Designing Landscapes: Using landscape fabric as weed blocker
There is often debate as to whether landscape fabric is effective as a weed blocker. Here I will discuss my experience with the material.
What is landscape fabric?
Landscape fabric usually comes in two forms. One is an actual woven fabric made from synthetic fibers such as polypropylene. This fabric has a weave through which water and air can pass through. This is considered the heavy duty stuff. They come in 3 feet or 6 feet wide rolls typically, although they can be ordered even wider than that. Landscape fabric has many uses but I will discuss its use as a weed blocker here.
The other forms are a thin plastic membrane made with a multitude of holes poked in it to allow air and water to penetrate and there is also plain old plastic sheet which is sometimes used as a weed blocker.
How do you use it?
The concept is pretty simple. You cover soil with a sheet to prevent weeds from growing up from below the weed blocker or rooting down into the soil from above the blocker. To keep the blocker from blowing away and to make it look better, you stake it down then cover it with mulch: wood chips, bark chips, rocks, etc.
Ideally, any weeds present before the weed blocker is applied can just be cut to ground level and covered by the blocker without removing their roots and the weed blocker will smother them.
Then, wherever you want your desired plant to grow, you cut a hole in the weed blocker and insert your plant’s roots through the hole and into the soil.
So how well does it work?
The basic concept is logical and it does initially work. If you have weeds that are really tenacious and like to come back from the roots, it does make sense to use a weed blocker to smother them for an extended period so they don’t come back.
However, I have personally found that over time weed blocker often becomes more of a hassle than a help and it is not good for the soil health.
Much like we’ve seen with thatch in the case of lawn grass, weed blockers form a barrier to healthy root growth for the plants we are trying to grow. Even though some amount of air and water can get through the weave or the holes, there is still a blockage of air, water, and nutrients by the fabric. That is the idea after all, to squelch the growth of plants. If the weave was too open, the weeds would grow through. When you make the weave tight enough to prevent weeds then generally the plants we are trying to grow will feel the negative effects as well.
We can’t have it both ways. We can’t make the soil inhospitable to weeds yet retain the hospitality for our desired plants.
In addition, the tenacity of the weed blocker sheet along with the weight of mulch can compact the soil below. I’m not sure what the mechanism is that causes this but pulling up fabric or sheet that has been down for several years usually reveals a hard soil surface, sometimes burnished and compacted into such a hard layer that it is actually shiny, especially if using plastic sheet. The soil is usually devoid of organic matter so you can expect very little microbial action is taking place and therefore very little natural fertility.
I believe we should be thinking about how to maximize the health of our gardens by improving the condition of the soil. This means we should avoid methods that selectively degrade areas of soil in order to control plants we perceive to be harmful. I believe weed blockers degrade the quality of the soil, so they should be avoided in our garden beds.
Rather than degrade, I would recommend improving soil health so that our desired plants can out compete the weeds by taking over the root space and by shading out areas weeds like to grow in. Weeds, by and large, populate open spaces.
So, here is a free pass to plant as many desirable plants in your garden as you can manage. Plant densely so your desired plants effectively become your weed blocker.
Human vs. weed
The Achilles heal of all degradation type control methods is that weeds by their very nature are best suited to inhospitable environments. That is what they do. They grow in the slightest accumulation of dirt in the gutters of a dilapidated house 20 feet off the ground. They grow in the gravel of a driveway shrugging off the occasional hit-and-run by the family car. When they get sprayed with enough herbicide, they adapt, become immune, and become “Superweeds”.
Simply put, weeds are bad-assed plants and their purpose is to fight soil injustice!
Weeds are Nature’s conquistadors of sorts. They infiltrate and colonize barren areas and the ironic part is; after many years the soil becomes more fertile and better suited to the plants we deem valuable. This is the work of the weeds. They are actually prepping the ground for later species, the ones we typically want. They are on our side if you can believe it. This progression from pioneer plants to climax species is well documented, but the problem with Nature’s progression is it works on a timescale that we perceive to be too long for our human needs. We are not a patient species, we tend to prioritize the short term at the expense of the long term.
Maybe we can envision the appearance of weeds as Mother Nature lending a helping hand and suggesting an easier way if only we would relinquish a little control (and less time and effort expended might I add). Sit back and observe more, reach for the kill-switch a little less.
