MSU Extension

Introduction

Modern production of several fresh vegetable crops is conducted with high inputs and high potential dollar returns per unit of land area. Vegetable growers continually look for ways to grow high-quality produce more economically, improve yields and extend the growing cycle. The value of many vegetables is high enough to justify microclimate modification through use of polyethylene film mulch. Commercial use of plastic mulch in combination with plastic drip tape began in the early 1960s and has since been termed plasticulture (Fig. 1).

Successful plasticulture depends on understanding how plastic mulch affects the microclimate and how this microclimate affects the crop, weeds, insects, diseases and other organisms. Plasticulture also implies significant changes in the production system, including specialized equipment, adoption of new fertilization (fertigation) and pest management technologies, selection of suitable crops and adoption of new cropping systems. Growers with a good record of growing vegetables on bare ground will have potential for greater success in plasticulture. Growers with no experience should start small and increase skills, acreage and equipment over time.

Plastic mulches contribute to reduced pesticide inputs by providing non-chemical alternatives to pest control. Other benefits include soil warming, reduced evaporation, increased yield and earliness, improved nutrient application and uptake, and reduced nutrient leaching. Benefits can be grouped into the following categories:

Crop benefits

• Earliness: Plasticulture-grown crops can be harvested 7 to 21 days earlier and get better prices.

• Greater yields: Marketable yield may be up to three times greater.

• Higher quality: Produce is cleaner and has increased size and fewer culls.

• Higher economic returns: Earliness, high yields and high produce quality improve the economic return to the farmer.

Pest management benefits

• Reduced weed infestation: Certain plastics prevent light penetration and weed seed germination/growth, and serve as a physical barrier to weed seedlings.

• Better insect management: Reflective mulches repel insects such as aphids and thrips.

• Potential decrease in disease incidence: As a consequence of better water drainage, incidence of soilborne diseases may be reduced. Many foliage and fruit diseases overwinter in soil and splash onto the plant during rain events. Plastic mulch acts as a barrier, limiting the number of spores reaching the plant. Virus diseases are reduced in plasticulture when reflective mulches are used to repel vectors (aphids/thrips).

Production management benefits

• More efficient use of fertilizers: Through fertigation, fertilizers are delivered directly to the root zone and on an as-needed basis, decreasing fertilizer use by as much as 50 percent.

• More efficient use of water: There is reduced evaporation from the soil surface, and delivering water directly to the root zone uses much less water than overhead irrigation.

• Possibility to double-crop: The same plastic can be used to grow a second or third crop with minimum inputs.

Environmental benefits

• Reduced fertilizer leaching: Lower water rates, especially in light soils, reduce risk of fertilizer leaching and groundwater contamination.

• Reduced soil erosion: Plastic mulch protects soil from being eroded by wind or rain.

How Do Plastic Mulches Affect Crop Microclimate?

The major reason for using plastic mulches in vegetable production is their ability to modify crop microclimate. Plastic serves as a physical barrier limiting moisture loss, and it changes the solar and thermal radiation balance affecting above- and belowground temperatures. When solar radiation reaches the mulch surface, it is reflected, absorbed or transmitted through the plastic to the soil. Reflected radiation may increase photosynthesis but will not be converted to heat. Transmitted radiation will warm the soil; absorbed radiation increases plastic temperature and heats the soil and/or air. The modified microclimate affects weed seed germination and development, crop growth and development, and populations of other organisms, including nematodes, insects and soil-borne diseases.

How Do Plastic Mulches Affect Plants?

Plastic mulches affect plants indirectly by changing water, heat and light balances.

Changes in Water Balance

Plastic mulches constitute physical barriers to water evaporation at the soil surface, potentially reducing drought effects. During rainfall, most water in a plasticulture system is channeled between mulch strips. Drainage area is therefore limited, which may lead to flooding between rows when heavy rain occurs on poorly drained sites.

Changes in Light Balance

Light modification is a factor of the quantity and quality of light passing through the plastic. For that reason, the planted crop is less affected than weed species. Light is necessary for photosynthesis. The fate of weed seeds depends on the amount of light they receive. Film properties such as thickness, color and opacity determine the amount of light reaching the soil. Low light prevents weed seed germination, and high light leads to high weed populations underneath the plastic. Clear plastic (high light transmittance), therefore, is associated with high weed populations; black plastic (low light transmittance) controls most weeds.