Plastic to prevent weeds? Stephen and the earthworms say “No”

Spring is the time of year when we all want to get out and “spruce up” our gardens. Because of time pressure we also want to create as maintenance-free environment as possible. One way (but not a good way) that we can attempt to improve the long-term appearance and cut down on future weeding is by applying a layer of plastic covered with mulch.  Plastic has come into our lives in many and various ways over the last few decades. It has been used as a weed barrier in gardens and in the nursery industry as a whole. In many instances it is an ideal barrier in both preventing weed growth and conserving moisture. However, there are some negative effects of using plastic as a barrier.

Plastic is a total barrier, it keeps weeds from coming through but it also keeps everything else both above and below from crossing the barrier. Air and water are unable to move down, and healthy bacteria and earthworms are unable to move across the barrier. A healthy soil is a mixed soil, one in which the natural environmental elements are allowed to move back and forth.  This is not possible with a plastic barrier. Normally, mulch is a very healthy ingredient to add to soil not simply because it looks attractive but because it adds organic matter. The organic matter, even though it is initially placed on the top, eventually works it way down into the soil through the action of earthworms, bacteria, and other natural processes. Organic matter contributes to a healthier soil through a greater water and nutrient holding capacity, greater aeration, and an environment which is generally more conducive for all living organisms. These characteristics are the key to a healthy soil. Without them, the roots of plants do not have an appropriate environment to survive and feed the upper part of the plant.

Plastic prevents all of the above processes from occurring. The lower layer of soil becomes totally isolated from the above. All of the naturally occurring processes which allow air and organics to move into the lower layers of soil are prevented. Earthworms and bacteria then have no reason to either stay in the lower area or come to the upper area. Earthworms not only help to create a healthy soil but they are an index of the health of the soil. The same is true of other bacteria but we can more readily see the earthworms.

Another negative effect of plastic is that it contributes to a two-layer root system. From my experience in moving boxwood which have been grown around plastic for a few years, I have noticed that there are roots both above and below the plastic. As mulch has been added around the plant and built up, roots have developed and created this two-layer system. It is interesting to note the difference between the soil above and below the plastic. The soil below is more compacted and dense and lacks the color of organics. The soil above is aerated and loose with the dark color of organics and even smells differently. The plastic has created two very different environments, isolating each, and preventing any interaction between the two.

Ironically, in putting mulch over plastic, the plastic’s original purpose of preventing weed growth is usually not accomplished. Weed seeds fall in the mulch and the rotting organics provide a wonderful environment for germination. Mulch without plastic will provide all of the positive aspects of applying plastic like water retention, while avoiding all of the negative effects of the plastic. Your garden environment will be filled with living organisms below the surface which are working constantly to improve your soil and give you a healthier environment for your plants.

Stephen D. Southall
English Boxwoods of Virginia

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