Q. In the summer of 2009 I had three thornless honeylocust trees removed from my yard, including the stumps. This year, dozens of little locust trees sprouted where the old roots are located. Mowing them did not prevent them from growing, and using Round Up did nothing either. How do I get rid of the sprouts short of digging up the roots?
A. Round Up (glyphosate) is not always the best choice for killing woody plants such as the suckers growing from the roots of those thornless honeylocust stumps. For one thing, it works best on green tissue such as leaves. Since you kept mowing the sprouts, there was little leaf surface to absorb the herbicide.
The ideal approach would have been to use a brush killer such as Ortho Max Poison Ivy & Tough Brush Killer (triclopyr) or Bonide Poison Ivy & Brush Killer (ester 2,4-D) to treat the stumps immediately after the trees were cut down. Known as cut-stump treatment, the undiluted product is applied with a paintbrush to the surface and sides of freshly cut stumps where it is absorbed into the their vascular systems and translocated out to the roots. You may have had a few re-sprouts, but nothing like you are dealing with now.
Some stumps, like this spruce tree stump, don't sprout
The stumps could have been ground out the following summer. Such targeted applications are more environmentally friendly since the herbicide is only applied to the target, rather than broadcast over an entire area.
Late summer and early fall are best for such treatments because plants are translocating the products of photosynthesis down to the roots for storage over the winter. They tend to absorb and move herbicides down to the roots most effectively at time of year.
Although it is getting late, you might try a targeted brush killer application. In this situation, you can apply the undiluted brush killer with a pump up sprayer rather than a paintbrush. Do not pour more into the tank than you will use since it is not good practice to pour the leftovers back into the original container. Mow the sprouts to create wounds, then immediately spray them with undiluted brush killer. It may not work as well as it would have a month ago, but it is worth a try.
Although this is a labeled application, be very careful since you will be handling the concentrated herbicide. Be sure to wear long sleeves, long pants, shoes and socks as well as chemical-resistant gloves as recommended by the label.