PREPARE YOUR SITE
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Smothering
Smothering, or tarping, is a method of killing existing vegetation by laying tarps or other opaque materials on the soil for a full growing season. This technique prevents light from reaching the vegetation, stopping photosynthesis and eventually killing mature plants as well as any germinating seedlings.
Silage tarp in the process of smothering an area of lawn, to be converted to a seeded meadow. Photo: PCA
A related technique, solarization (not discussed in this guide) uses a transparent tarp to heat up the soil and kill weed seeds and germinating seedlings; it is more intensive to install than opaque tarps and can be detrimental to soil health.
Where it’s appropriate: Smothering is a great option for smaller areas (less than ½ acre), and is very effective at killing lawn grass within a single season. If the existing vegetation includes many plants that have deep taproots or fleshy rhizomes, such as mugwort (Artemisia vulgaris), Common Reed (Phragmites australis), or Japanese knotweed (Polygonum cuspidatum), smothering can still work; but more than one season of smothering will likely be needed to fully kill the plants.
Where it’s inappropriate: Smothering doesn’t work well on large (multi-acre) scales. It’s not recommended for use directly under trees or woodland edges. It can also be an eyesore while in place over the summer.
Timeline for Smothering in One Growing Season
Spring: Plan to lay your tarp in mid-April through mid-May. You will leave the soil covered for at least five months.
Tarp Installation: First, mow the existing vegetation as closely as possible and rake off any significant organic debris to expose bare soil. If woody shrubs, briars, or vines are present they should be dug out from the roots and removed. Otherwise, disturb the soil as little as possible.
Weights: All tarps must be weighted down with heavy objects such as cinderblocks, sandbags, bricks, heavy rocks, old lumber or firewood, or tires. Weights should be placed every 3-6’ along the edges of the tarp and along any overlapping sections, immediately after laying the tarp.
Tip: Large tarps (greater than 20x50’) are most easily laid by a group of 4 or more people. Recruit friends!
Laying tarps on an area of lawn to be converted to native meadow. Photo: PCA
Summer: Some practitioners recommend removing the tarp for two to four weeks in midsummer to allow light to reach the soil, triggering weed seeds to germinate and then be killed when the tarp is replaced. This step isn’t mandatory, but will result in a cleaner seedbed.
Fall: Leave the soil covered until the moment you are ready to plant: late September or early October, if planting plugs or potted plants; or November to December, if planting seeds. Under the tarp, the existing vegetation will be dead and mostly decomposed, and the soil will be bare enough to plant seeds or container plants. You may rake lightly to remove dead vegetation, but do not till or disturb the soil, which brings weed seeds to the surface and stimulates their growth.
Keeping Tarps Dry: If your site is very flat or in a low-lying area, some practitioners recommend cutting slits or poking holes for drainage. Use a sharp blade to cut an “X” in the tarp - this allows drainage but prevents light from reaching the soil and causing germination. Drainage reduces the opportunity for mosquitoes to breed in standing water that may accumulate on the tarp; but note that punctures can reduce the potential to reuse your tarp in future. Small areas may be swept to spread and dry any puddles. Even very gently sloping sites will likely drain on their own. You can also wait to add drainage holes until later in the season, when and if you notice standing water.
Materials for Smothering
The most effective smothering material is heavy-duty black plastic tarp, 5-6mm thick and UV resistant, sometimes called silage tarp.These tarps are available in sizes up to 50x100’ and are durable enough to be reused for two to three seasons. Good options include the following: Example 1, Example 2, Example 3.
Cardboard is practical for smothering smaller areas. Cardboard sheets must overlap each other by at least 12” - weeds can find a way to grow through any gaps! Cardboard sheets break down when exposed to the elements; a double or triple layer holds up best. Overlapping cardboard sheets also require many more weights than a single-sheet tarp.
Lighter tarps from the hardware store, and landscape fabric, are not recommended. Light tarps are more likely to break down under the sun’s UV radiation, which can spread microplastics into the environment and create holes for weeds to grow through. Landscape fabric does not stop germination because it allows light and water to pass through.
Sheet mulching or “lasagna gardening” is a popular technique for establishing new vegetable or flower beds, but it is not recommended for native plant installations. In this process, multiple deep layers of organic matter are used for smothering, including compost, leaf litter, and wood chips. Too much organic matter can actually be bad for native plants that are adapted to growing in lean, poor soils. “Deep organic mulch takes years to break down and removes habitat for many ground-nesting bees that need bare soil to make their homes. If you have jumping worms on your property, heavy mulching also encourages them to proliferate.”
After a season of smothering, the lawn grass under the tarp is dead and ready to be re-seeded with native plants.