Early September is the time of transition from summer’s hot weather to the cool-down of fall. With the changes that begin to happen about now come a lot of questions from North Texas gardeners. I’ve assembled the ones I’m being asked most often.
How late can I apply pre-emergent weedkillers and still have good results against winter weeds?
You’re there now! You need to get Team, Dimension, Halts or Balan granules out in the next several days to prevent germination of annual bluegrass (Poa annua), rescuegrass and ryegrass. This is your only chance to deal with these grassy weeds. Once they germinate you will not have another opportunity until this time next year (next generation of the weeds).
Make an additional pass over the lawn to control broadleafed weeds such as henbit, chickweed, dandelions and clover. Apply Gallery granules for the non-grassy weeds. These products are most likely to be found at independent retail garden centers.
Why is my St. Augustine yellowed in big patches? It’s been that way for much of the summer.
In most cases that’s been gray leaf spot. It’s a fungal leaf spot that also appears on runners. The gray-brown lesions are BB-sized and irregularly diamond-shaped. Gray leaf spot has been especially troublesome this year, and many people have reacted by applying nitrogen fertilizer in an attempt to green up their turf. Unfortunately, gray leaf spot is exacerbated by nitrogen in hot summer weather, so those people have really added to their own turf troubles.
At this point, if you’re still seeing the disease actively attacking your lawn, hold off one or two more weeks before you fertilize your turf. If you’re one who applied nitrogen fairly recently, perhaps you shouldn’t fertilize again until next April. There are fungicides that will help with gray leaf spot, but it’s very late in the season to be applying them. Like some of the pre-emergent weedkillers, they’re most commonly sold at independent retail garden centers.
What is wrong with my roses? They didn’t bloom very well last spring, and they’ve looked worse and worse all summer. What can I do to help them now?
Odds are high that your roses have rose rosette virus. It’s a fatal virus that somehow has decided that the Fort Worth/Dallas area is where it needed to appear in epidemic proportions. It’s been so bad over the past five or six years here that there are almost no healthy roses remaining. Sales have dropped to almost zero. First symptoms are rank-growing, extremely thorny “bull” canes, but later they become stunted. RRV is transmitted great distances by wind-borne microscopic mites. There is no control for the virus, and there is also no prevention or control for the mites.
About all you can do is replace the roses immediately with some other type of shrubs or flowers. Hopefully the massive research now underway will find a work-around, genetic or otherwise, for this devastating virus.
Why are the leaves of my morning glories, marigolds and other plants turning tan, then dried and crisp?
Spider mites will do that. They feed on the undersides of the leaves, sucking the green color out of the leaves in the process. When the damage becomes severe you will see fine webbing in the leaf axils, but by then it’s usually too late to save the plants. If you see the tan mottling, thump a suspect leaf over a sheet of white paper. If you see tiny paprika-colored specks starting to move about on the paper, those are the mites.
Apply a general-purpose insecticide that is also labeled for spider mites to control them. Spray the bottom leaf surfaces as well as the tops. Check the plants a couple of days later to be sure you’ve gotten good control.
There is an insect pest that causes similar damage to another group of plants. Lace bugs attack pyracanthas, Boston ivy, sycamores, bur oaks, chinquapin oaks, azaleas, American elms and boxwoods, among others, turning their leaves the same pale tan color. However, on the backs of their leaves you’ll see black, waxy specks. Those specks are the excrement of the adult insects.
They began doing their damage in June, and there may not be any more of the adults still present on the plants. Systemic insecticides do a good job of preventing this damage if applied in early summer. There is no call to treat this late in the season.
“What is causing the rows of holes in the leaves of my cannas?”
That would be canna leafrollers. The larvae tie the leaves together while they’re still tightly rolled. They feed on the leaves much as if you drilled a hole through a rolled-up newspaper. When the leaves unfurl they look like they’ve been sprayed with a machine gun. A systemic insecticide applied as a soil drench in late spring usually will prevent this damage from happening.