After stump grinding the roots stay in the ground, use up the food stored in them and die, then rot over several years as soil fungi break them down, the fine roots first and the thick ones near the old trunk last. The exceptions are trees that sucker from their roots and trees whose roots have grafted onto a living neighbor of the same species, which can keep part of the system alive.
Grinding takes the stump, not the root system
Picture the stump as the hub of a wheel. Grinding removes the hub and the thick start of each spoke, the root flare, down to four to eight inches below grade. The spokes stay. They run out much further than people expect: NC State Extension says roots can spread three times or more the width of the canopy, so a tree with a thirty-foot crown may have roots ninety feet across. They are also shallower than people expect. For white oak, the Forest Service reports that nearly all of the main side roots stay within 21 inches of the surface. So after a grind, almost all of the root system is still there, cut off at the center.
Why the roots die
Roots run on sugar made in the leaves. Penn State Extension describes trees moving carbohydrate reserves underground in late summer and fall, which is why a root system holds a store of food. Cut the trunk and that store is all the roots have left. In a tree that sprouts, the store goes into shoots round the stump. Grinding takes out the root collar where those shoots start, so there is nothing to make leaves and nothing to refill the store. The roots use what they have and then die. Their cells stop working, the bark loosens, and the wood is open to the fungi already living in the soil.
How they rot, and how long it takes
Rot is the work of wood-decay fungi, helped by bacteria, insects and worms. Iowa State University Extension says how fast a stump rots depends on soil moisture, how many wood-decay fungi are present and the tree species, and that a stump left whole can take ten years or more. Roots go faster than a whole stump because they are thinner and wetter, but the big ones near the center still take years. Fine roots are gone within a season or two. Damp, loose soil speeds it up. Hard, dry clay and waterlogged ground both slow it down, because fungi need moisture and air at the same time.
Some woods last much longer
Species matters more than size. Purdue Extension rates black locust heartwood as very resistant to decay, which is why locust has been used for fence posts. A locust's roots and the core of its stump outlast those of most trees. Other woods give up fast, especially sapwood, the pale outer band, which has little natural protection. You will see it in the ground: the outer layer of a root turns soft first while the center stays firm. For a lawn it makes little difference. For a spot that has to carry weight soon, a slow-rotting species is one more reason to take the root ball out instead of grinding.
Exception one: trees that sucker from their roots
A few trees can grow new shoots straight from a root, with no stump needed. University of Arizona Extension names tree-of-heaven, honey locust, black locust and hackberry among them. On those, a ground stump can still be followed by shoots in the lawn, sometimes well out from where the trunk was. Each shoot that leafs out feeds the root it sits on and keeps that part alive longer. Mowing or clipping them as they appear starves them out. For a stubborn case, the guide on how to stop stump sprouts and root suckers covers the options, including labeled herbicides.
Exception two: roots joined to a neighbor
The second exception surprises people. Arizona's extension service points out that neighboring trees of one species sometimes fuse their roots, a process called root grafting, so two trunks end up drawing on one linked system. Where that has happened, the roots of a cut tree may be kept alive by the living tree next door, which goes on feeding them through the graft. You may never notice. Or you may see a stump in a row of the same trees that stays firm and grows a callus round the cut. The same joints are why herbicide put on one stump can travel to the next tree, so think twice about treating a stump in a row of trees you want to keep.
What you see above ground while they rot
Three things, all normal. The ground can sink a little over a big root as it decays, leaving a shallow groove running out from where the stump was. Mushrooms can pop up in wet weather: Clemson Extension says lawn fungi feed on decaying matter such as old roots and stumps. And the lawn can look a touch thin over the ground-out spot until the chips break down. One thing is worth knowing. NC State Extension warns that Armillaria, a root rot fungus, can persist in leftover roots and buried wood. See mushrooms on a tree stump for how to recognize it, and stump hole sinking for fixing dips.
Common Questions
Do tree roots die after stump grinding?
Yes, in nearly every case. Grinding removes the root collar, where new shoots come from, so nothing grows to feed the roots and they die once their stored food runs out. The two exceptions are trees that send up suckers from their roots and roots grafted to a living tree of the same species nearby.
Can roots left after grinding damage pipes or the house?
Dead roots do not grow, so they do not push into anything new. What they can do is leave small voids as they rot, so soil over a large root may settle a little. That matters under paving or a footing, not under a lawn. Roots from a living tree nearby are a separate question.
Does stump grinding remove the root ball?
No. Grinding takes the stump and the root flare below grade and leaves the root ball and the spreading roots in the ground to rot. Lifting the root ball out is a different job done with an excavator or loader, and it is only needed when a footing, slab or new tree is going in right there.
What to Do Next
For a lawn, the roots can be left to do their slow work after the stump is ground. For anything built, say so first. Call (828) 574-4474 or read about stump grinding, and a photo of the stump plus your town gets you a free quote.