The use of plastics -- to cover greenhouses and high tunnels, as
row covers, and for soil mulches – has resulted in a horticultural revolution
of sorts. Largely unheard of just two generations ago, it’s almost impossible
now to find a farm that grows horticultural products without plastic. Taken
together, the use of agricultural plastics for modifying crop microclimates
to enhance quality and yield is known as ‘plasticulture.’
Your markets, your climate and your pocketbook all play a role in
determining how much and what type of plasticulture to use. If you want
to push harvests up by a week or two in order to meet market demand and
fetch good prices, then plastic mulch by itself may be sufficient. To gain
additional earliness, plastic mulch as well as row cover may be required.
Obviously there has to be an economic benefit to using both methods in
combination because the cost is higher. More extensive, and expensive,
measures to extend the growing season, such as high tunnels and greenhouses,
are justified only with crops that have very high economic values per unit
The first use of polyethylene as a greenhouse cover in the United
States was in 1948, when Professor Emery Myers Emmert at the University
of Kentucky employed this material in place of glass, the traditional but
more costly greenhouse covering. Dr. Emmert is known as the father of agricultural
plastics in the U.S. and he developed the use of agricultural plastic through
his research on greenhouses, row covers and mulches. More recently, in
the Northeast, Dr. Otho Wells of the University of New Hampshire played
a key role in developing plasticulture techniques suitable to our climate
and markets and then he helped educate growers and colleagues about their
Let’s consider just one component of plasticulture: mulch. Plastic
mulch has been used in commercial vegetable production since the 1960’s.
The potential benefits of using plastic mulch include: soil warming, reduced
evaporation, increased yield and earliness, reduced nutrient leaching and
improved nutrient uptake. Of course, there are downsides, too. These include:
cost of the material and greater labor and equipment expense to apply and
remove the mulch, as well as disposal fees that can be significant.
There is a wide variety of plastic mulch on the market today that
can reflect, absorb or transmit sunlight. The color of mulch has a big
influence on how it affects the microclimate around a crop plant.
Black plastic mulch is typically used for spring-seeded crops because
it increases soil temperatures about 5ºF at a depth of 2 inches compared
to bare ground. Black mulches absorb heat from sunlight then pass it on
to the soil below. That’s why black plastic must be laid flat and tight
to the ground if it is to provide the maximum warming effect. A big advantage
of black mulch is that it blocks visible light, so it is effective at reducing
weed growth (except in the planting holes!).
Clear mulch is the most transparent to infrared radiation and warms
the soil to the greatest extent of all plastic mulches, increasing daytime
soil temperatures from about 7º to 13ºF at a depth of 2 inches.
For warm season crops, such as vine crops, earlier yields and higher yields
can be achieved with clear mulch. However, clear mulch is also transparent
to visible light, which contributes to weed growth under the mulch.
Unless herbicides are applied under the mulch, or the crop canopy covers
the plastic quickly, those weeds can become a big problem.
Infra-Red Transmitting (IRT) mulches warm the soil intermediate to
clear and black mulch. They are pigmented to reduce the amount of visible
light transmitted in order to reduce weed growth under the mulch. IRT mulches
are usually green or brown, and they contain specific pigments that let
them transmit a maximum of near infra-red radiation and a minimum of visible
light. Not all green or brown mulches are IRT.
The benefit of IRT mulch is highest early in the season. Late-planted
crops don’t usually show a yield improvement with IRT compared to black
plastic. Cloudy cool weather can also lead to very little difference in
yield between IRT and black mulch. Since IRT costs more than black plastic,
it is best suited to early plantings of tomatoes, peppers, and vine crops.
If you are using plastic row covers, or tunnels, with tomatoes and peppers,
the air temperature over the IRT will not be as warm as over black mulch,
reducing the risk of blossom drop caused by high air temperatures in the
White or co-extruded white-on-black mulch can reduce surface soil
temperatures slightly, by about a half a degree at a 2-inch depth, relative
to bare soil because it reflects most incoming radiation. This mulch is
useful when lower soil temperatures are desired for summer production.
A wide variety of other colored plastic mulches, including red, yellow
and silver have been developed. Each of these colors has distinct effects
on how light is reflected as well as the radiation balance in and below
a crop canopy. That can influence crop performance.
Studies of red mulch have not shown consistent results. Red mulch
sometimes increases yield of tomatoes, but not always. Different tomato
varieties also appear to respond differently to the red mulch. The effect
of red mulch, like that of IRT, depends on sunlight levels, so the variation
results in studies may be due to different light conditions, as well as
different temperature conditions. Red mulch does tend to increase soil
temperatures much like an IRT, so in early plantings it is likely to promote
earlier yields. When used with main season plantings, the effects of red
mulch may be more variable.
Yellow is generally very attractive to insects—that’s why it is the
color of sticky cards used to monitor pests in the greenhouse. I’m
not sure why a grower would want to use yellow plastic mulch except maybe
to create a ‘trap crop’ that lures pests out of other parts of the field.
Highly reflective or shiny aluminum plastic mulches have been shown
to repel certain aphids. They are therefore useful to reduce or delay the
onset of mosaic viruses in squash, melons, and other crops, that are spread
by aphids. These are the mulches of choice when insect-spread virus management
is your principal goal. Recently, a mulch has been introduced that consists
of a strip of black plastic down the center that is flanked on either side
by metalized reflective plastic. This plastic combines the advantages of
black plastic over the seed row, to help heat the soil, with the reflective
characteristics of metalized plastic for insect and disease management.
For more information on plasticulture and plastic mulches, see:
Penn State Center for Plasticulture: plasticulture.cas.psu.edu.
American Society for Plasticulture: www.plasticulture.org.