We have black locusts invading our black raspberry patch, as is well known, just cutting them doesn't really work long term. Has anyone had experience treating the cut stumps with Roundup or similar product?
The shoots in the garden are from 1/4 inch diameter to 1 1/2 inch diameter. the apparent parent tree is about 26 inch dbh and is only about 40 feet from the patch. Will the parent tree and others further away be in jeopardy?
Cut stump treatment to the small stems should kill them. It might however kill the parent tree 40 ft away also. I suggest either a dilute roundup (less than 10% active ingredient of glyphosate, and I’m guessing!) or a herbicide without glyphosate. If you use glyphosate, you could try a 2% or 3% dilution and see if that controls the suckers. See if you can find Pathfinder II (www.arborchem.com) or anything with Triclopyr (Garlon family). Triclopyr is less mobile than glyphosate, but still a bit mobile.
If you could avoid the raspberry, you could also spray the locust foliage as it sprouts from the cut stump.
The suckers are likely coming from the big tree. I suspect this will be a persistent problem, but you can lessen the annual work by controlling these root suckers with herbicides. As you found cutting will only stimulate more suckers. There is some risk of death to the parent tree, and that risk increases as you increase the surface area you treat at one time with glyphosate. The glyphosate will accumulate to the most actively growing portion of the tree which is likely the parent tree.
As a reminder, the May 20th webinar is on forest herbicides. See more information here
http://www2.dnr.cornell.edu/ext/forestconnect/web.htm and on this site in the events.
I agree with Peter's advice. 2,4,D may be another readily-available and less-mobile herbicide that could be used (but I personally don’t like using it due to the volatility/smell). You would also want to check to make sure that it's labeled for cut stump treatment on Black Locust. We’re treating suckers here on the farm that continue to creep into areas where we don’t want them. I cut them off with loping shears when ~ 1” in diameter and treat the cut surface. I haven’t noticed any flash kill yet on adjacent locust trees, but I’m not treating a large number of stems at any given time. Annual follow-up seems to keep things in check.