The drought takes a toll on trees

QUESTION:  I have a concern about two hackberry trees in my yard. They are quite old and beautiful, but the leaves are turning yellow and are falling off rapidly.  Is this due to the high heat or another problem? I would sincerely hate to see these lovely trees die.  

During drought, a hackberry’s leaves may turn yellow and fall. Mature, healthy trees should recover.

Let’s just say that while the high heat is a major culprit in the early-summer yellowing of a lot of things, it’s not acting alone. “It’s drought primarily,” says Alan Windham, the ornamental and turfgrass pathology expert at U.T. Extension.

“I actually witnessed this, this morning on hackberry behind my home while walking my dogs,” he said when I sent him this question earlier this week. “It was breezy, and yellow leaves were falling from the hackberry like a fall day.”

Most likely, this leaf-fall is not a fatal condition. “Most mature trees have survived many droughts, and should be able to survive this one,”Windhamsaid. “Regardless, I’m hoping for rain this week.”

Young trees that haven’t had a chance to develop an extensive root system are more vulnerable. An email alert from the Nashville Tree Foundation, which plants trees in public places and private yards on its ReLeafing day each November, has sent an alert with watering guidelines and new tips that you can read here to help young trees survive. Most important: water thoroughly, and water slowly to prevent run-off.

The long-range weather forecast may show some relief. Next week, it looks like temperatures will be in the more reasonable mid-to-upper 80s, with a chance of rain.

Coming up

July 14:  Middle Tennessee Iris Society rhizome sale, 8 a.m. – 3 p.m. at Ellington Agricultural Center. All types of irises available in a variety of colors, priced from $1 – $6. A great chance to add to your collection! To learn more: For additional information see our web site www.middletnirisociety.org.

July 17: Middle Tennessee Orchid Society meets at 7 p.m at Cheekwood. On the agenda: an orchid auction and discussion on Cattleya Orchids. To learn more, visit www.tnorchid.com.

July 17: Perennial Plant Society meets at Cheekwood in Botanic Hall. Refreshments and plant swap at6:30; meeting and program at 7. Guest is Kelly Norris, horticulturist, plant breeder and manager of Rainbow Iris Farm inBedford,Iowa will present “The Modern Plant Explorer.” The meeting is open to the public.

July 26: Farmer Jason performs at Family Night Out at Cheekwood, 6:30 p.m. The event is free with regular Cheekwood admission. www.cheekwood.org.

Fall — and aphids — in the air

QUESTION: Sometimes we see white, wooly-looking bugs in the air and all over the hackberry trees in the fall. At the same time, there is a black coating of something on everything under the trees. What is this?

These are Asian wooly hackberry aphids, which have been flying around Middle Tennessee for about a decade after making their way to the U.S. sometime in the mid-1990s. The aphid has a tiny mouth that resembles a beak, with which it pierces the leaves to extract the plant sugars. Then, like all aphids and
plant-sucking insects, they excrete a sweet, sticky waste product called honeydew, and in the heat of late summer, that substance grows sooty mold, the black substance you see on patio furniture, plants and anything else under a hackberry tree that is playing host to the insects.

I talked to U.T. Extension agent David Cook about the insects several years ago. He said that the feeding doesn’t cause serious damage to the tree. It’s considered mainly a nuisance pest.

If you feel the need to control the insects, Cook has suggested a systemic insecticide, applied around the tree’s root zone in the spring.

IF YOU THOUGHT last month seemed hotter than usual… you were right. Bobby Boyd at the National Weather Service sent me a list of factoids that made me sweat just to read them. Consider this:

August 2011 in Nashville was the 25^th hottest on record. There were 24 days when the temperature was 90 degrees or higher, and one day when it bubbled up to 102. That was August 3, and it broke the record for that date of 100 degrees, which was set in 1952.

The average temperature for August was 1.8 degrees above normal. The average high was 92; the average low was 68.9. Three of the 10 hottest summers in Nashville have occurred in the last five years.

All this makes these past few days of cooler weather even more welcome. Fall, ready or not, is in the air.