Q. I had a black locust tree cut down at the rear of my property two years ago. It was over 50 feet tall and about 18 inches in diameter. The stump is still there. Not long after it was cut down, shoots began growing from the stump and off the roots that had grown under my lawn. I have been treating them with an herbicide that only works about half the time. They grew all last year and are growing again this year. What can I do to get rid of them once and for all?
A. "Cut one down and 20 come to its funeral" is a common saying among workers who maintain utility rights-of-way in reference to suckering tree species such as black locust (Robinia pseudoacacia). Farmers prize black locust fence posts for their rot resistance and longevity. Unfortunately, that means that the stump and major roots will be around for a while since you did not have them ground out when the tree was removed.
The reason for your spotty success with herbicide applications may be a matter of timing as well as the herbicide you are using. Generally speaking, it is best to spray hard-to-kill species in late summer or early fall.
At that time of the year, carbohydrates produced via photosynthesis are moving to the root system for storage so the tree has the reserve to leaf out and bloom the following spring.
Herbicide applications at that time of year are taken down to the root system more easily, increasing your chances for good control. Spring applications may have little effect since those carbohydrates are moving up through the tree (“the sap is rising”), and the herbicides are not moved to the roots for a more complete kill.
It is too late for this now, but one way to reduce re-sprouts from suckering tree species is to cut the tree down in late summer or early fall, then treat the freshly cut stump with a brush killing herbicide such as Ortho Max Poison Ivy and Tough Brush Killer (triclopyr).
In such a case, the undiluted concentrate is applied to stump with a paintbrush as soon as possible after the tree is cut down. (You do not want to wait very long before treating the fresh cut, because it will begin to callous over and will not absorb the herbicide well). The vascular tissue absorbs and translocates the herbicide down to the roots. Although you would have some re-sprouts, there would not be as many as you have now.
At this point, the best approach will be to mow them through mid-July, then grit your teeth and allow the sprouts to grow. You need a decent amount of leaf surface to absorb the herbicide so that enough of it is taken in and translocated to the roots for better kill. Spray the sprouts with a diluted triclopyr solution (follow label directions) in late August or early September.
It is a selective herbicide that will not kill your grass. You may need to make a second application two weeks after the first if you see still see undamaged sprouts, and you may need to repeat this treatment next year to get complete control.
This cut stump approach works for tree-of-Heaven (Ailanthus altissima), sumac (Rhus spp.), poison ivy (Toxicodendron radicans), wild brambles (Rubus spp.), and multiflora rose (Rosa multiflora).