A big tree, a lawn full of roots breaking the surface, a mower that keeps scraping them. The obvious fix is a few wheelbarrows of topsoil and some grass seed. It’s also the one tree specialists keep advising against. Here’s why, and what they put over exposed roots instead.
In short
- Piling soil on exposed roots can keep water from reaching them, and roots grow back through it anyway, Davey Tree says.
- Arborists recommend a layer of organic mulch, 2 to 3 inches deep.
- Gravel over landscape fabric and stepping stones are other options. Concrete isn’t.
- Never cut exposed roots: it can make the tree unstable, or kill it.
Why soil is the wrong fix
The question keeps coming back, and Martha Stewart recently put it to arborists. The tree care company Davey Tree explains the main risk: “If you use the wrong soil or pack on too much, the tree won’t get enough water to stay hydrated.” The fix doesn’t even last, it adds, since the roots will likely grow through the new soil before long.
There’s a reason roots sit so close to the surface. Eric North, a certified arborist at the Arbor Day Foundation, told AOL that “surface roots frequently form when soils are heavily compacted or there is a high-water table.” Oxygen and moisture are most available in the top layer of soil, so burying roots pushes the tree to grow new ones closer to the surface. The exception is erosion: when soil has washed away from roots that used to be buried, adding it back makes sense.
The Arbor Day Foundation’s blog gives the same warning about heavy loads: “If you suddenly smother them with a load of soil, you risk damaging the tree.” If you really want to raise the level, it advises adding small amounts of soil gradually, in stages, and giving up on the idea of a full perennial bed on top.
What arborists put on them instead
- Organic mulch. Davey recommends 2 to 3 inches, which looks tidy and protects the roots while keeping them moist. For a large network of roots, the Arbor Day blog suggests a 3-inch layer.
- Gravel, with care. Lay landscape fabric first, then spread no more than two inches. Davey warns gravel can compact the soil and heat the ground in summer.
- Stepping stones, to make a path through the roots instead of walking on them.
- Not concrete, which blocks oxygen and water, and which the roots will eventually crack.
Grass or flowers can work if you find shade-tolerant varieties that need little water. Davey adds that they often die anyway, because the canopy blocks the sun and the tree takes most of the water.
The limit
Don’t take the saw to them either. Davey says pruning exposed roots “often makes the tree unstable or unhealthy,” and done wrong, it can kill the tree. For compacted soil, North adds that a certified arborist can loosen it with pressurized air with minimal damage to the roots. For a big tree near the house, or roots lifting a walkway, have a certified arborist take a look.
Mulch can turn against you: here’s the mulching mistake that turns protection into poison. Roots explain more than you’d think, like why fig trees were kept away from the house, and if a tree is already gone, see the natural method to remove a stump.
A few inches of mulch, no soil, no saw. That’s the trick.
