out by hand. If you were to apply a herbicide to kill them, it would be carried back to the mother tree and do damage to it as well. Instead, use your sharpshooter spade to slice into the soil at a 30-degree angle. Sever the sprouts and pull them out.
4. “Are borers causing these holes in the trunk of my live oak? What should I do?”
The holes are not caused by borers, but instead by woodpeckers or sapsuckers. The birds will move around the trunk drilling holes that make the trunk look like it’s been strafed by a machine gun. It doesn’t suggest that there are any insects in the trunk, nor do the holes invite insect invasion. The birds may come back to feed on sap that flows from the wounds, but there’s really little harm done unless they riddle the trunk in complete rings.
You can’t do anything to harm the birds because they are protected species, but you can discourage their activity by applying the very sticky Tree Tanglefoot, or if you don’t want all that mess, by sealing the holes with an aerosol pruning paint.
5. “My live oak has moss growing on the trunk. Is this any cause for worry?”
These are lichens, and they are of no concern. They are a communal growth of algae and fungi that nourish one another. They gain only support from the tree trunk and limbs — no water or nourishment. In fact, you’ll find them growing on “mossy” boulders at the stone yards in town. People pay extra to get stones with a full population of lichens.