PGMS Member Area > PGMS Knowledge Center

March-April, 2002

All pavement and hardscaping installations are critically dependent on the integrity and stability of base materials below the surface. Any disruption or movement of the base material or underlying structure will cause deficiencies, such as rutting, heaving, or displacement of paving surfaces to quickly appear on the surface. These defects represent a hazard for vehicles, wheelchair users and pedestrians and can be expensive to repair.

A recent study cited defects directly attributed to tree root growth as the sixth most common cause of premature sidewalk failure in municipal applications. For this reason, root barriers, which prevent the growth of opportunistic tree roots under infrastructure components such as sidewalks and roadways, have been increasingly recognized as essential components of hardscaping installations. Over time they have proved to be effective in eliminating deficiencies in new construction and in retrofitting and repairing existing installations.
Proper detailing and installation of root barriers is required at locations where tree roots intersect with hardscaping components to ensure successful long-term performance.

SURROUND STYLE PLANTING APPLICATIONS

The first root barriers made their debut in the early 1970s. The first configuration employed was a “surround style” barrier. These surround barriers could be purchased in one-piece, or they were assembled in rolls or panels which were connected together on-site.

Early research showed that the smooth walls of the surround sound barrier often lead to circling of the roots. The young tree roots would hit the side of the barrier and instead of growing in a downward direction they would grow in a horizontal path. This eventually resulted in a circular root configuration that could eventually “girdle” the tree. This kind of root growth pattern could eventually constrict the vascular system of the tree and inhibit growth. Root barriers are now available with ribs that encourage the downward growth of roots. In addition other features, which are now available, assist in deep watering and continuing oxygen exchange in the soil.

One of the most crucial factors for the long-term success with root barriers is ensuring the installation of the top edge of the root barrier above grade. It is also important to choose a root barrier of sufficient size to accommodate the growth of the tree as it matures over time. An undersized barrier will restrict root growth and compromise the tree’s ability to maintain a firm anchor in the soil. Additionally, if the root barrier is too small it will be difficult or impossible to properly backfill between root ball and barrier. This condition causes air pockets to become filled with water, creating conditions that can lead to decline of root growth. In general, barrier installation is more successful if the soil is properly prepared. Specifically, the soil should be loosened well below the bottom of the barrier to encourage deep rooting. If hard and compacted in later years the roots will grow out from the bottom and simply turn upward.

As well as proper soil preparation, accommodations for adequate soil moisture below the barrier should also be made at planting time. If adequate moisture levels below the barrier are not maintained the roots tend to grow near the surface where moisture is readily available.

1) The root barrier panels interlock without any adhesive or extra assembly strips. 2) Next, the assembled barrier is placed in the trench. 3) Finally, the contractor backfills around the barrier.

INSTALLATION

• In instances where a tree pit is located within an area to be surfaced the sub-grade should be prepared according to the design specifications.
  
• The initial planting hole as shown in the diagram. (Consideration should be given to installation of drainage devices or amendments to correct any adverse soil or planting conditions.)
  
• It is also recommended to dig an additional 12” to establish the largest possible rooting area below the depth of the barrier. This area should be re-compacted, either with the original excavation materials or soil amendments as necessary. This ensures that the roots will be able to penetrate the ground below the barrier, yet will not settle below grade with watering.
  
• Assemble the appropriate number of panels with the vertical root deflecting ribs on the panel facing inward toward the root ball.
  
• Next, place the barrier in the center of the planting hole, with the double top edge of the barrier positioned at the top of soil grade, (if there is no tree grate.) With a tree grate, place the barrier just below the lip that the tree grate sits on.
  
• Install the tree inside the barrier. Backfill soil inside the barrier to a level ½ inch below the double top edge of the root barrier. This will ensure that roots do not grow over the top of the root barrier.

LINEAR INSTALLATION

• When the tree pit is located beside a sidewalk or area to be surfaced, install the base and geotextiles where specified.
  
• Determine the correct number of panels to be used. Depending upon the actual planting plan and the number of trees involved, the length of linear barrier will vary. As a general rule of thumb take the anticipated mature canopy diameter of the tree and add 2 feet.
  
• Choose the barrier that best suits the application. For most applications 18” root barriers are generally considered a sufficient depth. 12” root barriers can be used as an alternative choice for non-aggressive trees. For curb and gutter protection, or for more aggressive roots, however, 24” barriers are generally a better choice.
  
• Excavate the area where pavements are to be laid. Place the barrier in a trench, at the edge of the excavated area, with the vertical ribs facing toward the tree and align in a straight fashion.
  
