How to kill a tree roots

Could some one give me some advice please.
I have had a tree cut down about 8 weeks ago leaving just a 5/6 inch high stump and around 10/12 inches in diameter it was starting to affect a 5 feet retaining wall
I have drilled about 30 holes three quarters of an inch in diameter and about 5 inch deep in the top of the stump to put in Roundup solution
I have bought some Roundup Concetrate to kill the roots, will I need to fill up these holes more than once and how strong should the Roundup solution be to make sure I kill the roots

 

I use SBK, and plug the holes in case it effects wildlife, not sure it would .

I think neat glyphosate might work also, but depending on tree type you might need to do it more than once and if its a suckering type tree, you might need to spray any suckers.

It would have probably been best if you had had the stump ground out.

 

What sort of tree? Something like ash is likely to sprout (but not if you've treated it with glyphosate! You'll only need to do that once, and a few ml of 360g/l glyphosate should suffice). You can buy granular potassium nitrate to pour into your holes; you'll need to keep the stump covered so that it stays moist to hurry the rot up.