8 ways to use black plastic in the garden

When you think of black plastic in the garden, you probably picture the large yard and garden contractor-sized trash bags. These are the ones people use to bag up leaves in the fall, haul grass clippings away and carry off the carnage of tomato and pepper plants after the harvest. But black plastic plays a much bigger role in gardening than holding trash.
Plasticulture refers to the use of plastic materials in agricultural and horticultural settings for crop production. Black plastic film, also known as polyethylene, comes in a variety of thicknesses for use as plastic mulch and row coverings. Some gardeners have adapted plasticulture to home garden use by covering the entire soil surface with it and creating holes for planting or covering the open space between rows of plants to reap a host of benefits.
1. Warms the soil for earlier planting
After the soil has softened in the spring, cover it with black plastic to help raise the soil temperature for earlier planting. The plastic absorbs the thermal energy from the sun, heating the soil up to five degrees Fahrenheit. In cool spring temperatures, this can mean getting seeds and plants into the ground up to three weeks sooner if the plastic can get soil temperatures between 65 and 75 degrees F.
2. Speeds up seed germination
Warmer soil temperatures and trapped humidity under the black plastic can speed up seed germination considerably. Seeds need optimum temperatures and high humidity to promote hormone production and enzyme activity for germination. Often the higher temperatures related to good germination cause soil moisture to evaporate and germination to slow. Using black plastic mulch as a soil covering increases the soil temperature and maintains high humidity levels to drive quicker germination.
3. Retains more moisture in the soil
Covering the soil with black plastic film significantly reduces evapotranspiration rates. This not only means a decrease in the amount of irrigation or natural rainfall needed, but research shows higher soil moisture levels also equate to better growth and higher yields.
4. Keeps soil warmer on cold nights
During the day when the sun is shining, the black plastic traps the thermal rays and heats the soil underneath it. An elevated soil temperature protects roots from cold damage.
5. Extends the growing season in the fall
Not only does black plastic encourage an earlier start to gardening in the spring, but it also prolongs the fall growing season by keeping soil temperatures higher as air temperatures begin to dip. It also acts as an insulator to maintain more consistent soil temperature, protecting roots from the abiotic stress of fluctuating weather.
6. Acts as a weed barrier
The black plastic acts as a weed barrier in a couple of different ways. Some seeds need light for germination to occur, especially small seeds that come from many weeds. Blocking sunlight from reaching the soil helps prevent the germination of many weeds. If they do germinate and sprout, the lack of sunlight causes seedling death because they can't photosynthesize. In research trials, growing tomatoes in soil covered with black plastic resulted in only six weeds per square meter, whereas the control had an average of 99 weeds per square meter.
7. Reduces soil erosion
Covering garden soil with black plastic prevents soil erosion that occurs through irrigation events or natural rainfall. The plastic helps to control and slow the runoff of surface waters; that means less soil is carried away through erosion, minimizing damage.
8. Sterilizes the soil to kill unwanted pests and diseases
Repeatedly using the same garden soil or potting mix over and over again can lead to a buildup of weed seeds, fungal spores, harmful pathogens and other detrimental garden pests. A process called solarization — in which black plastic is laid over the soil to heat it to temperatures that cause sterilization — is used to rid the soil of such problems.