Film mulches include such manmade material as black, clear, and colored polyethylene, weed-block fabric, aluminum foil, roofing paper, paper coated with aluminum foil, biodegradable paper, and some experimental degradable plastic mulches. Not all of these mulches are popular enough to be commercially available; the most common manmade mulches used by home gardeners are black and clear plastic film and weed block fabric. They are available in rolls at nurseries and garden supply centers.
Plastic mulches modify soil temperature, conserve soil moisture, control weeds (black plastic), prevent root injury because the soil is not cultivated, maintain good soil structure by preventing crusting and compaction of the soil, reflect light that repels certain insects (colored plastics), and improve soil moisture-holding capacity.
Watering and Plastic Mulches
Watering through plastic mulches is not a problem, and gardeners who have drip irrigation systems save water. Lay the drip system down before the plastic, and spread the plastic over it. Be careful not to cut the drip lines when punching planting holes in the plastic.
To irrigate with a sprinkler system, cut upside-down T-slits in the plastic every foot or so for water to run in. If the water puddles on the plastic, punch a couple of holes in the bottom of the puddle. Another way to water is to lay soaker hoses under the plastic. Leave the end exposed and attach a hose to it to water.
With furrow irrigation, cover the beds with plastic and leave the furrows exposed.
Soil Heat and Plastic Mulches
Plastic mulches have pronounced effects on the soil they cover, and ultimately on the plants’ grown. But there are considerable differences in the way black and clear plastic deal with the sun.
The sun’s rays that reach the earth are absorbed or reflected back to the sky, depending on what kind of surface they fall on. Most of the sun’s energy is carried by “short” rays — rays in the visible part of the spectrum. The soil absorbs short rays, but reradiates its stored heat but in a different wavelength — as “long”, or infrared, rays.
The short rays of the sun travel through glass, translucent paper, clear plastic film and other transparent or translucent materials. But these materials block the long rays, trapping heat inside. This is called the “greenhouse effect.”
Clear plastic mulch allows the short rays of the sun to enter the soil and traps heat there, effectively increasing soil temperatures by 10 degrees or more. The plastic also traps warm air under the mulch, adding to the heating effect.
Black plastic gets very hot on a warm sunny day as the film absorbs heat. But the plastic gives back a great deal of heat to the air above it rather than transferring it to the soil. The insulating air pockets between the film and the soil surface retard most of the heat transfer to the soil. The soil temperature under black plastic is generally only 2 degrees to 6 degrees warmer than uncovered soil.
Usually made of spun polyethylene, this porous material allows water and air to pass, but blocks weed growth. It has little effect on soil temperature, nor does it prevent water loss from the soil. As are most film mulches, it is unattractive, so is usually covered with an ornamental mulch. It is most useful under permanent mulches, such as gravel or river rock. With a weed block underneath, only enough gravel to cover the weed block is needed, saving money and labor.
To keep corners and edges from working their way to the surface, stretch the fabric and staple it down with garden staples, usually available where you buy the weed-block fabric.
Weed-block fabric is also useful as a weed block under patios, paths, and other permanent pavings where weeds might be a problem.
Black Plastic Film
The mulching material most often used by home gardeners is black polyethylene film, which is available in rolls 3 or 4 feet wide and 1 or 1.5 mils thick. Black film prevents weeds from growing under it because it excludes the light needed for their growth.
A mulch of black plastic film increases yields and speeds up the ripening of melons, eggplant, peppers and summer squash. In areas where temperatures early in the growing season are less than ideal for these warm weather crops, yields of muskmelon in experimental plots have been increased fourfold over the yields of unmulched plants. Make sure that the soil is damp when the mulch is applied so that there is sufficient moisture for good plant development under the mulch.
Clear Plastic Film
The major differences between clear and black plastic mulches are that clear plastic mulch promotes weed growth and heats up the soil.
Clear plastic has its own uses in the garden, especially in cool climates. A layer of clear plastic over the soil helps with warm season crops such as tomatoes, melons, peppers, and all the squash family. The warming of the soil promotes 10 to 14 days earlier maturity and higher yields of tomatoes. If the air temperature gets into the 90’s, the weeds will probably be killed by the heat under the plastic. If not, remove the plastic and weed by hand.
In hot-summer areas, clear plastic will probably warm the soil to the point where it kills plant roots. However, it can be used as a temporary mulch in the spring to warm the soil, then removed or replace with another mulch when hot weather arrives, or cover it with a thin layer of organic mulch, such as sawdust.
In the hottest part of summer, clear plastic film heats the soil enough to kill weed seeds, fungi, and insects. This pasteurizing process is called solarizing, and can be used to prepare soil for planting or destroy soil-borne pathogens.
Combining Film and Particle Mulches
Landscapers frequently save money on expensive mulches by laying them over a layer of black plastic or weed-block fabric. This method is effective, with the plastic stopping weeds and the particle mulch protecting the plastic from both tearing and sun damage. However, it’s difficult to remove the plastic later to work in the soil, and ragged corners tend to emerge from the mulch with time. If you can afford it, it’s better to put down enough particle mulch to control weeds by itself.
Planting in Plastic
Plant rows of seeds in plastic mulch by making a series of slits, about 2 feet long, to plant through, leaving small bridges of uncut plastic between slits to keep the mulch together. Dribble a little sand through the slit to cover the seeds. For planting larger seeds through plastic mulch, an old-fashioned corn planter (sometimes called a jabber) does a very nice job. Simply push the jabber through the plastic and use it as you would in cultivated soil.
Transplants may be planted through X’s cut in the plastic, or use a small bulb planter. Sharpen the cutting edge with a file, then cut through the mulch cleanly with a twisting motion and continue on to make a planting hole in the soil.
Colored and Reflective Mulches
There is some evidence that colored mulches promote the growth of plants grown in them. Research by the Agricultural Research Service of the USDA showed that red plastic increased yields of tomatoes and other crops grow in them. Red mulch of the right shade for this use is available from specialty gardener’s supply outlets.
Reflective mulches, such as aluminum foil and white plastic, might promote faster growth by reflecting more light to the leaves. They may also repel some insects.