Fabric Mulch Friday, March 15, 2019
Author(s): Steve Rasmussen, District Forester
Mulch fabric, sometimes referred to as “weed barrier” or "black plastic", has become a useful tool in establishing conservation tree plantings in Nebraska and across the Great Plains region. The material is a black polypropylene fabric with the appearance of tightly-woven burlap. It is recommended and used most heavily in locations with droughty soils and areas that receive 24 inches of precipitation or less per year. This fabric is intended to degrade within 5-10 years.
Proper installation is critical for the fabric to provide the most benefits and not adversely affect the tree planting. With the expense of the fabric and installation, customers have a right to expect the correct job done during installation. The fabric should be installed as soon as possible after the trees are planted to ensure adequate water conservation and/or weed control.
For best results, the fabric should be installed right after the trees are planted and have a funneling effect to direct water to the seedling.
Fabric should be stretched tight and lay flat on the ground without a deep "furrow" in the tree row that holds the fabric off the ground. If the fabric is too far off the ground around the seedling, then hot air and rodents can easily get under the fabric, damaging the seedling. If the tree planting machine leaves a furrow, the ridges on each side should be packed down. A slight furrow (so the fabric is less than 2 inches off the ground) is good to help funnel water runoff toward the base of the seedling. As a tree matures, the fabric should be slit and opened up. This allows the tree the freedom to continue growing without the fabric girdling or constricting the stem.
Fabric needs to be pinned or held down within 6 inches of the seedling to help direct the water to move and flow into the cut area by the seedling. Funneling the water to the base of the seedling increases available moisture. The use of soil is discouraged since heavy rains will wash everything off the fabric. Also, weed seeds can germinate in the soil on top of the fabric.
Edges need to be firmly secured by pins when installed by hand or rolled under the soil when installed by machine. Both methods keep the wind from blowing underneath, working it loose and flapping in the wind.
Where the end of one roll and the new row overlap, this splice should be held down with pins; again to keep the soil from washing away. In areas of sloped ground over 30% where water erosion could start under the fabric, one method would be to create slight water turnouts or water bars should be erected across the furrow to divert water from eroding the downslope. Another method is to leave a 1-2' gap when starting a new roll.
During installation, the seedling should be cut and the seedling pulled out from underneath the fabric within 60 seconds during hot, sunny days to reduce heat damage to the seedling. The fabric should be cut in the shape of an “X” or “T” (“U”) and not a single slit so the fabric does not rub on the seedling.
The cuts should be less than 12 inches with the seedling centered. Larger “holes” cut during installation decreases the effectiveness of having the fabric since winds and hot temperatures can then take moisture out of the ground next to the seedling. Also, the larger openings allow for weeds and grasses to grow close to the tree seedling. Improper cutting during the fabric installation can cut the seedling or "girdle" the tree as it grows larger.
The water conservation fabric does not degrade as well as advertised; and not at all if the fabric gets covered by soil, blowing sand, leaves, grass, etc. Even exposed to sunlight, the fabric rarely degrades well enough for the tree stem to rip the material. The greatest risk of tree failure with the fabric is having the tree “girdled” at the base with the tree trying to grow around the fabric. Rather than assume the fabric will degrade, the landowner should expect to have to cut the fabric around the base of the tree before the fabric starts girdling the tree.
Weed seed can still germinate and grow in the cut area next to the tree seedling, limiting growth. Weeds should be pulled when small and kept out of the area by the seedling. If allowed to grow too large, the weeds will choke out the tree seedling or when the weed is pulled out, the tree seedling also may get pulled out.
After the initial establishment time of 3-5 years (depending on tree type and growth rate), the fabric should be cut at right angles to allow for larger tree stem size as the seedling grows. This new opening should be at least 18 – 24 inches for most shade trees and conifers. Larger growing trees like cottonwoods and silver maples may need larger openings. Making the cut larger at this time is ok since the tree seedling roots should be 24 – 36 inches out away from the base and the tree is large enough to shade weeds and other competition.
Once the seedlings have grown and as trees have become established, the fabric can be removed by ripping it away from alongside the tree stems or pulled off the tree row. At this point the trees are strong enough not to be pulled out during the ripping. Cutting the fabric down the center of the tree row prior to or during the pulling process will help separate the material and keep the fabric from damaging the tree stem as it is pulled away.
Water conservation fabric is a useful and proven addition to conservation tree plantings on droughty and stressful sites. With proper installation and management, tree seedling survival and growth has been demonstrated to increased growth. However, without proper installation and awareness of the potential girdling risks when the fabric does not degrade, it can cause early decline and even death for trees as they grow larger and try to develop natural trunk swell at the base.
Contact your county USDA Natural Resources Conservation District, local area Natural Resources District or Nebraska Forest Service District Forester office for more information on mulch fabric use and general conservation tree and shrub plantings.