The following is an article by Ron Hill "Root Barriers In The Urban Forest" Ron is a Consulting Arborist and Forensic expert specializing in all aspects of tree root barrier technology.

Invention of Root Control

The 1974 patented invention of the 90-degree root deflecting rib molded to impervious, smooth walled (plastic) root barriers set into motion the new technology and science of tree root management.

The excavation of the initial test trees provided dramatic evidence that a tree root tip could be successfully redirected downward along the barrier wall & rib into the prevailing soil environment beyond the bottom perimeter of the barrier.

The co-inventors of Deep Root, Leondard Albrecht and Maurice Morman, were the initial principals of the Deep Root Corporation, which has been considered the leader in state-of-art root control technology over the past 20 years.

Overview - Root Barriers Today

If you are from California chances are root barriers are an integral component of you city code or engineering specification, initiated to control directional root growth patterns of urban trees. However, in some regions of the U.S., root barriers are still viewed with curiosity and skepticism.

Root Barriers have been around for nearly two decades and today there is a strong movement eastward from their birthplace in the west. Currently barrier products are marketing in the U.S., Canada, Europe, Hong Kong and Australia

Field testing of root barriers has been done by various groups such as the U.S. Department of Forestry, Forest Service, U.S. Department of Agriculture, the University of California system as well as regular use and specifications by hundreds of cities and landscape architects.

The City of Pasadena, home of the Rose Bowl and Tournament of Roses Parade, has specified root barriers for 18 years to help maintain pedestrian safe environments. The City of Los Angeles has planted thousands of trees in root barriers over the past 14 years, per code. The L.A. code was re-written in May, 1982, just one year after the city conducted their own excavation of test plantings.

The city of Santa Barbara has been a pioneer is root pruning and retrofit root barrier installations over the past 15 years.

A conservative estimate of over 1.5 million trees are growing in root barrier planting systems with as many as 20% of those in linear or retrofit applications.

The Root Barrier Planting System

Because the living tree root system requires specific survival elements, a Planting System was invented along with the root barrier product to promote the elements essential to healthy tree root establishment.

Without a well-designed and professionally-installed planting system that incorporates the elements of soil exploration & preparation, proper backfill procedures, deep watering and oxygen & gas transfer, root control would be improbable if not impossible. Without these essential planting elements, the 90-degree root deflecting ribs on the tree root barriers would function as obstacles of survival rather than vehicles of guidance, directing the exploring root tips downward for deep anchorage and subsequent lateral growth into the surrounding soil environment.

"The integral planting systems associated with plastic tree root barriers cannot be over-emphasized regardless of the planting style selected."

Types of Root Barriers

There are basically 2 classifications of root barriers manufactured today:

1. "Impervious Plastic Barriers" produced in (expandable) self-locking panel sections with integral molded components and 90-degree root deflecting ribs.

2. Fabric Root Barriers- Allows the transfer lateral movement of water and nutrients while suspending roots at the barrier (without releasing toxic chemicals or vapors into the soil or atmosphere).

3. "Chemical-release barriers" with the active herbicide "Treflan" (molded into pellet form) supported by a "host" deotextile fabric. The product is produced in rolls.

The "impervious plastic barriers" are guaranteed to emit the Treflan gas/vapor into the soil for a period of 15 years. This type of barrier functions by stopping cell division at the root cap as the root tip grows into the vapor zone, thus blocking root growth. The host product, a geotextile fabric, has limited root-deflecting capabilities and does not possess the basic elements for extended root control.

Treflan is classified & registered as a hazardous chemical in many regions of the U.S. The product literature discusses the special handling instructions and product use restrictions that you should be aware of if you intend to use this type of product.

As you pursue additional research into root barriers you will quickly discover that there is a direct relationship between quality, product integrity and manufacturer's support. If a barrier manufacturer cannot produce supportive product specifications, planting instructions and testing documentation, it would be prudent to seek a reputable manufacturer.

Root Barrier Planting Styles

There are 2 distinct barrier-planting styles inherent in barrier systems; they are referred to as "Surround" & "Linear." The surround style is defined as "an enclosed planting system where the barrier surrounds the new tree root ball, with an open bottom."

Root control was born out of the "surround planting style, and today it is still the most popular as well as economic method of achieving quality root control.

The Linear style planting is defined as "an expansive, open planting environment with parallel root barriers positioned along
adjoining lateral hardscapes." The linear configuration is used in new tree plantings as well as in the root pruning application of mature trees.

Although the prospects of an expansive Linear style planting may overwhelm the Surround style system, there are several considerations to fit the appropriate planting style to your particular needs.

• Do you have a budget for the additional costs associated with the Linear style planting?
• Which planting style will best suit the overall needs and health of the trees selected?
• Does the physical planting width justify the additional costs of a linear application?
• Will the planing site accommodate heavy pedestrian traffic?
• Will the soil condition enhance the planting style you desire?

A thorough evaluation of your planting requirements will ultimately be your greatest asset in determining the planting style that you will incorporate into your project or municipal specification.

There are three primary functions performed within a tree root barrier-planting environment. I generally refer to these as Golden Rule of Root Control because all root barrier applications can be weighed against these primary functions:

root barrier planting system must address the immediate and long-term m

"A Model" Root Barrier Product

Beyond the sprinkler head there is no other product in the landscape that is subject to more human and natural abuse than the root barrier planter. The problem arises from the fact that the top edge of the barrier must be positioned and maintained 1/2 to 1 inch above grade to prevent root overgrowth.

