GEOTEXTILES AND ROOT PENETRATION

INTRODUCTION

Geotextiles (also known as engineering fabrics) are often used in conjunction with vegetation in erosion control and slope stabilization work. While this conjunctive use of vegetation and geotextiles serves many useful purposes, questions have arisen as to whether such use limits root development and/or the ability of roots to penetrate across the plane of the fabric. Roots should have no difficulty penetrating across natural, woven fabrics with large openings, such as coconut (coir) or jute fiber netting. The problem lies instead with woven and non-woven polymeric geotextiles, which tend to have relatively small openings.

This penetration concern has been voiced in particular when geotextiles are used in the following manner:

•  As a filter course beneath vegetated rock armor (riprap) on a streambank,

•  As a drainage blanket behind a live brushlayer reinforced earthen fill, and

•  When live brushlayers are placed on geotextiles in vegetated, mechanically stabilized earth applications.

In each of these cases, the live cuttings are placed either directly against or in close proximity to a geotextile.

Considerable evidence can be found both in support of, and against, root penetration across geotextiles. The nature, basis for, and reliability of this evidence are briefly reviewed herein. Few, if any, carefully controlled and documented research experiments have been conducted on this issue, at least not in the context of cuttings used in soil bioengineering applications. Most of the reported research has been directed towards investigating the efficacy of geotextiles in preventing either tree root penetration into drainage media or preventing vegetable root penetration into toxic subsoils. Nurseries have also investigated this issue from the point of view of using geotextiles to wrap and contain root balls.

Geotextiles or engineering fabrics are used for a variety of purposes or functions, viz., separation, filtration, reinforcement, and erosion control. Root development is generally compatible with geotextiles for soil reinforcement and erosion control; however, geotextiles may be potentially incompatible for filtration and separation. Both of these functions require a geotextile with relatively small apertures (effective pore size) to allow the passage of water, while excluding the transfer of fines. This same property or requirement also impedes or militates against root penetration.

Root penetration across a geotextile boundary is not governed solely by aperture size. Other factors, such as type of vegetation, root size, soil type, moisture content on the other side of the geotextile, light conditions, etc., play an important role as well. Roots exhibit a property known as "edaphoecotropism" (Vanicek, 1973), which essentially means that they grow into hospitable regions and avoid stressful micro-environments (e.g., mechanical obstacles, lack of moisture, light, lack of oxygen, saturation, etc). These considerations have just as much effect on root development and penetration as the presence of a porous/permeable separation layer, viz., a geotextile. See Special Topic “Bio-Adaptive Plant Responses” for further discussion of this propensity of plant roots.

Controlled experiments (Miljostyrelsen, 2003) with a variety of vegetable plants have shown that geotextiles only impede and do not prevent penetration of roots. Experiments were conducted with selected non-woven geotextiles whose effective pore size varied from 55 to 180 mm as shown in Table 1. Plants used in the experiments included carrots, chicory, green pea, and lucerne. The plants differed in their ability to penetrate across the geotextiles, with chicory being the most effective, and green pea the least. The geotextile with the smallest pore size, 55 mm, was the most effective at impeding or delaying root penetration, as shown in Figure 1.

TABLE 1: Properties of Selected Non-Woven Geotextiles Used in Root Penetration Study
( from Miljostyrelsen, 2003)

Interesting root penetration findings (Morgan, 2003) have been reported for tree root balls wrapped with geotextiles. Some nurseries grow oaks, maples, pines, cypress, crepe myrtles and other species aboveground in containers made entirely of fabric or geotextiles. In general, lateral roots are "air pruned" at the geotextile interface whereas vertical (sinker) roots penetrate across the geotextile into the moist ground below. This behavior results in several benefits, namely, irrigation or watering needs are reduced because the vertical roots are able to extract moisture from the ground below, and air pruning of the lateral roots eliminates girdling while at the same time promoting a fibrous (branching) growth at the pruned root tip.

The main evidence against root penetration is the simple fact that geotextiles are widely promoted and marketed for this very purpose, i.e., as a root barrier. Numerous web sites and marketing brochures tout the advantages of using both woven and non-woven geotextiles manufactured from polyethylene and polypropylene as root barriers. Some of the marketing literature concedes the superior qualities of high density polyethylene (HDPE) membranes in this regard but maintain that porous geotextiles can still be used as root barriers in situations that also require a barrier that will pass water. This marketing literature does not directly cite aperture size and instead refers to such properties as a very tight weave or high specific weight per unit area. In some instances, the geotextile is impregnated with copper salts or an organic herbicide to improve its resistance to root penetration.

Figure 1. Penetration of carrot and lucerne roots through different geotextiles described in Table 1
( from Miljostyrelsen, 2003)

TECHNIQUES FOR ENHANCING ROOT PENETRATION

Several techniques can be employed for enhancing root penetration and development across geotextile boundaries:

•  Select the maximum effective pore size or aperture opening consistent with filtration and separation requirements.

•  Drive live stakes and poles through the geotextile. If necessary, use a steel rod to create a pilot hole.

•  Make slits in geotextile sheets at selected locations to facilitate root growth and penetration. Use this method when live cuttings are placed on the geotextile, e.g., in vegetated mechanically stabilized earth applications. The slits will not significantly compromise the reinforcing function of the geotextile because the tensile forces in the plane of the fabric will act parallel to the slits.

•  Use horizontally discontinuous, vertical drainage blankets or chimney drains behind earthen brushlayer fills to allow roots to more easily grow into the natural ground behind without the need to penetrate across a geotextile drainage layer.

The jury is still out on the question of whether or not geotextiles prevent root penetration across the plane of the fabric. Carefully designed tests and research studies are required for a definitive answer. Available evidence suggests that root growth and penetration may be impeded but not prevented entirely. Only geotextiles with very small apertures or effective pore sizes will function as effective root barriers. Several techniques can be adopted to improve or facilitate root penetration.

Miljostyrelsen (2003). Geotextiles as root barriers.http://mst.dk/udgiv/publikationer/2003/87-7972-497-3/html/samfat_eng.htm

Morgan, D.L. (2003). Focusing on fabrics. American Nursery Magazine, May 15, 2003, p. 32

Vanicek, V. (1973). The soil protective role of specially shaped plant roots. Biological Conservation 5(3): 175-180.