Having spent the better part of Saturday removing plastic and landscape fabric from a client's yard, this seems a good time for a reminder of why they're a bad idea — if my aching muscles can manage the typing.
This particular landscape has had a problem with intermittent flooding for years, and it was the search for the supposed leaky pipes that led to the discovery of the real problem: plastic and landscape fabric had prevented irrigation water from reaching the soil, resulting in runoff. It had been flooding for years, but there was enough gravel on top of the plastic and fabric that only occasionally did the water rise high enough to be visible and run into the street.
Large sheets of plastic and landscape fabric are typically laid on top of soil in an effort to prevent weed growth. Unfortunately, they don't work, and they cause a host of other problems along the way.
First, we'll take a look at why they don't work for weed control, and then we'll examine some of the other problems they cause.
In theory, plastic and landscape fabric control weeds by preventing weeds seeds from contacting the soil and forming roots. What this doesn't take into account is the creation of a new soil layer on top of the plastic or fabric. While we tend to think of soil as something permanent, and the creation of soil as something that happened in the past, in fact new soil is being created every day. The specific process that concerns us here is called "wind deposition" and if you have ever seen a dust storm, you have seen it happening. All that dust in the air eventually settles out when the wind dies down, and when it lands on top of the plastic or fabric it creates new soil for weed growth; the landscape from Saturday had at least two inches of soil on top of the plastic and fabric. In some areas there was enough new soil that small plants had been planted without the fabric beneath them ever being discovered.
If you're skeptical about new soil forming in such quantities, try this: don't vacuum or dust the interior of your house for an entire year, then measure how much soil you have on your living room floor. Send me an e-mail next year and let me know how that works out for you.
To add injury to insult, plastic and landscape fabric create problems in addition to not solving any problems. First, they prevent water from reaching the soil. While new-generation landscape fabrics claim greater water and gas (air) permeability, remember that water always follows the path of least resistance, and if there is even the slightest slope, the rain or irrigation water will flow over the surface of landscape fabric, rather than seeping through. In the Desert Garden we need all the water we can get, and anything that creates a barrier between water and our soil creates a terrible waste.
The second problem created by plastic and landscape fabric is the same as the first, but turned upside down. There will always be some moisture in covered soil, if only from seeping in at the edges, or from the holes cut for plants. As this moisture rises in the soil column, it becomes trapped immediately underneath the plastic or fabric, creating a permanently moist soil zone. Permanently moist and covered soil tends to form a "dead zone" where none of the activity that normally takes place in soils can occur. Even our tough desert soils are normally a hot-bed of activity: rain falls, the winds removes old and then deposits new soil, organic material falls on the surface and decays, the worms crawl in and the worms crawl out. You can see this for yourself: walk out into your garden and carefully make a footprint in an area of unplanted soil; in a fairly short time, that footprint will be gone. When we uncovered the soil on Saturday, we found footprints and tire tracks imprinted in the moist compacted soil, as fresh and clear that day as on the day they were created, more than a decade ago.
The third problem created by plastic and landscape fabric is a direct result of the other two: because water can't penetrate the soil, and because a moist zone forms at the surface, the roots of the trees and other plants proliferate in the only available area of moisture and grow in a shallow layer just beneath the landscape fabric; instead of anchoring the plants deeply in the soil, the roots spread out on top of it. On Saturday, we found wrist-sized roots laying on top of the soil at distances twenty feet from the trees to which they belonged; these particular trees are small and fairly sheltered, but larger trees in a more open space would likely have blown down by now.
And last: in all but a very rocky or very sandy soil, the constant pressure of a smooth fabric or plastic layer on top of damp soil compacts and "burnishes" the soil surface to such a degree that the pores of the soil are all but eliminated, impairing the gas and moisture exchange capacity of the soil even after the offending material is removed. Such a "closed" soil only re-opens after a lot of time or a lot of effort.
Weeds have been with us always, weeds will be with us always, and there is no silver bullet to kill them dead forever; anything that claims to be — whether physical, mechanical, or chemical — is a guaranteed misfire.