MDV...There are two trees together....a species which tends to root-sucker if disturbed (Robinia is a classic for this)...and you're wanting to remove only one, but keep the other....AND theres a good chance they're root-grafted. IS that correct?
Any translocating herbicide has the potential to move into the specimen you wish to keep...(and kill the non-traget).
Option....continual supression of the regrowth ....it will eventually exhaust itself. You could try something phytotoxic....boiling water comes to mind immediately...if you have the patience to do it, that is. Boiling water poured directly on the re-shoots...before they get too big and woody.
and here some other food for thought...
The scenario is Willow (or other) removals where herbicide is undesirable.
I had a case of an commercial Organic Vegetable Garden where herbicide use was not an option (contravenes 'Certification' status)...Stump grinding was out too...these Willows were standing in a watercourse ....so dozing over was out too.
In fact it was quite important to retain root-mass.... for bank binding...at least for an interim period anyway.
The solution, after much research...and as recommended by Dept. Natural Resources....cut off all vegetative growth to a log abt. 1m high...make numerous vertical cuts into the face of the stump, such that the injuries would be suceptible to pathogens entering.
Return 6mths to inspect...remove re-growth if it occurs ... take off about 200-300mm from the stump and re-injure...
Continue in this way .. inspect, remove regrowth...re-injure until the decay wins.
This is the certified Organic Garden...the willow infested creek is in the background...(there were a couple of Black Willows in the group too).
Organic beds.jpg Willows.jpg
All up there would be about 50 trees...and in another area there were Coral Trees.
Willows 2.jpg
The water quality is so bad...because the week before I visited, the had owner spent the weekend on a tractor pulling the trees out...wholesale.