In an era of rising eco-consciousness, homeowners are gradually moving away from lawns and other heavily irrigated landscapes.
The trend is toward xeriscapes, landscapes that are designed for climate they are located in, with plants adapted to thrive on little more than rainfall, and gardens that use efficient irrigation systems like drip systems or, in some cases, no irrigation at all. That is definitely a laudable option in my book—when I install gardens, it’s practically the only thing I consider unless I’m asked to do otherwise. Not only is it better for the environment, a site-appropriate garden is more resilient and easier to maintain.
But one unfortunate detour in the path towards xeriscaping is a tendency for installers to plot out gardens with vast swaths of landscape fabric. The material, made of polypropylene plastic, is spun into threads or ribbons and woven into a cloth purported to allow air and water through, then topped with gravel or mulch. The stated purpose of the fabric is to prevent weeds from emerging between the plants, which, in a popular xeriscape style, are placed relatively far apart in holes punched strategically in the fabric.
There are a number of low-water garden plants that appear to do ok in landscape fabric, and you have seen them because they are the popular plants to install in road medians and around parking lots, or other rarely-maintained but busy public spaces, where landscape fabric is ubiquitous, or at least has been for a couple decades at the time I am writing this. And indeed, for the first year—or at least at the moment that the installation service collects payment from the property owner—it appears that the fabric is doing its job keeping plants comfortably spaced and blocking the weeds between them.
Many modern xeriscape styles, especially those I see around Denver, seem to exist on the idea of saving water by using a lack of plants. The few that are in place consist of some yucca or a small patch of daylilies, a fernleaf yarrow or two, some sedums, a couple stands of ‘Karl Forster’ ornamental grass, and a big patch of Russian sage, all arranged as islands in a sea of gravely rock. The absence of plant coverage means weeds would have ample opportunity to grow, but are kept at bay, at first, by the fabric beneath them.
That’s not the case the year after. Again, you have seen this before—how often do the gardens around the edges of parking lots keep looking great for long? There are always weeds, and probably a few patches where installed plants have died, and the opening is crammed with cheatgrass because the drip emitter is still trickling water in that spot. Even an untended irrigation ditch or rural highway shoulder, receiving no planting or maintenance whatsoever, probably has prettier vegetation than the berms around a suburban parking lot, middle school yard or boulevard median, where fabric is purportedly keeping things nice. Some lucky properties might get a second or third year out of their fabric, but ultimately, inevitably, the fabric will fail, and the garden will go to chaos unless it’s in an affluent area where it is frequently re-installed.
Ironically, a garden bed covered in landscape fabric is like a curated space for noxious weeds
The types of weeds that we have learned to dread most as gardeners are species that have followed human inhabitation around the world—as invasive species, thriving on the peculiar alterations we repeatedly make to land.
Despite the wide variety of weed species we deal with, there are actually some pronounced ecological similarities between the most common garden weeds. They usually resent shade and do not thrive in competition from diverse neighboring plants, but spread eagerly in monocultures like lawns or crop fields or areas cleared by frequent mowing, herbicides, foot traffic or construction. Common weeds are resistant to chemicals that kill other plants, mainly because they have all been sprayed so many times and only the survivors, bearing genetic mutations for resistance, have passed on their seeds. Common weeds dominate dense compacted soils low in oxygen—humans are great at creating these compacted soils along streets, in lawns, in foot-trafficked areas and construction sites—and are tolerant of salt and other common pollutants found in developed areas.
The weeds germinate in two main phases depending on the species—late winter, when nothing else in the garden is active (we don’t often think of this time as meaningful for plant growth, though it is in the wild), or at the onset of hot temperatures in late spring when long days supercharge fast growth. The weeds reach maturity within a few weeks, faster than most other plants do, as a function of the frequently-disturbed settings they are adapted to. They quickly flower and drop more seed, or anchor their roots into deep soil before anything else can displace them.