• Install surfacing materials in specified manner. Follow the same procedures that you would normally. Be sure to keep the barrier’s double top edge at least ½” above grade, or at grade, to ensure roots do not grow over the top. Plant the tree(s).

ROOTS, WATER and SOIL

Once the tree is planted attention needs to be directed at roots, water and moisture movement in the soil. Trees have three primary forms of roots: roots near the tree base, transport roots and the ephemeral, absorbing roots. These transport roots carry food to the root tips, as well as water and essential elements to the leaves. The longer a transport root survives, the bigger it becomes. Roots grow where soil resources are most plentiful. If that area is near the soil surface, that’s where the roots will grow.

The two resources most important to tree-root growth are oxygen and water. Oxygen is available only near the soil surface in large air-filled soil pores. Tremendous competition exists for this oxygen between tree roots and other plant roots. Because of the ease with which roots pull water from the soil, roots sense and grow toward areas where plenty of water is available. The quandary is that water supplies must be located in the same soil areas where atmospheric oxygen also is available. Soil organisms quickly use the dissolved oxygen in soil water and,as a result, it is not as readily available to tree roots as atmospheric soil oxygen.

Atmospheric oxygen is essential to tree life. The aboveground portion of a tree has no problem finding oxygen in the air for respiration. (Oxygen content in the atmosphere is about 21 percent.) For roots, however, the plight of getting enough oxygen is severe.

For unconstrained root growth, the soil atmosphere must contain more than 15 percent oxygen. As soil-oxygen levels fall below 5 percent, root growth stops. Oxygen levels of less than 2 percent lead to root decline and death. The three major problems that cause inadequate soil oxygen are:

• Competing organisms
  
• Soil compaction,
  
• Water-filled pores (saturation).
  

All of these problems lead to an oxygen-limited condition of the soil: an anaerobic condition. Under anaerobic soil conditions, different types of microorganisms-primarily bacteria-take over the soil. The anaerobic organisms produce toxins and consume or infect roots. Several tree root rots thrive at low soil-oxygen levels.

Warmer temperatures disrupt oxygen use by tree roots. As soil and air temperatures increase, so does oxygen demand in a tree and in the surrounding soil. For every 18 degrees F increase in temperature, oxygen demand doubles for both tree roots and other soil organisms. Increasing temperatures cause tree roots to respire faster, which uses food and oxygen more quickly

An additional example of root-oxygen problems occurs on recreation sites where foot and vehicle traffic have compacted the soil, collapsing the soil air pores. To maximize landscape performance, some landscape managers add composted organic matter and nitrogen to the soil, along with continuous irrigation. The result is a rich mess of oxygen-demanding microbes fueled by organic material and nitrogen.
As these organisms use oxygen, and oxygen is not easily replenished because of water filling all available pore spaces, more portions of the site become anaero depleted. Under these same conditions, however, tree roots decline and die.

WATER

Roots’ search for water holds similar problems. Too much water and the tree drowns (suffocates). Too little water and the tree starves (desiccates).

Continuous soil saturation or flooded conditions lead to low soil oxygen and, thus, major tree-root problems. Unregulated, poorly adjusted and improperly zoned irrigation can all cause root damage, especially in warm weather. Saturated soils also are prone to mechanical damage, which reduces aeration and thus lessens trees’ ability to survive well there. As water fills and occupies all available soil pores, any activity on the soil (walking, driving vehicles or parking, for example) disrupts soil structure. These activities result in rutting, puddling and compacting of the soil, which lead to root injury and death.

Tree roots are very aggressive, growing near the soil’s surface in search of water, nutrients, and oxygen. They can extend underground, spreading outward, two to three times the diameter of the tree canopy. As the tree grows, the roots grow, becoming larger and larger, exerting tremendous pressure on concrete and asphalt.

MOISTURE MOVEMENT IN THE SOIL

Damage can also be caused by the effect tree roots have on the moisture movement in the soil. Where tree roots extract large amounts of water, the sub-grade may shrink and cause damage to structures. Typically, the damage is not caused by direct physical pressure from the roots themselves but by radial growth of tree roots in the immediate environment. Gravel layers in sub-base materials can create high humidity and aggressive tree species will take advantage of the enhanced conditions to increase their root growth.

Structures, which rely directly on the sub-grade for support, such as retaining walls, are susceptible to changes in soil moisture levels that can cause differential movement within the subgrade.

While root growth can create expensive and time-consuming problems, preventative methods such as installing root barriers can make these problems someone else’s.