Sheet mulching is one of our favourite techniques, and we use it all the time to halt the progress of weeds, build garden beds on top of lifeless soil, and to convert a lawn into a garden quickly and cheaply.
Sheet mulching is so simple that lots of people are skeptical that it works. But it absolutely does, and it's an excellent technique to have in your gardening toolbox.
Indi and I have both established pretty much our whole gardens using this technique. It is a chemical-free way of killing off weeds or existing grass and keeping them suppressed, all while feeding soil life with organic matter. And it is very cheap and easy.
Before we get into the nitty gritty, I want to quickly explain the connection between sheet mulching and establishing a no-dig garden bed for vegetable growing -- because they are very similar. Both use a biodegradable barrier (usually cardboard) on top of existing soil.
In the case of sheet mulching, the cardboard is topped with mulch/ wood chips. If you're planting trees or native tubestock, for example, you can pierce a hole in the cardboard and plant into the soil below. We use this technique when establishing orchards or any new perennial garden area.
For a no-dig vegetable garden bed, you top the cardboard with a thick layer of compost instead, then plant or seed directly into this. By the time the new seedlings' roots have reached the cardboard layer, it will be well on its way to decomposing, and the new garden bed will become continuous with the garden's native soil. In the meantime though, any existing weeds or grass have been starved of sunlight and died, leaving you with a weed-free bed.
We'll get more into no-dig gardening and its many benefits in another post. For now though, let's focus on sheet mulching.
Step one: Collect your cardboard. Aim for twice as much as you think you'll need! Avoid anything shiny or coloured. We usually collect our cardboard from skips outside shopping centres and hardware stores. Ask first if you can.
Step two: Remove the sticky tape and any plastic labels from your cardboard. This will be MUCH easier if the cardboard has been left outside in the rain for a week or two. Staples are ok; they'll eventually break down.
Step three: Prepare your sheet mulching area. If it's lawn, mow or cut it short, leaving the clippings lying on top. If it's weeds, you might want to pull out the biggest ones or just flatten them. If it's bare soil, perhaps stick a garden fork in a few times to aerate it. Dig out the roots of any unwanted trees or large plants.
Step four: Layer your cardboard over the whole area, with plenty of overlap and no gaps! (If there are gaps, the weeds will push their way through.)
Step five: Wet down the cardboard so when you water your new garden the water can soak in to the soil below.
Step six: Lay mulch (our favourite is screened tree mulch from SA Composters) over the cardboard at least 10cm thick and up to 15cm.
Step seven: Plant! Pull the mulch away at each planting site, pierce a hole in the cardboard, dig your hole and add some compost before planting and watering in.
If you are laying irrigation, put it on top of the cardboard and under the mulch. If you have sheet mulched over particularly pernicious rhizomatous grasses like kikuyu, some weeds will come through -- just pull them out as soon as they appear and over time the rhizomes will die. It will be much easier to manage, but still takes active management. Either way, you may need to top up the mulch every year or so, with 5-10cm.
Unfortunately, sheet mulching alone is not enough to suffocate couch grass as it is just so vigorous. If you have couch, your best bet would be to cover it with an opaque tarpaulin for several months before sheet mulching, or dig it out and then sheet mulch.
You can also use jute mat instead of cardboard if you are mulching on a steep slope. However, we have found the jute mat to be less effective against oxalis (soursobs).
You can also combine sheet mulching and no-dig garden beds when creating a new garden area. Just lay the cardboard, then add compost to the new beds and mulch around them. Below we found an innovative way to do this when establishing unusually shaped beds with mulch pathways on an existing lawn.
Another hot tip, if you're spreading a lot of mulch, is to get yourself a mulch fork. They're much quicker and easier to use than a shovel when moving mulch.
Sheet mulching really is garden magic. Go forth and mulchify!