These weeds—you know them as bindweed, dandelions, thistle, quackgrass, cheatgrass, sorrel and others—are good at overcoming the barrier weed fabric presents to them. They can germinate in the duff layer on top of the fabric and grow in soils that are extremely shallow (such as cheatgrass), or they send their taproots down through the fabric (examples include mallow or curly dock), or they manage to gain foothold under the barrier and spread on long runners that can grow several feet in all directions, searching for an opening. Bindweed and thistle are obvious examples of weeds that advance underground, exploit seams or holes in the fabric, and, left unchecked, can form dense mats on top of it with deep, unreachable roots burrowed in the low-oxygen soils beneath. Meanwhile, desirable plants—many of which can be quite hardy in the face of competition or drought—do not contend well with the surface barrier, and can’t fill in to displace the weeds. Groundcovers are limited to the size of the hole in the fabric that was cut for them, so they more resemble cute miniature plants than the sprawling spreaders they are selected to be.
The above are reasons why landscape fabric doesn’t work as well as we hope it would. But I’d also like to address some ways fabric could be actively harmful, at least in terms of missed opportunity to create a garden with ecological value.
How unnatural barriers block healthy processes in the garden
See, long-lived plants that colonize the soil actually improve it structurally and chemically, to make the soil layers much more porous, absorbent of water and efficient at recycling nutrients. This benefits the plants directly as the soil’s inhabitants, but it also has secondary benefits for the whole system. Plants’ deep roots periodically die and are replaced by new roots, leaving long-lasting channels in the earth, which water can trickle down and oxygen can move through. The leafy and woody debris plants drop on the soil surface feeds microbes that bind silt and clay particles to bits of organic material, forming granules that give soil a grainy, light texture even if the inorganic components are otherwise very fine and sticky. The worms and burrowing insects that feed off of dead leaves and roots stir the soil and make larger channels, crossing the soil profile in many directions.
All of this activity, in aggregate, drags organic material—often referred to in shorthand as carbon—deep into the soil, where it is sequestered for many years, and helps draw heavy rainfall into the deep layers, and also gradually wicks deep water back up towards the surface when the top layers are dry.
All of this further serves to capture heavy rain and hold it on site, or gradually feed water to aquifers, with minimal runoff. Limiting runoff has the obvious effect of reducing flooding, but also means our urban pollutants (mostly nitrates and sulfates) can be processed by soil bacteria and rendered harmless before they reach natural waterways. When land is covered by deep, porous, biologically active soil, streams are fed by rainwater moving through soil over weeks rather than runoff pouring over the land in hours, and thus streams will not swell as intensely in storms or dry to a trickle during brief droughts. Deep, uninterrupted soils also mean that water stays put in deep layers of soil for a long time, and plants can draw from the reserves between rains, so there is less need for irrigation, less loss of plant life during dry spells, and more biological productivity on the whole.
Landscape fabric interferes with all of this. When soil particles mesh with the fabric, it forms a sheet that is far less permeable to water and oxygen than the fabric initially was. Between holes punched in the fabric for plants, the soil surface is relatively lifeless and hard, although lifting a sheet of fabric might reveal the thin, white etiolated stolons of bindweed and Canada thistle corkscrewing in all directions just beneath it.
The benefits of open, mulched soil with no barriers extends to animal life as well. Beneficial insects—both carnivores, which eat other insects, and detritrivores, which eat dead leaves, stems and fungi—overwinter below ground and benefit from free movement between soil and the sky. Small birds, in turn, rely on an abundant supply of earthworms, beetles, grubs and arachnids in the mulch layer. This supply of protein is crucially important for them raise their young, as you will see when they arrive to pick through the garden and burrow into the duff layer with their heads. Soil creatures will also come up to eat dead plant material or to drag it into their burrows to moisten it so they can eat portions as they decay, cleaning up the garden and recycling the nutrients. All of this life and activity depends on a barrier-free continuum between the sky and the soil—two realms that are not really as separate on Earth as they seem.
With this, I hope it’s clear that landscape fabric can cause more problems than it solves. A relatively recent invention, landscape fabric is a strategy that, though logically understandable, doesn’t hold up in practice—like so many of our attempts at micro-managing nature. Instead, using organic mulches like wood chips, or other natural materials such as large stones, can change the structure of a perennial garden favoring desirable plants over opportunistic weeds. In turn, it allows the plants themselves to control their habitat, feeding wildlife and forming a garden that does not deteriorate with time, but grows only more well-established and resilient.