The root flare, also called the trunk flare, is the part of a tree where the trunk widens as it meets the soil and splits into its main roots. Stumps are measured across the flare at ground level because that is the true width the grinder has to remove, and it is often much wider than the cut face higher up.
What the root flare is
Look at the base of a healthy tree and you will see the trunk spread out as it reaches the ground, like the foot of a column, with the tops of the big roots showing as ridges running off into the soil. That spread is the root flare. NC State Extension describes it as the part of a tree where the trunk meets the ground and flares out, and notes that on a properly planted tree you should be able to see the flare and even the tops of the major roots. Arborists sometimes call the same zone the root collar.
Why a stump looks smaller than it is
A tree crew often cuts a stump a few inches up the trunk, above the flare. The cut face looks like a neat circle, and that circle is the number people give on the phone. Below it, the flare can be half as wide again, with root knuckles reaching out further. The grinder has to take all of that below grade, not just the circle on top. That is why a stump described as eighteen inches can turn out to be well over two feet at the soil line, and why that gap matters for the quote and the time.
How to measure across the flare
Pull back grass, mulch and leaves from the base of the stump until you can see where the wood meets the soil. Lay a tape across the widest point at that level, from the outside of one flare root to the outside of the one opposite, and read the number. Do not include roots running away under the lawn, only the spread of the base itself. Snap a picture with the tape still lying across it. If the stump is oval, measure the long way. The guide to measuring a stump before you ask for a price covers the other pictures worth sending.
Why the flare has to be ground out
Grinding only the cut face and leaving the flare in the ground is the most common shortcut in stump work, and it shows. The flare roots stay at or just under the surface, where the mower hits them and grass struggles over them. On many hardwoods the buds that send up sprouts after cutting sit at the base where trunk meets roots, so a flare left in place can also sprout. A proper grind takes the whole flare below grade, out to where the big roots turn down into the soil. That is why the footprint of a ground stump is wider than its cut face.
When the flare is buried
Some stumps have no visible flare at all. They go straight into the ground like a post. NC State Extension notes that when a trunk enters the soil like a telephone pole, the flare is buried too deep, usually because the tree was planted too low or mulch was piled against the trunk for years. On the stump, that means the real width is hidden under soil or old mulch. Dig back around the base before measuring, and say in your message that the flare looks buried. The grinder will have to chase the flare down, and that is worth knowing before the price is set.
What the flare tells you about the tree you are keeping
If you are grinding one stump among trees you want to keep, look at their flares too. A visible flare is a good sign. A buried one, or roots wrapping around the base, can mean trouble later. NC State Extension describes roots that grow in piled mulch and then thicken around the trunk as stem-girdling roots, and calls them a frequent cause of early tree death, especially in new housing developments. Keep mulch pulled back off the flare of the trees that stay, and if one looks like a pole going into the ground, ask your county Extension office about it.
Common Questions
What is the difference between the root flare and the root collar?
They are two names for much the same place: the base of the tree where the trunk widens and joins the main roots. Root flare or trunk flare usually describes the shape you can see. Root collar is the term often used for the zone itself, which on many hardwoods holds the buds that sprout after a tree is cut.
Why does my stump go into the ground like a pole with no flare?
The flare is probably buried. Trees planted too deep, or with mulch heaped against the trunk for years, end up with the flare below the soil. Dig around the base to find it before measuring, because the real width of the stump is down there.
Does the root flare have to be ground out with the stump?
Yes, if you want the spot to behave like lawn. A flare left in place keeps hard roots at the surface for the mower to hit, stops grass from growing over them, and on some trees can still sprout. A proper grind takes the flare below grade along with the stump.
What to Do Next
Got the flare measured? Email the picture with the tape across it, a shot of the way in and your town to jbdumpsters@outlook.com, or phone (828) 574-4474. The stump grinding page covers what happens next.