To kill tree roots, kill the tree that feeds them: cut it and treat the fresh stump at once with a herbicide labeled for cut stumps, or treat the standing tree, so the chemical travels down with the sap into the root system. Roots of a tree that is already down and not sprouting are dying on their own, and products poured on them months later do very little.
Roots live on what the leaves send down
Roots do not make their own food. Leaves make sugar and send it down, and the roots store it as starch. Cut the tree and that supply stops, but the stored reserve does not vanish. That reserve is what pushes up sprouts round a fresh stump or out of the lawn. Once it is spent and nothing green is left to refill it, the roots die and rot. So for most trees, cutting the tree and keeping it from leafing out again kills the roots in time. Two kinds of tree complicate this: species that throw up shoots from their roots, and trees joined underground to a neighbor.
Herbicide has to ride the sap
A herbicide reaches roots only by moving through living tissue. Penn State Extension explains that cut-stump treatment targets the sapwood, the lighter band just inside the bark, so the chemical travels down into the roots. NC State Extension says to treat the outer inch on bigger stumps, to get water-based products on as quickly as possible after cutting, and to use oil-based products within a two-hour window. Season matters as well. Penn State says spring treatments are often ineffective because sap is flowing upward, and prefers late summer and fall, when trees move their reserves underground and carry the herbicide with them.
Three ways to get it in
Cut-stump treatment goes on the fresh face of a stump right after the saw. Hack-and-squirt is done on a standing tree: NC State describes cuts made in a ring round the trunk, each one sprayed, which suits a few trees in a small area. Basal bark treatment is sprayed on the lower trunk, and Penn State lists it, with hack-and-squirt, as the better choice for tree-of-heaven, which resprouts heavily after cut-stump treatment. Products differ in what they are labeled for, how much to use and where. The label decides every one of those things, so read it before you cut, not after.
Why late treatments disappoint
Most people go looking for a root killer once the tree is long gone and the stump is grey. By then the wood is not moving sap, so anything poured on it stays near the top. Iowa State University Extension says herbicide poured into drilled holes or over a stump well after the tree was removed is not beneficial for decay, and mainly ends up in the soil. It also advises against products containing picloram, sold as Tordon, in a home landscape. If an old stump is still sprouting, the sprouts are what can carry a herbicide down. If it is not, the roots are already on their way out.
Shared roots and the tree next door
Here is the warning that matters most. University of Arizona Extension notes that trees of the same species growing near each other can share a common root system through root grafts, and that herbicide applied to one can travel to the others. Suckers that come up from the roots of an adjacent tree are part of that tree. Its advice is not to put herbicide on any shoot that rises from the roots of a tree you mean to keep. Before you treat a stump, look at what stands near it. A row of the same species along a fence line is exactly where this goes wrong.
Roots from a tree you are keeping
Sometimes the roots you want gone belong to a healthy tree: in a vegetable bed, a sewer line, under a walk. Killing those roots harms the tree they feed. Cutting them has a cost too. Clemson Extension's notes on oak diseases name damaged roots, from building work or trenches, as one reason oaks fall to root rot and cankers. Roots travel as well: by NC State Extension's figure they can reach several times as far as the branches, so a tree across the yard may still be in your bed. Options are cutting a few roots carefully, moving the bed, or taking the tree. For lines and drain fields, see tree roots and septic systems.
After the roots are dead
Dead roots are harmless under a lawn. They rot over the years, the soil above can settle a little, and that is the end of it. The stump is a different matter, because it sits there for a decade or more. Once it has stopped sprouting, stump grinding takes it below grade in one visit so the spot can go back to lawn. If the reason you wanted the roots gone is a slab, a footing or a new tree, killing them is not enough. The root ball has to come out. For what dead roots do next, read what happens to roots after stump grinding.
Common Questions
What kills tree roots permanently?
Killing the tree they belong to, with nothing left to sprout. A herbicide labeled for cut stumps, applied to the fresh cut right away, travels down into the roots of most trees. Without herbicide, cutting every sprout as it appears starves the roots more slowly. Root-suckering species and trees joined by root grafts need more care, and the label rules either way.
Will cutting down a tree kill its roots?
Eventually, for most trees, as long as nothing green grows back to feed them. The roots live on stored reserves for a while, which is why many cut stumps sprout. Remove those sprouts or treat the fresh cut and the roots run out and die. Some species keep sending up shoots from the roots for a season or two.
Can I kill one tree's roots without hurting the tree next to it?
Usually, but take care with trees of the same species standing close together. Their roots can graft, and University of Arizona Extension warns that herbicide can then move from one tree into the other. Do not treat suckers that come from a tree you want to keep, and ask your county Extension office before treating near a valued tree.
What to Do Next
Once the roots are dead, the stump is the part still in the way. A picture of it beside a shovel or a boot, plus your town, gets a free price for grinding it below grade. Ring (828) 574-4474 or email it. Tree still standing? Send that photo too and we will talk through the options.