North Texas summers bring out worst in shade trees. How to take care of the problems

Gardeners stuff their pockets with questions this time of year. Heat and dry conditions bring out the worst in our plants, notably shade trees, and here are some of the things that are front and foremost on people’s minds.

“My tree’s leaves are turning yellow. Do I need to add iron?”

That depends on the tree species and the exact symptoms you’re seeing. Iron deficiency will only impact trees that require acidic soils to grow successfully. That would include East Texas pines, dogwoods, true pin oaks (Quercus palustris), water oaks, occasional pear trees and magnolias and cherry laurels. (I’m probably leaving out a few, but those come to mind first.) If you have one of those trees, and if the yellowing is most pronounced out at the ends of the branches on the newest leaves, and if the veins of those leaves remain dark green longest, that does sound like iron deficiency.

There is a warning that you need to heed when you start trying to correct iron deficiency in large trees. Adding iron (and soil acidifier) to the root system of any of those trees is probably just going to be a short-term fix. The tree will continue to grow larger, and the amount of iron required to correct the problem will become greater and greater. In most cases you’re better off replacing the tree with a species that’s better adapted.

But wait! There’s more! Yellowed leaves will much more commonly be due to low humidity, dry soil, and high temperatures. Trees simply can’t pull water through their systems fast enough to keep up with the demands and they start shedding those yellowed leaves. (Plants with iron-deficient leaves do not shed their foliage in early August.) Plants under moisture stress exhibit it first on the oldest leaves farthest down the branches toward the trunks. If that’s your problem, be more attentive to your trees’ water needs.

“How often should I be watering my large shade trees, and what is the best way to do so?”

If you have turfgrass or landscaping beneath large trees, and if you’re watering those plantings to keep them growing and vigorous, your trees will probably be able to compete adequately for the water they need.

In the off chance that you need to water a large tree that’s alone in the landscape, it’s best to use a soaker hose. The most critical roots are those out at the drip line. To explain, think of the tree’s leaf canopy as an umbrella. The “drip line” is where water would be dripping off the umbrella. Position the soaker hose in a long spiral there. Adjust the spiral by a couple of feet every week or two as you repeat.

Gimmicky devices are of little value in watering large trees. In that category I’d include rods you push into the soil. You would miss most of the surface “feeder” roots with them. The rather expensive refillable trunk water bags would certainly go onto that list as well. Except for fairly small, brand-new trees, that’s not where the water needs to be applied.

“What is the mist that is falling from our pecan trees?”

That’s casually known as “honeydew.” It’s the secretion of insects after they feed on the sap of trees’ leaves or twigs. It will be most noticeable when you’re looking toward the light in late afternoon or early evening. You’ll often be able to feel the tiny droplets hitting your face or arms, and you’ll certainly see them accumulating on the windows of your car or your house.

Think back just 11 short months ago. That honeydew from pecans and crape myrtles coated walks, streets, patio furniture, rooftops and all other horizontal surfaces late last summer. Most of us found our shoes sticking to our walks and driveways as we tried to step out for the paper, walk the dog, or move the hose. Insects responsible back then included aphids on the large shade trees and pecans, also on crape myrtles. Crape myrtle bark scales also put out large amounts of honeydew when they’re present, as do lace bugs on chinquapin oaks and several species of elms. All are reasonably easy to control if you treat early in their life cycles.

“My 3-year-old red oak’s bark is cracking and pulling away from the trunk. Should I be concerned?”

Yes. That’s probably a sign of sun scald damage to the trunk. I’m getting more questions about this damage this year than I’ve ever had before.

I looked at 20 oaks and maples in a city park that were planted two years ago. (It takes about that long for the problem to manifest.) A recent caller to my radio program described multiple Shumard red oaks lining a rural drive. Sunscald and associated borer invasion. may end up killing all of those trees.

Sunscald appears on the west sides of the trunks because that’s where the sun bakes the trunks the first summers that new trees are taken out of the protective nursery environment and planted into the wide-open exposure of our home landscapes.

You normally see vertical cracks developing in the bark the second or third year. Soon thereafter the bark separates and peels away from the wood of the trunk. At that point the phloem, the cylindrical conduit for sugars manufactured during photosynthesis in the leaves, has been so damaged that the food supply to the roots is interrupted and the roots die.

The sad thing is that all of this damage and the loss of the tree could have been prevented by use of paper or plastic tree wrap to shield the trunk from the intense rays of the sun from the day the tree was planted. It takes only five minutes and a few dollars to protect any tree, yet nurseries and landscape contractors fail to suggest this one final step in the transplanting process.