Grab a saw and make a fresh stump cut. Have full strength round-up with you. As soon as you make the fresh cut, use the round-up generously on the outer 2-3 inches of the stump all the way around the stump. That is the cambium and it will immediately suck the the round-up into the stump and into the roots.
i prefer on a warm day, going into the warmest part of that day,
Aiming to command to slurp as much RoundUp into the cold blooded system by the patient.
i also tic-tac-toe hash-tag cuts to
>> expose more surface area
>> and give system of nooks and crannies for catching and hiding water and biologicals
>> and leaving micro fissures into wood separating single fibers, rather than smooth clean finishing cut.
else, the exact opposite/inversion to opposite paradigm of Doc Shigo prescription for clean cut that does not promote rot.
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Buttermilk supposedly is good bio inoculation to do few times a yr.
>> But any bio-active probably better than none
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Burning roots underground risks fire finding mass of mulch, other decaying pockets.
Tho fastest burn is by force feeding oxygen as fuel w/blower!