Do root barriers hurt trees? What the research found

What did the studies find?

In four field studies, trees planted with a root barrier mostly grew as well as trees without one. The bigger risk is the trench beside an established tree, because it cuts every root it crosses.

Which rings slowed growth?

Two of nine kinds of ring, both in the six-year UC Riverside study. Unmarked results were not significantly different from no ring.

Ring size alone didn't decide it: the 10 in pot didn't slow growth, and the widest ring did. The authors judged the slowdowns too small to spoil the trees' looks. Fig trees grew as tall with every ring.

Where do the roots go?

Roots that reach a barrier turn down and pass under the bottom edge. In the 10-year sidewalk study, the roots under the walk were fewer and deeper. In the three ring studies, many came back up near the surface within about 1 to 3 ft past the ring.

Sidewalk Shield is designed to turn roots down and away from the paving. No study tested it, and all four put the barrier in at planting, so the trench cut no roots.

What is the real risk: the barrier or the trench?

The trench. Most tree roots grow in the top 12 to 18 in of soil and reach past the drip line.

Sidewalk Shield goes 18, 30 or 36 in deep, so its trench cuts nearly every root on that line. Cut roots cause two problems:

• Less water and food. Leaves can thin, wilt or brown and branches die back, often on the side that was cut.
• Less anchoring. In a Bartlett study of young willow oaks, cuts within 3 times the trunk diameter made the trees measurably easier to pull over. A cut right at the trunk lowered the force needed by 35%.

How close to the trunk can the trench go?

It depends on the layout. A straight run goes along the edge of the sidewalk, curb or driveway, so the paving sets the distance. A ring stays at least half the distance from the trunk to the mature drip line: for a tree that will spread 40 ft, the drip line is 20 ft out, so the ring stays at least 10 ft from the trunk. Where a tree root barrier goes covers both layouts.

Next to an established tree, talk to an arborist first, because the trench cuts the roots on that line. Take the numbers below to that talk. They are limits from tree research and Iowa State University Extension, not ProTech rules. Iowa State measures the trunk 3 ft up and advises against cutting roots over 2 in thick.

Example: a street tree with a 16 in trunk stands 40 in from the walk, and the barrier goes along the walk. 3 × 16 in = 48 in, and 48 − 40 = 8, so a trench just under 40 in out is at least 8 in inside the research limit. 1.5 × 16 in = 24 in, so it is outside the zone where Iowa State says removal may be wiser. Take both numbers to the arborist: the trench will cut roots that help hold the tree up.

Is a straight run or a ring better for the tree?

Next to an established tree, a straight run. It cuts roots on one side; a ring cuts them on every side.

For a new tree, the trench cuts no roots yet. Run the barrier along the paving you want to protect. With paving on more than one side, a ring can go around the tree, at least half the distance from the trunk to the mature drip line.

If your plan or city detail differs, follow the plan. Never set a ring tight around the root ball or line the planting hole with it. At UC Riverside, trunks filled and often split the 10 in pots within six years.

How do I dig the trench with less harm to the tree?

1. Call 811 before you dig. It gets buried utility lines marked.
2. Mark the line along the paving edge, centered on the trunk, or where your plan or city detail shows it. A ring goes no closer than half the distance from the trunk to the mature drip line. At an established tree, have an arborist check the line.
3. Keep machines and soil piles off the root zone. Heavy equipment packs the soil, and piled soil cuts off air to the roots.
4. Inside the drip line of an established tree, expose roots first, by hand or with an air knife (a compressed-air digging tool). Don't cut roots there with a chain trencher or excavator; they tear and pull them. Outside the drip line, or for a new tree, a trencher is fine.
5. Cut exposed roots with a sharp saw or pruner, so the ends are clean. Then dig the rest of the trench, to the barrier depth minus ½ to 1 in.
6. Keep cut roots moist. If the trench stays open more than 8 hours, cover it with damp burlap.
7. Set the barrier vertical, with the top edge ½ to 1 in above finished grade and joints overlapped 12 in and clamped. On a straight run, where there is room, you can lean it about 10°, top edge toward the tree. Set a ring vertical. Backfill in 6 in lifts.
8. Water the tree in dry spells and prune any branches that die.

The tree root barrier install manual covers each step. Depths and prices are on the Sidewalk Shield page.

When should I call an arborist?

Before any barrier trench next to an established tree. Call one when:

• The trench would come within 3 trunk diameters.
• Roots over 2 in thick cross the line.
• You want a ring around a planted tree.
• The tree already has thin leaves or dead branches.
• It's a street tree. Some cities, such as Portland, Oregon, require a permit to cut its roots.

What should I ask the arborist?

• Which roots on the line can be cut, and which must stay.
• What share of the roots the trench will cut. Duke University's campus standard caps it at 30% for mature trees.
• Whether a different layout, or no barrier, is better for this tree.

For a street tree, also ask the city forester about permits and standard details. Find an ISA Certified Arborist near you.

Who is a root barrier not for?

• A slab that has already lifted. A barrier won't flatten it. See why tree roots lift sidewalks.
• A mature tree with no room. If the only line is inside 3 trunk diameters, ask an arborist about other options.
• A small tree with paving 4 ft or more away. The UC Riverside authors judged 4 ft of space about as good as a ring over six years. Large shade trees need more: Iowa State asks for a rooting area at least 10 ft wide.

Common mistakes

• Thinking a narrow trench is harmless. It cuts every root that crosses the line.
• Setting a ring tight around the root ball. Trunks can fill and split a tight ring.
• Judging the tree next month. Symptoms of root damage can take months or several years to appear.

FAQ

Will a root barrier make my tree blow over?

Not in the one pull test we found. In a Bartlett study, young green ash grown in 18 in deep rings took slightly more force to pull over than ash without rings. Their deeper roots likely explain it, but the gap was not statistically significant. Cutting roots close to the trunk is what weakens a tree.

Does a root barrier stunt a tree?

Usually not, in the research. Trunk growth was mostly the same with or without a barrier. Two of nine rings slowed some trees: a 14 in nursery pot and a commercial ring about 2 to 2.5 ft across. Sweetgum in them grew about 5.5 ft and 3 ft less over six years.

Does the plastic itself harm the tree?

Sidewalk Shield is a sheet of HDPE, made in the USA. It stands on edge, so rain and air still reach the root zone from above.