Black Locust (Robinia pseudoacacia)
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).