arborvitae root barrier

Asked August 12, 2020, 4:57 PM EDT

How deep would a a barrier need to go and how far from the tree does it need to be in order not to disturb the plant?

Kitsap County Washington

1 Response

Hello and thanks for contacting us.

Research shows root barriers don't stop roots from growing or crossing a border. They can limit root growth however, so that it takes longer for roots to grow around them. The roots that make it around a barrier tend to be smaller for longer, so there is that advantage.

Root growth (spread and depth) depends in part on the soil texture and structure around the plant. These can make it easier for root growth (sandy soil, good structure) or inhibit root growth (compacted soil, heavy clay, poor drainage). Root spread also depends on how long the plant has been in the ground; the longer it has been there, the farther out the roots will grow, up to twice the diameter of the canopy. If possible, you could measure the distance from the trunk to the edge of the leaves and then double that, installing the root barrier at a distance from the trunk equal to twice the distance from the trunk to the edge of the leaves, sometimes called the drip line.

Most tree roots are found in the top 12-18" of soil, however arborvitae roots are found up to 24" below ground. You may want to consider installing the barrier as deep as 24-30" if possible.

If the idea suggested above can't work, then hear is a different way to determine where to install the root barrier. When digging into the root zone of a tree, there is a formula to apply. (Part one: Trunk circumference at 4.5 feet from the ground / pi (which is 3.14) = trunk diameter) Measure around the trunk at 4.5 feet off the ground. We're going to pretend it's a 10" circumference trunk. Divide that number by pi (which is 3.14). In our example then, we're dividing 10 by 3.14 which gives us 3.18471 inches, which is the approximate diameter of the tree trunk at 4.5 feet off the ground.

(Part two of the formula: Trunk diameter at 4.5 feet off the ground x 1.25 = critical root zone.) Now we multiply by 1.25, so 8.18471 x 1.25 = 3.980, which we'll round up to 4.

Next we replace inches with feet, so in our example we have 4 feet as our answer. This is the critical root zone around the trunk, so we should not do any digging within 4 feet of the trunk. Digging in this zone could do severe damage to the plant and increase the chance that it fails (dies or falls over).

Determining the critical root zone does not mean the rest of the roots are fair game. You want to limit the amount of root damage done by installing the root barriers to less than 30% or the roots maximum, with hopefully less damage than that.

I've attached a picture to hopefully help you visualize the critical root zone and root system of a tree.

Please let us know if this was too confusing, and we can talk more about this issue. Thank you for contacting us!