Combining Mulch with Geotextiles for Landscape Weed Control

Most landscape maintenance programs rely on hand weeding, herbicides, and mulches (alone or in combination) to suppress and control weeds. Black plastic has traditionally been used by many landscapers to enhance the effectiveness of organic and inorganic mulches, but several studies have reported adverse effects on landscape plant growth due to its use.

Within the past few years, a new group of synthetic materials has been introduced for use with mulches for landscape weed control. These geotextiles (also called landscape fabrics or weed barriers) have one major advantage over plastics -- they permit the exchange of water and air between the soil and the atmosphere. Several reports have been published on the weed-suppressing effects of these geotextiles, with mixed results. While most are fairly effective at controlling annual weeds, control of perennial weeds is often poor. The manufacturers recommend applying a 2- to 3-inch layer of mulch atop the geotextiles for aesthetic reasons, and as a result, numerous problems have been observed, particularly when organic mulches (pine bark, pine straw, hardwood bark, etc.) are used.

The major problems that have been observed when geotextiles are topped with mulches are:

• Weed growth in the mulch layer when organic materials are used

  Weeds begin to grow after the mulch is installed, even when all existing weeds have previously been eradicated from the site. Contamination may originate from weed seed carried in the mulch or introduced via wind or irrigation water. Mulch depths need to be kept to a minimum, not only for economic reasons, but also to prevent weed growth in the mulches. As mulch depths increase, the need for geotextiles decreases to the point that mulches alone are as effective for weed control as mulch/geotextile combinations. Even with proper mulch depths, some hand weeding or herbicide use will be necessary. Weeds are easier to pull from shallow mulch depths, but if the weeds are successful at penetrating the fabrics, they begin to grow very rapidly.

• Mulch shifting and exposure of the geotextiles

  One criticism of plastic has been that it lacks an adequate coefficient of friction or roughness to keep mulch from sliding or washing off, especially on sloped areas. The same problem has been seen with many geotextiles that have slick or smooth rather than rough surfaces. Once mulches shift and expose the geotextiles, not only is the appearance of the landscape diminished, but if the geotextiles lack ultra-violet inhibitors, they may begin to decompose, giving way to weed growth.

• Lack of mulch replenishment and exposure of the geotextiles

  Due to the decomposition of organic mulches, they must be periodically replenished to avoid exposure and possible decomposition of the geotextiles.

While root surfacing is not uncommon in landscape mulches and forest-floor leaf litter (due to the moist, yet well-aerated nature of this soil covering), we had not thought about it occurring with the mulch-covered fabrics, where the same moist, well-aerated medium is created. Surfacing also occurred under the black plastic and black plastic/mulch plots (presumably due to low oxygen content in the soil under the black plastic), but roots did not grow into or through the plastic. There was also a considerable increase in vole tunnels under the fabric/mulch coverings.

Where we see a potential problem is in landscapes that are periodically reworked or replanted, where sections of fabric might need to be lifted and/or removed. This could severely damage large portions of landscape plant root systems if this phenomena proves common.