The elements of nature present an even greater challenge in the attainment of quality root control.

The natural dynamics of hydraulic root expansion, invasive root systems and tree trunk mass, coupled with daily exposure to ultraviolet rays, soil chemicals and constant climatic changes, are practically immeasurable.

Based on observations of trees growing within tree root barrier systems over the past 20 years, it is clearly obvious that quality products and systems must prevail. The highest standards of quality in the selection of plastics and the manufacturing process must me maintained in order to meet the challenges of product degradation brought about by time.

These elements are:

• Longevity- Root barriers must be manufactured from a plastic material capable of maintaining integrity in the ground for two to three decades. Dynamic plastic properties exhibiting superior strength, flexibility and adaptability to extreme temperature changes is critical to the long term success of a barrier planting system. Polypropylene & Polyethylene exhibit superb, durable, plastic properties.

• Ultraviolet Protection- Because of the constant exposure of the top edge of the root barrier, positioned 1/2 to 1 inch above grade to prevent root overgrowth, the product must contain maximum UV inhibitors.

• Manufacturing Integrity-The superior product would be an injection molded section or panel consisting of integral molded root deflecting ribs, joiner, anti-lift devices and reinforced edges.

• Containment Longevity- The product must contain a positive self-locking (non-gluing) joiner of superior quality and strength, capable of withstanding the dynamic forces of trunk and root expansion for two decades or longer.

Examining the Track Record of Root Control

One of the first major articles discussing the potential of "root barriers" appeared in the Journal of Arboriculture 1984, Vol 8 pp 223-234. In his summary the author W. Douglas Hamilton opened the door to the consideration of open-ended root containers.

In June 1985, The Journal of Arboriculture published an article by J. Alan Wager titled REDUCING SURFACE ROOTING OF TREES WITH CONTROL PLANTERS AND WELLS - Vol 11, No. 6. The article described the first major scientific testing or root controlled plantings utilizing "plastic tree root barriers & planters" as one of the test devices.

More recently Phillip A. Barker, a Research Horticulturist with Urban Forestry Research, presented a paper to the international workshop, "The Landscape Below Ground." The reception to this latest research on root barriers was excellent. The conference was presented by the Morton Arboretum in Lisle, IL October 1993. The proceedings are published in the new book "THE LANDSCAPE BELOW GROUND," available through the I.S.A.

Root barriers have received a good deal of attention in the area of research. Overall, the reports are very positive and support the use of root barriers as a method of reducing root mass and development in the upper soil region around the perimeter of the tree.

These research papers and test data provide current information to aid you in your root barrier planning and planting.

Tangible Benefits of Root Barriers

Considering long term research and observations of planing systems, there are several tangible benefits inherent in quality root control that we may consider in our evaluation of tree root barriers.

• Vertical 90-degree root deflecting ribs have proven to break up root girdling, thus producing a healthier tree.

• Root Barriers expand the variety of street trees that can be planted and managed

• Buttress roots that normally flare off from the trunk are blocked by physical contact with the root barrier and are redirected downward, beneath the barrier wall, ultimately reducing the potential of hardscape displacement and damage.

• When used in the root pruning application, tree root barriers redirect the new root growth and faster stabilization of the soil

• The versatility of root barrier products, coupled with the diversification of planting styles, broadens the scope and potential of root management

• Although a root barrier planting system and curb & gutter repairs by promoting deeper rooted tree root systems, the ultimate benefit may be the reduction of accidental tripping over raised sidewalks and the legal costs associated with personal injury lawsuits.

Tangible Benefits of Root Barriers

Misconceptions such as the "bonsai tree effect" or "root barriers popping out of the ground" stem from lack of knowledge of root barrier systems and technology.

There are genuine concerns about planting trees within root barriers in hard clay soils. However, common-sense measures such as soil modification, adequate water drainage & control and the use of variable sized barriers can lead to successful tree root management even under adverse soil conditions.

The "living" results achieved over the past two decades in root barrier management and the potential dollar savings in hardscape repair are dramatic. We must view root barriers wit5h an open mind and move beyond the myths and misconceptions that lead one away form the big picture.

Here are some rules of thumb that will be of value to you in your planning and research.

• Most landscape architects and municipal specifiers specify root barriers if the tree is to be planted within 5 to 6 feet of the hardscape.
• Expanding tree well cut-outs to maximum sizes -- 4,5, or 6 feet square will provide more open soil volume for root expansion.
• The Surround Style system is designed to train the young root tip growth downward and then outward beneath the edge of the barrier.
• The Linear Style installation protects the hardscape from the exploring tree root tips after they have traveled through the soil to encounter the barrier.
• A tree can be installed within a dual planting system incorporating both a surround & linear tree root barrier.

Root Barriers In the Urban Forest

Having explored some of the aspects of root barriers, it is easy to see that the subject is anything but cut and dried. There is no bottom line to root control. Nonetheless, three root barriers are here to stay.

As arborists, foresters, city planners and urban tree managers, it is our responsibility to see how tree root barriers can fit into our own urban environment.

An assessment of the disposition of our own streets and street trees without the influence of root barriers may be the revelation needed to stimulate further investigation into their use.

Tree root management utilizing (90 degree) ribbed, plastic tree root barriers is more than a concept, it is a proven technology and growing science that must be explored to the fullest extent possible.