Landscape Fabric: A Cautionary Tale

Feb 18, 2023
Do those nice, neat rolls of “weed blocking” fabric seem like the perfect solution in your battle
against unwanted vegetation? Visions of freeing oneself from this perennial struggle are
enticing and the task seems simple enough: just lay it on the soil, cover with an attractive layer
of mulch and the foul demon weeds that lie in wait will wither and die in the darkness. At last,
you will be free from the tyranny of weeding! Hang on--there's more you should know about
this stuff! Although the promise of a neat, easy to maintain landscape may be hard to resist, the
reality isn't quite what it's cracked up to be, and you may end up doing more harm than good.
First, a bit of history. Landscape fabrics were developed originally for commercial use in large-
scale agriculture operations and to provide stability for structures such as retaining walls. Weed
suppression is accomplished by blocking sunlight from reaching seeds present in the soil.
Manufacturers claim that the materials are permeable, help to retain moisture, prevent weeds
from growing, and can therefore reduce the need for chemical herbicides.

Does it work? Landscape fabric does help to suppress weeds, but the effect is only temporary.
When freshly laid down, fewer weeds will appear on the surface because seeds that are already
present in a particular area are, indeed, blocked. Over time, however, new, and ever-
enterprising weed seeds will be blown or dropped by birds onto the surface where they will
proceed to germinate just as before. Meanwhile, the worst and most determined weeds will
opportunistically seek tiny holes in the fabric and utilize pinpoints of light to sprout through the
fabric-mulch layer and emerge triumphantly on the surface. You might admire their tenacity
before pulling them out but unfortunately, the tough fabric covering makes it difficult to get at
the root--a crucial aspect in the battle against weeds. Most likely, a bigger hole has also been
created in the process, making it easier for more weeds to germinate.

Is it permeable? Landscape or “weed blocking” fabric is made of tightly woven fiber, usually
polyester or plastic, both of which are derived from petroleum. There are different grades and
thicknesses, which will have correspondingly different degrees of permeability. Initially, they
are somewhat porous, allowing a certain amount of water and air to move through the fabric.
Unfortunately, permeability decreases in short order as the small holes that create porosity
gradually become clogged with dirt and debris. This is where the trouble starts.

Does it help conserve water? Moisture trapped in the soil beneath the landscape fabric barrier
will have a harder time evaporating so initially, plants might require less water. But, as you will
soon see, retained moisture can become a problem.

What goes on below the surface? While not visible to the naked eye, healthy soil teems with
billions of beneficial organisms that depend on the movement of water and vital gases (oxygen,
carbon dioxide) between the atmosphere and the soil. As permeability decreases, these
components become more and more restricted. Deprived of oxygen, carbon dioxide and water,
soil microorganisms die off, leading to a downward cycle. Natural processes start to shut down
and plants start to appear less and less healthy. Once uncovered, the degraded soil may appear
cracked, compacted, and will very likely smell rotten. This is because the natural process of
decomposition has been interrupted and the healthy microbial community has died off.

Rooting for roots. Plants that are surrounded by increasingly clogged landscape material have a
hard time, too. Vegetation that is planted in good, properly irrigated soil grows deep roots in
the process of seeking out nutrients and moisture. This leads to healthy plants that are more
resilient to stress. Conversely, plants that are surrounded by a cover of landscape fabric (which
can be bone dry even after a deep, soaking rain) soon spread their roots out closer to the
surface. Eventually, roots may appear at the edge of the landscape fabric but despite a
gardener's best efforts, plants in this scenario will not do well.

Mulch: always good? Landscape fabric is often topped with a layer of organic mulch such as
wood chips. Although mulch is normally a very good thing and a top dressing of it certainly
looks more attractive than naked landscape fabric, it can't do what it does when it is in direct
contact with the native soil and becomes counterproductive. Why? Instead of the usual process
of decomposing and adding valuable organic matter to the soil, the mulch particles just break
down on top of the fabric and add more particulates to clog things up even more.
Still in the weeds? If you are determined to get the upper hand in the battle against weeds,
consider sheet mulching and other beneficial practices that will minimize time and effort and,
at the same time, help your garden to thrive.

For more information on weeds and weed suppression techniques, check out these resources:
Ali Williams is an El Granada based UC Master Gardener who promotes sustainable practices.
This article was edited by UC Master Gardeners Maggie Mah and Cynthia Nations.

By Ali Williams
By Maggie Mah
By Cynthia Nations

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