Some dandelions in your beds are okay. See how they are feeding the early bees, see how they stay green in a drought with their deep roots bringing up water and nutrients from the deep soil below? Some blackberry invasion is okay. See how delicious those berries taste come Autumn? They are full of antioxidants and other healthy compounds and the birds enjoy them too. Some English Ivy climbing the fence and the tree is okay. See what beautiful soil it has left behind after we finally got around to converting that ivy patch to a garden bed?
Of course, I wouldn’t go so far as to advocate for weeds. I’m merely suggesting that they are not a mistake in Nature’s long term system and we need to carefully examine the damage caused by methods of degradation vs the actual damage caused by weeds which is usually very little.
Like doctors, the gardeners should take the Hippocratic oath to:
Primum non nocere: First, do no harm.
So to circle back, due to weed’s inherent tenacity, even when we apply weed blocker, the weeds still come back. The slightest accumulation of dust in the mulch above the weed blocker allows the weeds to root. Then, the smallest of holes in the weed blocker allow the weed’s roots to penetrate the blocker, as in the picture below, nullifying the blocker’s effect.
The weeds will be back because that is their nature.
Meanwhile, all weed blocker material deteriorates and eventually ceases to serve its purpose. When the act of weeding does need to start again, after the honeymoon period of successful blocking, there is a tendency to poke holes in the weed blocker in the course of weeding with tools. It’s inevitable.
Worse, once the weeds triumph over the weed blocker, the blocker itself becomes a nuisance to the gardener trying to weed the area via the usual manual methods. Instead of just pulling weeds you’re also pulling weed blocker because the weed’s roots are tangled up in it.
In a somewhat comical fashion, pulling up a weed may suddenly become an unraveling of your garden bed; large sections of weed blocker coming along for the ride. After a short bout of tug-of-war, human vs. weed, you end up with a circle of torn blocker fabric in your hand and a ring of frayed blocker sticking up from under the mulch. Do you effectively “sweep it under the rug” (cover the ripped blocker over with nearby mulch) or do you proceed to rid the entire bed of the no longer effective weed blocker; an unplanned renovation project?
Biodegradable Weed Blocker (i.e: Sheet Mulch)
I do believe there is a case to be made for bio-degradable weed blocker instead of the plastic weed blockers. This would be a non-petroleum based product such as paper or cardboard. The intended result of this “sheet mulch”, as it is sometimes called, is to give your desired plants a head-start with the sheet mulch acting as weed blocker. Then, as the desired plants mature, the sheet mulch decomposes and you no longer have the barrier to good soil health. That’s how it’s supposed to progress anyway.
A few difficulties with this method are:
- Sheet mulch is a fair amount of work to assemble and install. Usually more work than the plastic weed blocker since the sheets usually comes in small sizes to which you must collect and then assemble into a mosaic of overlapping pieces to cover your garden beds, then cover over with mulch before the wind blows them away. The labor and expense of sheet mulching should be weighed against its effectiveness. Sheet mulch is usually best for small areas.
- Is sheet mulching material toxic? It’s hard to say, it depends where you source it. I have heard white paper (bleached) is more toxic than brown and cardboard may use toxic glues, but it’s hard to track. I haven’t come across any studies on its effects on garden soil. It would seem to me brown butcher paper or other food grade material is the best choice but it’s just a guess.
- If you don’t keep the sheet mulch area well moistened, it wont decompose as you intended. Extra watering may be required, not for the plants but for the sheet mulch.
- Sheet mulch can make it harder to water your plants because multiple layers of sheet are really resistant to water passing through. Its true that you might get more water to your plant by watering near the base of the plant where the hole is poked through the sheet mulch but that can tempt crown rot and usually a plant’s feeder roots eventually get well beyond the canopy of the plant which is under the sheet mulch.
In summary, there are many things to think about when choosing to use a weed blocking method for your garden. Weed blocking fabric is effective for the short term but has soil health consequences. They often cause more work when they start to fail and can actually make weeding harder at that point. There are some advantages for temporary, biodegradable weed blocker, also known as sheet mulch. When growing a garden, I believe one should be focusing on maximizing soil health, plant health, and overall system resilience while doing no harm. Weeds do have value and a use, it is up to us to understand them better. A mature healthy garden full of plants covering bare ground is naturally resistant to weeds as that is making use of Nature’s natural progression.
– By James Young
James Young is the owner of Blue Wheelbarrow Landscaping in Edmonds.