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Black plastic mulch can kill grass and weeds to make space for a vegetable garden or new border. It’s an eco-friendly way to clear land without having to use herbicides.
Once the excitement of starting a new garden wears off, the questions begin. One of the most common is ‘How do I convert a piece of lawn, or overgrown allotment, into a productive garden?’ The best way to begin is to remove any unwanted features and vegetation, especially perennial weeds. If your goal is to grow organically, there’s an easy way for you to do this — kill weeds with black plastic. The technique is called sheet mulching, and once you lift the plastic, your land will be clear for cultivation.
This is exactly how I created my own vegetable garden and it saved me an incredible amount of time and energy. It’s the main way that I recommend clearing land without using herbicides since it’s suitable for organic gardening and also, pretty easy. The before and after photos below show how I covered the entire area and then gradually peeled the plastic back. Bit by bit I transformed a weedy plot of land into a beautiful and productive veggie patch. You can see more of my garden, and learn about two other types of sheet mulch in the video at the bottom of this piece.
How to Use Black Plastic to Kill Weeds and Grass
- Mow the area or strim it so that the weeds and plants are low to the ground. Remove any visible perennial weeds.
- Apply a 2-3″ layer of organic compost (optional)
- Lay heavy-duty black polythene plastic sheeting flat on the ground and weight or peg it down.
- Pop fine holes in the plastic to allow air and moisture through (optional)
- Leave for 2-3 months in summer or six months in winter
- Lift the plastic, remove slugs and New Zealand flatworms, and dig up perennial weeds
- If you’ve not applied organic compost first, you will need to apply a good layer now. Plant directly into the compost and you’ll get a crop that very first year.
Heavy-Duty Black Polythene as a Weed Suppressant
After clearing most of my plot using this method, I had just one last weedy corner to tackle. The photos in this piece are from that corner, and to help get it ready for growing, I covered it, and the compost pile beside it, with a layer of heavy-duty black plastic. This is the kind of material that you’ll find used as pond or roof lining and will survive the elements. Thin black plastic, like bin-liners (garbage bags), is not suitable since it easily rips and shreds. Blue tarps and clear or light-colored plastic aren’t great either since some plants will still grow under them.
You can get heavy-duty black plastic (polythene/polypropylene) sheeting in other colors. I’ve seen it in a light blue color before and this is also fine for using to kill weeds and clear land. It can be more of an eyesore than black, but if it’s thick enough, no light will get through it either.
Leave the plastic on the ground for 2-3 Months
Once the plastic is laid out and weighed down, you just leave it and let it do its work. Because the dark color stops sunlight from getting to the plants below, most of the plants die off. Grass and annual weeds are the first to go but hardier weeds can take longer. In warmer months it can take as little as two months for the plants underneath to die and rot down. In winter leave the plastic for at least six months.
Some weeds will survive and even after a year of being covered, the dock on my plot is still alive. They show up as white and yellow stems as I lift up the plastic so they’re easy to spot and dig up. Slugs and other pests are easy to see when you lift the plastic too. Take the time to remove and destroy them and you’ll save yourself the pain of them multiplying and eating your veg.
Dead plants can be dug back into the soil
After removing any last weeds, the ground under the plastic could be ready for immediate planting. That is if you’ve applied a layer of organic compost to the ground before you put the plastic down. If you haven’t, you can apply it to the bare soil now. The best way is as a 4-6″ layer of mulch, to create a no-dig garden. That mulch will also cover any seeds that are lying at the soil’s surface and will stop them from germinating.
You might also spot dead plants at the soil surface. Desiccated or slimy grass and weeds. Apply the mulch directly on top of them since they won’t grow. Worms and other soil bacteria in the compost will break them down into nutrients in no time at all. If the plants look fleshy still, then they could still be alive. That was the case with dock weed in my garden and I manually dug them, and their long taproots, up. Any tough weeds like that I put into a container and either dispose of them or leave them to eventually break down.
Cons of using black plastic to kill weeds?
In case you were wondering, the plastic only dries the soil out when used in very large sheets. In smaller areas, like the one I’ve just dug over, the soil is moist and worms and other soil creatures seem unaffected. There’s also, information floating around on the internet about how plastic sheet mulch can negatively affect soil organisms and the garden. If any reduction of soil life does occur, it is only temporary, as I and many others have seen in a real-world scenario. The study that folks refer to is one conducted by Washington State University; a study that may be flawed due to the testing method.
Regardless of any studies, sheet mulching with black plastic works well to clear the land and help create an organic vegetable garden. I just wouldn’t recommend that you use it as more than a temporary solution. If you wanted to use something to protect the soil of beds each winter, cover with a material that’s durable but light and air-permeable. Something like landscaping fabric. If you use the plastic woven mesh type, just make sure to melt the edges with a lighter or creme brulee torch. If you don’t, it will shred and turn into a mess.
From Weedy Land to New Garden
In an area that was just weeds is now a new slightly raised bed. I used timber salvaged from my old plot to build it and filled the inside with soil and compost. You don’t need to convert the land into raised garden beds afterward — that’s just my choice since I garden on a slope. What you will need to do is add a 3-6″ layer of organic matter such as garden compost, rotted horse manure, and rotted mushroom compost to the soil. You can simply spread it on top and let the worms do their work. From weedy land to garden bed, all it took was black plastic, garden compost, and a bit of time.
Another great thing about using black plastic sheet mulch is that it can be reused many times over. Use it to extend your garden, or lend or sell it to other gardeners. At our allotment site, we cover any vacant plots with black plastic and some of it is five years old. When you kill weeds with black plastic you’re saving effort and starting your new garden in an efficient way.
How to kill weeds with black plastic
- Large stones, weights, or pegs
- Garden fork
- Organic compost
- First, clear the area that you'd like to turn into a vegetable bed or entire vegetable garden. Cut the grass short, and dig up any shrubs or perennial weeds.
- It's best to next lay two to three inches of organic compost on the ground. You can also spread the grass clippings, leaves, and any other finely chopped organic material. If you don't lay this layer, the ground will be bare soil when the process is finished and will need more work.
- Spread the black plastic sheeting over the entire area you wish cleared. A second person helping will make this step easier, and avoid laying the plastic on a windy day.
- Weigh the plastic down at all four corners, along the edges, and throughout the center. Ensure that wind is not going to pick the plastic up and send it sailing to the next county.
- If you'd like to increase the amount of air and moisture reaching the soil beneath, pop small holes in the plastic with a pitchfork/garden fork. Doing this can help the soil biota beneath.
- Leave the plastic in place for however long it takes to kill the grass and weeds underneath. In the summer it may only take two to three months, in the winter it can take double that. If you have a lot of tough weeds underneath, it may take up to a year.
- Peel the plastic back, either all at once or a part at a time, to begin growing in your new growing space. If you've laid compost down first, the ground will be tilled by worms and soil bacteria and ready for planting.
- Watch the video below to see how to use heavy-duty black plastic to kill weeds, alongside two other types of sheet mulch. Shows further before and after shots.