I just moved into my house last summer. There was a small tree (3" dia.) growing in one of my flower beds. I cut it down last fall, leaving a stump about a foot high to perch a birdhouse on. Now it keeps sprouting new shoots along the sides of the stumps. I cut them off, but new ones appear. How can I discourage the new growth? I don't want to use a killer because it's in a flower bed.