Since your neighbor has cut the tree down, she must have reluctantly come to terms with losing her Aspen tree. Cutting the tree can encourage even more suckers and it could re-sprout, so the ideal situation now is to kill the stump. Ideally she would have cut the tree and painted it immediately with herbicide. If this did not happen, and the cut is now healed, you might ask her to try cutting the remaining stump and painting it with triclopyr or glyphosate or grinding the stump out. Page two of this guideline describes how to treat the cut stump with herbicide.

Even if the stump is removed, cutting the suckers in your yard and painting them with triclopyr will help manage your sprouts and eventually weaken the rest of the root system.

Regarding the risk this poses to her birch tree; there is conflicting information about whether triclopyr can harm neighboring trees.

According to Penn State Extension “herbicides containing the active ingredients glyphosate or triclopyr…have practically no soil activity and pose little risk to nontarget plants through root uptake.” On the other hand, the National Pesticide Information Center says that triclopyr is mobile in soils.

Another publication from Penn State Extension states “Caution: Herbicides can potentially kill or injure nearby trees through root grafts. Restricting treatments to tree species different from desirable crop trees will minimize this concern. Herbicides containing glyphosate (e.g., Rodeo) and imazapyr (e.g., Arsenal AC) are more prone to movement through functional root grafts than herbicides containing triclopyr (e.g., Garlon 3A and Vastlan).”

This publication from Washington State states that “Unrelated plants are unlikely to form root grafts [and] field research indicates that glyphosate and other translocatable herbicides do not cross root grafts in healthy trees.” However, “Fungal vectors can breach root grafts through degradative enzymatic activity [and] Root grafts that have already been breached by fungi may serve as conduits for herbicide translocation as well”.

Ultimately, you may need to speak with her about management options and the potential risk to her birch tree. As always, when using herbicide read the label carefully and follow the directions exactly.

Unfortunately, dealing with Aspen suckers is quite challenging and requires diligent management over time and ideally the removal of the parent tree.