Preventing Tree Root Damage in the Right-of-Way

Trees have two kinds of roots — permanent and temporary.

Permanent roots are the stout, woody ones flaring out from the trunk. While serving several functions, they are essential to the structural stability of trees. When a tree is young, these roots are developing underground. As with the trunk and branches, diameter increases with each annual growth ring. Under certain growing conditions, this increasing diameter becomes apparent as the roots “break through” the soil surface.

Temporary roots are non-woody, approximately 1/16 inch in diameter and in healthy trees, are constantly being replaced. In the ROW, they are found primarily in the upper inches of soil where moisture, oxygen, and minerals are sufficient to support growth. In looser soils, they’ll be found more deeply. Temporary roots are particularly dense and biologically active right at the base of a tree’s trunk.

Roots grow where they can. In forest soils, root depth may reach several feet, but never achieves the “mirror” of above-ground growth that many of us were taught. In urban/suburban sites, soil compaction limits infiltration of moisture and oxygen, resulting in a strikingly shallow, wide-spreading root system. Frequently, 80-90% of roots are found in the top 9-12” of soil.

Damage typical in the ROW:
Three types of damage are the most common threats to curbside tree roots.

Mower blight:
Where diameter has developed to the point that roots breach the surface of the soil, scalping by mower blades becomes an issue. This chronic, weekly wounding is, at a minimum, a stressor — rather than directing resources toward maximum canopy growth, the tree must expend energy on wound closure and decay management. With constant damage, wound closure fails. Over an extended period the tree begins to lose resilience and the ability to withstand other stressors like drought or infection by pathogens, diminishing service life.

Leveling the area with the addition of a few inches of soil is not the solution. Some species of trees absolutely require the high levels of soil oxygen that are only found in the upper 4-6 inches, so with time new surface roots will appear.

The better strategy is to eliminate turf in the area and mulch the bare soil. This immediately solves the mower conflict and over time, alleviates soil compaction and improves oxygen diffusion.

Parking off pavement:
This is where a good intention can have terrible consequences. It is most likely to happen in areas where we have no curbs (like the Poage Farm neighborhood) or where we’ve got those gentle, S-curve curbs that don’t chew up your wheel covers like traditional L-angled curbs. In trying to keep a parked car from slowing traffic on a two-lane street, drivers are increasingly scooching their offside tires past the pavement onto the turf and tree roots below.

As mentioned above, soil compaction impairs proliferation of tree roots. When a vehicle is snuggled up under a large tree for the benefit of shade, feeder roots are damaged or killed outright, stunting above ground growth. Structural roots are bruised and crushed threatening tree stability over time as they rot. The tree will need to be removed before it fails. Its service life was shortened, and it’ll be decades before a similar pool of shade is available at that spot.

Herbicide use:
When used according to label directions, herbicides can be used safely around trees. However, careless use poses risk of damage or even death.

Maintaining a generous mulch zone will nearly eliminate the need for post-emergent herbicide use near your street tree, reducing risk. In the 4” halo around the trunk that should remain mulch-free (link?), and for weeds that take hold in the mulch, hand-pulling or spot-treatment will be needed from time to time. Take care not to wound the trunk or roots if using a weeding tool. If spot-treating with a “natural” burn-down herbicide, take care to keep it off of young, thin-barked trees. Older trees with thick, corky bark are of less concern.

For proactive weed control in your mulch, a pre-emergent herbicide will cause the failure of 60-80% of weed seeds that blow in and start to germinate. These products are safe for trees. Those available in homeowner quantities are granular products which must be watered in to activate. They include Fertilome’s Gallery® and Preen Extended® (somewhat broader spectrum). Both are equivalent to products professionals use.

Further reading:

Healthy Roots and Healthy Trees — — 3 pp. & illustrations — ~650 words — ~700 words — ~225 words

Fast FAQs
Just the basics
Info for newcomers

Organizing Concepts
What is the Urban Forest?
What is a Public Tree?
Who Cares for Public Trees?
Who Prunes Right-0f-Way Trees?
Why Care About Public Trees?
How is Public Tree Management Funded?

Preventing Soil Compaction in the Right-of-Way

Glossary of Terms