Solarization or sheet mulching uses a plastic tarp over an area to heat up the soil and suppress weed growth. The plastic covering blocks access to water, therefore killing all vegetation underneath. The process also kills some plant pathogens and pests and makes the soil release nutrients stored in organic matter. This will be beneficial for any plants you grow in the future. Solarization is not entirely environmentally friendly due to the use of plastic; however, you can reuse the same covering multiple times.
Factors to consider before solarization
Soil type and color
The soil should be well-draining and not too heavy, as black plastic can retain heat and moisture, which can be detrimental to certain types of plants.
Darker-colored soils, such as those that are high in organic matter, will tend to retain more heat and moisture than lighter-colored soils. This can lead to overheating and drying out of the soil if the weather is hot and sunny. Lighter-colored soils, such as those that are sandy or have a lot of gravel, tend to be well-draining and may not retain as much heat and moisture. They are more suitable for sheet mulching with black plastic in sunnier weather.
Season of the year
Weather conditions should also be taken into account, as black plastic can exacerbate heat and dryness in hot, sunny climates. Ideally, you should sheet mulch in partly cloudy and sunny weather.
Type and color of the plastic sheet
Black is often preferred for killing weeds, but you could use bright-colored, as long as it is thick. The results may not be as good as black plastic, especially in overcast conditions.
The type of weeds in your garden
If the area is heavily infested with perennial weeds, such as dandelions or thistles, it may not be effective to use black plastic as a mulch. These types of weeds will likely continue to grow after the plastic is removed. It’s best to remove as much of the weed growth as possible before laying down black plastic. If the area is mostly composed of annual weeds, such as crabgrass or pigweed, black plastic can effectively kill the weeds and prevent new weed growth.
Follow these steps to use black plastic sheeting to kill weeds:
1. Prepare your land
Identify the area of your yard for weed control. Remove any stones, twigs, or other debris that might puncture the plastic.
You should make the land as smooth as possible to ensure that the plastic covering does its job effectively.
Till the soil with a manual tiller or pitchfork. Smooth out the garden bed to ensure that there are no lumpy areas.
You can add a 2-3-inch layer of organic compost on the bed to enrich the soil.
2. Measure and cut the sheet
Measure the space available to determine the size of plastic to purchase. You can choose clear plastic or thin black tarps, typically around 2-4 millimeters thick. The plastic should be at least 6 inches longer in all directions compared to the size of your garden bed.
3. Spread the sheet
Lay the plastic sheet flat on the ground. Do this on a non-windy day. The plastic should extend beyond the boundaries of the plot.
4. Weigh down the plastic
Use bricks or rocks to weigh down the plastic. Alternatively, you can use pegs across the edges and within the central areas to keep the plastic intact. Take care to avoid tearing the plastic.
You can include a perimeter trench around 1-2 feet deep and broad for the plastic cover to extend. This prevents weeds from the edge of the garden bed from surviving from any remaining roots.
You can poke small holes in the plastic to allow air and moisture to escape. This might let some weeds break through the covering. When this happens, you can uproot them or push them back under the cover.
Heavy-duty black polythene sheets are best for solarization. It is preferred for colder climates as it absorbs heat. Clear plastic is recommended if you need the sun to heat the earth. It is best used in warmer temperatures. Thin plastic, like garbage bags, is unsuitable since it easily rips and can be blown away by the wind if not properly weighed down.
How long does it take weeds to die under black plastic?
Once you have laid out and weighed down the plastic, leave it in the ground for six to eight weeks. The dark color will prevent light from reaching the plants below; therefore, most will die off.
Grass and annual weeds die faster, but perennials can survive longer under the plastic sheet. If you live in warmer areas, leave the sheet on for two to three months. The plastic should remain on in colder climates for at least three to six months.
The best time for solarization is during late winter or early spring when the weeds are first established. Some weeds will survive after a year of being covered. Lift the corners of the sheet after around six weeks to check whether the weeds have died.
Pull out dead plants and other hardy weeds and place them in a compost pile. Apply mulch on top of grass or desiccated weeds to further suppress their growth. Solarization is considered complete when all vegetation under the plastic covering is dead.
Advantages of solarization
- Solarization is used as a pest control measure. Solarization has varying impacts on different pathogens and pests depending on the time it is on the ground.
- Solarization is effective against bacteria and fungi that cause plant diseases. Some pathogens will be resistant to this process and may require additional treatment.
- Solarization boosts soil health. The process helps to increase the nutrient content in the soil, which makes your plants stronger against pathogens and boosts yields. Studies have shown that it can reduce soil salinity, essential in conserving agricultural productivity and soil fertility.
- Soil solarization conserves topsoil temperatures, creating better conditions for seed germination and root growth.
- The process reduces the use of chemical products, which is beneficial for sustainable land management.
Disadvantages of solarization
- Covering the soil kills soil bacteria and other beneficial microorganisms. Research has shown a decrease in microbial activity in the ground after solarization. The microorganisms can die due to the high temperatures, but they can re-establish after removing the covering.
- The land will be out of production while solarization occurs. The long treatment period can be detrimental to your farm’s productivity.
- Solarization is ineffective against perennial weeds and some grasses.
- The use of plastic is bad for the environment. Although the plastic covering can be reused multiple times, you might have to treat your soil for plastic residues.
- Solarization might be less effective in regions with high winds, too much rain, or fog. The dependence on weather elements can hinder the success of soil covering.
This weed-killing technique can be applied to any size of garden bed. You are required to purchase a plastic covering, lay it in your yard, and leave it in place for a while. The plastic sheet will clear out weeds from areas that can be tough to reach by hand. Solarization is an inexpensive and efficient way to control weeds across different seasons.