Preventing Concrete and Hardscape Damage with Root Barriers
By Erik Skindrud, regional editor
Surrounding root barriers can be used on hillsides—away from hardscapes—to protect trees from runoff. The layout here uses a barrier that has been hand-cut at the hillside’s angle to route runoff away from the pine tree’s trunk and roots.
Developed 30 years ago in California, root barriers have revolutionized planting near sidewalks, driveways and retaining walls. They are physical barriers of flat plastic or synthetic fabric placed around or near the tree at right angles to the surface. The devices force roots downward, away from vulnerable surfaces.
The results of planting without root barriers are familiar and dramatic. Everything goes well for several years, until the tree reaches a tempestuous adolescence. Then roots bulge and push for the surface in a search for water and nutrients to fuel the tree’s rapid growth.
Workers fit sections of 24-inch polystyrene root barrier together for a surround-style tree planting. Holes should be dug to keep at least one inch of barrier above soil surface—to prevent roots from sneaking over rim.
If growing near concrete or other hardscapes this can have serious consequences. Sidewalks buckle, curbs heave up and retaining walls suffer cracks that require time-consuming and expensive repairs. By one estimate, root damage cost California cities and counties more than $62 million in 1999 in street and sidewalk repair. That total doesn’t count pruning, replanting or worse—possible legal consequences when pedestrians or skateboarders trip and fall because of broken concrete.
All this makes root barriers worth their cost, which is modest in comparison.
The best root barrier protection is achieved by combining linear and surround methods. The worker at center is surrounding the tree planting with a root barrier. The hardscape at left, in this case a curb, can also be lined with a plastic barrier in a linear fashion to produce complementary, double protection.
Root Barriers—The Importance of Preparation
The introduction of root barriers freed architects and planters to use a wider variety of species. Before their introduction, planners had to limit damage by excluding shallow-root types like carobs, elms, maples, myoporum and sweetgums. One solution was to root-prune these and other trees on a regular basis, but many species don’t tolerate the practice well.
In 1974, Leonard Albrect and Maurice Morman, the co-founders of today’s Deep Root Partners, L.P., pioneered and patented the first root barrier. At first, it was hard to convince arborists and planters that the invention would work or was even necessary. Some thought root barriers would prevent root growth and cause “a bonsai tree effect.” Others were sure the barriers would pop out of the ground as the tree grew. There were some problems with movement during the early years, but before long the bugs were worked out. Even so, some industry reluctance, especially on the east coast, persists.
A comparison of surround and linear use of plastic root barriers. “Surround application” (left) works well with individual trees. “Linear application” (right) is more economical with rectangular planting beds. The two approaches are often combined for maximum root protection.
“There still are some misconceptions,” said Graham Ray of Burlingame, Calif.-based Deep Root. “Some people think (root barriers) contain roots and stunt the growth of trees—but the roots are actually directed down and away where they find more nutrients and water.”
Getting the roots to travel down and out requires some careful planning, however. Contractors would do well to keep in mind a saying attributed to Deep Root sales pioneer Ron Hill. Now retired, Hill repeated the following whenever teaching a new audience about the root barrier concept: “The root barrier product and system combined is only as good or effective as the application and installation procedures associated with the product’s use and intention.”
A boxed tree is about to be placed in a corner prepared with a root barrier. Note correct placement of the barrier at the top edge—the barrier should slightly protrude above the surface to prevent roots sneaking through.
In other words, contractors should take their time and do it right the first time. For example, areas containing clay or other dense soils need to be prepared to a sufficient depth to accommodate root growth. Also, a deep-watering irrigation system will be needed to nourish a healthy root system in dry areas. Without these measures, Hill warned, root barriers “function as obstacles of survival rather than vehicles of guidance” for root systems.
Types of Root Barrier Systems
Two general categories of root barriers are used today—rigid plastic and cloth-like fabric. The first is the Deep Root type, also manufactured by Villa Landscape Products, Inc. of Placentia, Calif. and Century Root Barriers of Anaheim, Calif. This root barrier design is assembled by fitting together plastic segments. An advantage to this design is durability, longevity and resistance to bigger roots.
The second kind of root barrier is manufactured from polypropylene fabric. Tennessee-based Reemay, Inc.’s Biobarrier incorporates a root-blocking chemical called trifluralin into the material. The substance prevents roots from penetrating by stopping cell division in the root tips. The Canadian firm Texel markets another non-rigid fabric barrier that is not impregnated with the chemical called Tex-R. Fabric root barriers have one obvious benefit—they let water and nutrients penetrate but may be less resistant to tough, fast-growing root systems like bamboo, explained Jerry Dunaway of Biobarrier.
Close-up view shows a hole for the irrigation line. Carefully sizing the hole prevents roots from breaking out.
Trifluralin works by creating a narrow chemical barrier within the soil that can prevent root growth for 15 years or more. Users should carefully follow the instructions on the label. The chemical is also used to prevent the growth of grass and broadleaf leaves.
The Tex-R fabric barrier is non-toxic and stops roots while allowing water and nutrients to pass through. Like Biobarrier, it is installed completely beneath the ground and is not visible after installation.
One caution with the rigid system is its above-ground character. Ron Hill, who helped pioneer the system, was well-aware of this pitfall. “Beyond the sprinkler head there is no other (landscape product) subject to more human and natural abuse,” he wrote.
Filling a trench after installing a linear root barrier. Installing the barrier to a depth of one to one-and-a-half feet will keep most species of trees under control. Linear installation is the right choice along sidewalks, driveways and retaining walls.
One should evaluate the claims made by each manufacturer along with the specifics of the planting area before selecting a product.
Root Barrier Planting Styles
Whatever the choice of product, there are two styles of installation for root barriers. The styles can be used separately—or together for extra root-growth protection.
The two styles are linear and surround (see diagram on page 49). As the term implies, the surround method encircles the tree, usually to a depth of about 12 inches. The surround method can be used when a tree is grown in a confined space like a square cutout in a concrete sidewalk.
The linear style places a longer segment of root barrier material along a concrete or other hardscape edge. This method can give the tree access to a larger soil area but will also require more material at a correspondingly higher cost. Many projects today employ the linear and surround methods in tandem.
Root-barrier panel detail. Note vertical ribs, which direct roots down to water and nutrients instead of allowing them to bunch near edge. Years of experience have prompted companies to add UV-resistant sunblock components to help panels stand up to years of exposure.
The Future of Root Barrier Systems
New materials and manufacturing techniques may result in root barrier systems that are cheaper and lighter while at the same time more durable than present methods. One area for improvement is cold-weather durability, as barrier systems are currently not as widely used in the colder parts of North America.