Dear Neil: Can a walnut tree growing near a vegetable garden affect it?
A: In terms of sucking water and nutrients out of the soil, yes. In terms of shading garden plants that would really do better in full sun, yes. In terms of dripping oils that might inhibit good growth into the soil, probably not. That's called allelopathy, and it really gets overplayed. If you're preparing the soil properly, the tannins from walnuts and pecans won't matter nearly as much as the shade and loss of water and nutrients.
Dear Neil: I saw your comment that letting grass get tall weakens it. My St. Augustine lawn is being mowed twice per month, and it gets pretty tall between mowings. Is that bad for the grass?
A: It's very bad. My guess would be that the lawn is badly scalped-looking after each mowing. The truth is, St. Augustine and bermuda should be mowed every 5 or 6 days during the summer, St. Augustine at 2 to 2.5 inches and bermuda at 1.25 to 1.5 inches. Mowing close will keep them low and dense.
Dear Neil: We have several red oaks in our yard. Every one of them has branches that are turning white and dying out. Is this some kind of oak wilt? It has not affected live oaks. Should we have the trees sprayed?
A: It really would help to be able to see the trees, but it doesn't sound like oak wilt. There is an insect gall that bothers red oaks. It will cause small branches to die from the tips, back 10 or 15 leaves. The leaves turn light brown/tan and remain attached. If you look at one of the twigs closely, you'll see a swollen "knuckle" on the branch. If you see that, that's the gall. There are no sprays to control or prevent galls, but the good news is that they don't do any major damage.
Dear Neil: What would cause St. Augustine to turn yellow this time of the year?
A: Three things come to mind, including iron chlorosis in alkaline soils: yellow blades with dark green veins. Control it with additions of iron/sulfur, but keep iron products off masonry and painted surfaces that could be stained. Second, and more likely, is gray leaf spot (yellow, singed-looking blades in washes across the lawn). Lessen it by waiting until September to fertilize with a high-nitrogen or all-nitrogen lawn food, also by applying a labeled lawn fungicide. Third would be chinch bugs (drying, dead-looking parts of the lawn, always in the hottest, sunniest part of the yard). Apply a labeled turf insecticide to the entire lawn to eliminate chinch bugs.
Dear Neil: Should I be removing long, spindly branches from my new crape myrtles? Do I train them as shrubs or trees?
A: Prune crape myrtles as little as possible. However, unsightly branches can be removed entirely (leaving no stubs). Crape myrtles are all shrubs genetically, but we choose to train some of the larger types to grow as small accent and patio trees.
Dear Neil: I have a Texas persimmon tree that is losing its bark in long chunks. What does that mean, and is there something I should be doing to protect the tree?
A: That's probably just the normal exfoliation of the bark of your persimmon. Many trees do that even more dramatically, including sycamores and crape myrtles, among others. It's probably no cause for concern, especially if the tree looks healthy otherwise.
Dear Neil: When, and with what do I fertilize my fig tree?
A: Don't fertilize it much at all. Figs compete favorably for nutrients, and adding more fertilizer will often weaken the plant. Keep them mulched and moist, and they'll take care of the rest.
Dear Neil: Do Bradford pears ever have fruit?
A: Yes, sometimes. They'll be marble-sized (or less), the same as its mother (callery pear) has, just not as many. If the tree bears fruit that's any larger than that, it's not a Bradford or any of the other ornamental pears.
Dear Neil: What can I do to cope with nematodes in my vegetable garden? They're killing my tomatoes.
A: There is no perfect cure for these microscopic soil-borne pests. They'll cause swellings on the tomatoes' roots, and they'll lead to very early death of the tomato plants themselves. Elbon cereal rye will trap many of the nematodes in its root system. Plant it in October, and plow it under in January. Choose nematode-resistant varieties. The label will have several initials following each variety's name. The initial "N," when it appears, indicates nematode resistance.
* If you'd like Neil Sperry's help with a plant question, drop him a note in care of The Eagle, P.O. Box 3000, Bryan, Texas 77805. Or e-mail him at mailbag@sperrygardens.com.
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