I want to dig up weeds, and place landscape fabric with mulch over a backyard of about 300 square feet so I can focus on other parts of the garden, and the smaller, but needy front lawn. It was suggested that black plastic be used before the whole process, because the summer heat in west Toronto will help minimize weeds. What does the hot sun and plastic do to prepare the ground for the process? In another part of the TBG response from a master gardener, it suggests that the plastic use can negative impact by accelerating weed growth(https://www.torontomastergardeners.ca/askagardener/how-can-we-keep-weeds-from-regrowing/) I feel caught between a lot of weeds and a hot place. Thank you.
Thank you for contacting the Toronto master Gardeners with your inquiry.
Covering the ground with thick black plastic is an effective way to kill weeds. At first the weeds will begin to grow rapidly due to the heat under the black plastic however over time they will die from lack of food. Essentially, the weeds will starve since they cannot produce food through photosynthesis and eventually will exhaust their food supply and starve. This method takes time and will only work as long as no light reaches the plant.
Another effective non-chemical technique to eradicate weeds is through solarization. According to the University of California Integrated Pest Management
“The method involves heating the soil by covering it with clear plastic for four to six weeks during a hot period of the year and when the soil will receive the most direct sunlight. Plastic tarps allow the sun’s radiant energy to be trapped in soil, heating the top 12 to 18 inches to temperatures lethal to a wide range of soilborne pests; including weeds, plant pathogens, nematodes, and insects. When properly done, the top layers of soil will heat up to as high as 140°F, depending on the geographic location. Soil moisture is important in this process, as wet soil conducts heat better than dry soil. Moisture also makes soil pests, weakened by the heat, more vulnerable to attack by beneficial soil microorganisms during and after treatment.”
In addition, solarization stimulates the release of nutrients from organic matter present in the the soil. It is especially effective for treating garden soils, where the intent is to plant vegetables, herbs, and flowers.