You would have seen rolls of plastic “weed matting” for sale in the hardware store or garden center. Don’t buy it though- We’re about to re-learn that just because it’s for sale, doesn’t mean it doesn’t suck.
Spoiler: sheet mulch instead.

Just because it’s for sale,
doesn’t mean it doesn’t suck.

Adrian

You can be forgiven for believing the sales pitch which would have you believe that because you are covering your weeds with plastic and plastic lasts for a million years… your garden will stay weed free for a million years. Initially it will look like it has worked.

Unfortunately, reality has a few more details and exists on a larger timescale that the time it takes to hand over a fist full of your money to a mega-corporation.

Weeds are weeds because they do well in hostile environments. You would have noticed them thriving in situations with very little water or soil, like in cracks or piles of dirt on a path that hasn’t been swept. By covering up your garden soil, with plastic and a layer of mulch, you are changing the conditions of you garden to favor weeds only. All weed matting is destined to be covered in a thick layer of only the most robust and obnoxious garden weeds. Many of these will penetrate the weed matting and not only will you have to pull out all these weeds manually, they will be all connected together by large sheets of slightly deteriorating plastic which will have to be thrown out along with all the attached topsoil and and all the nutrients bound up in it.

You can also buy recycled cardboard weed matting. Using cardboard is a great idea- buying it, is not.

Sheet mulch instead

This is the process of smothering a large area (normally grass), with something like cardboard and then mulch. The vegetation underneath is deprived of sunlight and eventually dies. The steps are as follows:

1. Mow the living crap out of it. Use the mower on the lowest setting or a trimmer to mow down to soil level.
2. Wet the area. This helps the vegetation to rot in the darkness and it helps lime stick to the leaves of the grass and weeds. Dry grass can survive a long time.
3. Sprinkle lime across the surface of the grass. This creates an alkaline environment unfavorable to plant life.
4. Lay wet cardboard and overlap-leaving no gaps for grass to grow through. Wet the cardboard by submersing in a wheel barrow or baby pool.
5. Cover with 20cm of coarse, woody mulch.
6. Monitor in the following months for regrowth. This should be about 10% of original growth so repeat the steps to reduce growth to 1% and eventually eradicate unwanted grass. The quick planting of desirable species will lead to less space and sunlight for future weed growth.

Special credit to Lachlan Storrie from the original school of anti-weedmatting: Tree Frog Permaculture.