Author: Sam Angima
Publish Date: Fall 2009
Local foods awareness in Oregon has increased tremendously over the last five years. Our local schools are among institutional groups that are now looking at expanding the local foods in their breakfast and lunch menus in the “farm to cafeteria” programs. As these programs expand, what does it mean to our local growers? It might mean expanded markets for fresh vegetables and local meats. The dilemma however is that schools are closed in summer when our produce is abundant while there is scarcity of fresh vegetables in the fall and winter terms. Also there is competition for locally grown food between consumers and schools in the fall and winter as backyard gardens are not producing enough at this time.
How do we as local growers maintain year-round production without necessarily increasing our expenses or consumption of non-renewable resources? There is no magic bullet but a good way to start is to try to understand how heat and cold affect plants and therefore utilize ways of protecting crops from extreme heat and cold and enhance growth of crops for quicker but higher quality end product under adverse conditions. In the fall and winter, supplying the necessary heat is one of the limiting factors in plant growth. Use of a combination of mulches and high tunnels and being able to move them to desirable locations is one way of insulating against heat lost and capturing solar radiation.
In order to reduce external sources of heat to allow for crop growth, manipulating the ground as a heat reservoir and radiating body can greatly lengthen, at minimal cost, the extension of the growing season. Heat from the ground protects crops at night when covered with row covers or plastic mulches. As the ground gets cooler in the fall, it radiates less heat, reducing metabolic activity in the plant which translates to less growth. Utilizing plastic mulches in the fall on soils can change ways in which heat is captured and stored for fall growth on crops.
Plastic mulches have been used commercially on vegetables since the early 1960’s. Plastic is laid over a soil bed generally 4-6 inches high and 30 inches wide, with a slope of 1.25 inches from the center to the edge. Soil under the plastic mulch usually remains loose, friable, and well-aerated. The color of a mulch determines its energy-radiating behavior and its influence on the microclimate around the plant. Color affects the surface temperature of the mulch and the underlying soil temperature. Also the degree of contact between the mulch and soil quantified as a thermal contact resistance influences performance of the mulch. If air space is created between the plastic mulch and the soil by a rough soil surface, soil warming is reduced. What are the different types of mulches?
Black Plastic Mulch
This is the most widely used option in all the plastic mulches. It is an opaque blackbody absorber and radiator. Much of the solar energy absorbed by black plastic mulch is lost to the atmosphere through radiation and forced convection. The efficiency with which black mulch increases soil temperature can be improved by optimizing conditions for transferring heat from the mulch to the soil. Because thermal conductivity of the soil is high relative to that of air, much of the energy absorbed by black plastic can be transferred to the soil by conduction if contact is good between the plastic mulch and the soil surface. Therefore, it is recommended that as you prepare your fall beds, the soil tilth should be fine and slightly compressed to take advantage of this concept. Soil temperatures under black plastic mulch during the daytime are generally 3-5° F higher at 2-4-inch depth compared to those of bare soil.
Clear Plastic Mulch
Clear plastic is recommended for fall crops due to the high gain in heat units by the soil. It absorbs little solar radiation but transmits up to 95% with relative transmission depending on the thickness and degree of opacity of the plastic. If you look closely, the underside of clear plastic mulch usually is covered with condensed water droplets. These droplets are transparent to incoming shortwave radiation but do not transmit outgoing long wave infrared radiation, therefore, tremendously increasing the heat gain in the soil. On clear sunny days, soil temperatures under clear plastic mulch are generally 6 to 14° F higher at a 2-4-inch depth compared to those of bare soil. Weeds can be a big problem under clear mulch but in the fall, the physiology of most weed seeds prevents them from germinating and competing with crops. Also it is recommended to use transplants for fall crops thereby eliminating germination problems under plastic cover.
Red Plastic Mulch
Red plastic mulches have been used substantially in tomato production because some experiments have shown up to a 12% increase in marketable fruit yield compared to other plastics. However, when environmental conditions for plant growth are ideal, the red plastic mulches’ influence on yield is minimal. In terms of soil warming, they perform like black mulch but cost-wise are more expensive.
These mulches are intermediate between black and clear plastic mulch in terms of increasing soil temperature. The color of these mulches can be blue-green or brown. The advantage of these plastic mulches is being able to warm soil temperatures without the weed problem of clear plastic mulches.
White, White-On-Black, or Silver Reflecting Mulch
These plastics are not recommended for fall crops because they can result in a slight decrease in soil temperatures. They usually reflect solar radiation back into the plant canopy and are usually used in summer when soil temperatures need to be cooler.
Despite the low cost of black plastic mulches, the overall cost of disposing used mulches is quite high. That is why there is a move to develop biodegradable plastic mulches. Biodegradable plastics are made with starches from plants such as corn, wheat, and potatoes. They are broken down by microbes. Field studies on tomato and pepper crops have shown that biodegradable plastic mulches perform just as well as polyethylene film, and they can simply be plowed into the ground after harvest. Another advance in this technology is the photo-degradable plastic mulch that also comes in tinted brown, black and clear. It degrades with sunlight with time but the section in contact with the soil does not degrade and has to be removed later. Other mulch options (and some are still under development) include paper mulch, planters paper, and recycled Kraft paper.
It is important to remember that growing crops in the fall under protection requires soil warmth as well as above ground warmth coupled with providing enough light for photosynthesis. Ultimately, the market price and prevailing markets of produce will determine the profitability of growing late fall crops. What is certain is that more and more people are demanding locally grown foods and those growers who innovate to capture this market will benefit from investing in late season crop production.[Content credit: ATTRA Season Extension Techniques for Market Gardeners, Center for Plasticulture at Penn State University and North Carolina A&T State University Black Plastic Mulch Program]