I've had a black walnut tree voluntarily growing in my lilac bush. I've repetitively cut it to the ground but it keeps coming back & it's beginning to affect the lilac. What can I do to kill the walnut without harming the lilac. Is it possible to drill holes in the walnut stump and pour some kind of stump killer in the holes? The lilac is part of a tall long hedge so digging it out and replacing it is not really an ideal option.
Scrape the top of the stump until the wood at the top is fresh. Paint the fresh cut with Roundup and repeat until it stops sprouting. This process will not hurt the lilac. It may take several applications.