The Sheet Mulching Technique

In Permaculture There Is a Technique Called The Sheet Mulching Technique, It Is Almost (But Not Quite) Identical To The No Dig Gardening Method That Feeds The Soil & Increases Yields.

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The Sheet Mulching Technique is based on natural processes such as the forest or woodland floor where organic matter is regularly added each year (e.g. autumn leaves) and the organic matter is slowly incorporated into the top layers of soil via natural methods such as earthworms and other detritus devouring organisms.

The action of some species, such as earthworms also leads to some of the organic matter being pulled deeper into the soil profile. The addition of organic matter in this way improves drainage, increases moisture retention, improves aeration and feeds the soil organisms. This, in turn, feeds the vegetation growing in the soil. The Sheet Mulching Technique mimics nature and can be closely copied in gardening systems such as No Dig.

How To Start The Sheet Mulching Technique

In its simplest form, and there’s no need to make it complicated, a sheet of organic material is used to cover the existing soil, eg cardboard, and compost is then added in a layer above. The cardboard is easier to apply if the soil is reasonably level and vegetation is cut back to soil level, but this isn’t imperative., it just makes the process easier to follow.

Cardboard is my preferred material for the first layer as it decomposes after a short while, allows air and water percolation, but stops the growth of most weeds. The layer of organic material added above the cardboard layer is preferably well rotted. However this again isn’t imperative and I often add freshly grown grass as the top layer.

Whereas well rotted compost is permeable to air and water grass has a draw back. Too much fresh grass soon forms a plug-like mass of rotting material that not only smells, but also doesn’t let the air through. However if a thin layer of grass is used, and it is allowed to break down for a while, a second and subsequent layers can be added over time. I have found this to be a very effective means of weed control whilst feeding the soil and hence crops. Ideal crops to be grown whilst adding more grass clippings are those in wider rows as it is easier to apply the mown grass. I soften do this when using a leeks crops as the first crop after changing from grass or similar. Leeks have the added advantage in that they are in the ground a long time and soil disturbance is minimised.

Using grass as a sheet mulch in
The Sheet Mulching Technique — Using grass as a mulch

The Sheet Mulching Process & Considerations

Textbooks often give strict instructions on gardening techniques. My view is that they are for guidance only and there is no need to adhere strictly to them if we understand the reasons they are included. Below is a list of sheet mulching instructions with my comments. Apply the rules as you see fir once you understand them.

Flatten the soil and remove all vegetation. … Do this only if necessary. The less the soil is dug and interfered with the better. Let the soil bacteria and fungi start to grow, digging only damages them.
Check soil pH and adjust if need be. … In theory this makes sense, particularly if the soil is extremely acid. But in many cases it wil not be necessary. The soil pH will naturally change as the Sheet Mulching process continues. Very chalky, alkaline, soils are almost impossible to change ands I wouldn’t bother trying. Remember that overall pH is not the same as that around the plant roots. Plants and fungi have the ability to modify pH within the mycorrhizal layer. Close to plant roots pH is often different to the overall level.
Irrigate if the soil is dry. In the UK I rarely find this necessary. Cardboard placed on most soil soon soaks up moisture as does the organic material above in all but very dry conditions. Not irrigating saves unnecessary work!
Add a layer of compost. … this is sometimes known as the weed barrier and should ideally be 3-4 inches deep when added. It will soon settle to halve that depth and work really well. I have problems with terms such as weed barrier because it is far more than a weed suppressing layer. In reality it is the layer that the soil microfauna soon colonises and is also the layer that feeds the soil and plant. It does more than stop weeds .. though it does that very well.
Add weed free soil. …NO, No No I say. Why bother moving soil in a natural system? It’s madness in my view. A layer of cardboard and organic matter is enough. Anything else is pure folly.
Add a thick layer of woody and or leafy material in an attempt to mimic the forest floor. … NO I say again. OK so technically this and the last layer is what some people describe as sheet mulching. But to me it is full of extra steps that are so unnecessary.

In my view all that is necessary are the two layers, cardboard (you could use paper or similar) and a good layer of organic material. Keep it simple. Remember it is people, not nature, that make gardening difficult.

In a food forest we can adopt a different method. It’s very simple. Just add woodchip to the surface, with or without cardboard. We’ve done this in our own food forest to successfully wipe out couchgrass and it has worked very well.

More Sheet Mulching Considerations

Cardboard can be replaced with any organic material that will cut out light, but is air and water permeable. Carpets are not suitable as they are very thick, slow down water and air penetration and often contain non-organic materials. eg plastics.

Very thick layers create anaerobic conditions. Avoid them.,

You can apply sheet mulching at any time of year provided the soil isn’t too wet to work on. It can be set up months ahead of when you want to plant. But if I did this I would probably add a green manure to keep the soil alive.

Some perennial weeds are not killed right away. If weeds such as bindweed re-emerge you’ll need to patiently remove top growth every few days. This will eventually kill bindweed … though most people don’t have patience to do it properly then complain the method doesn’t work. It works if you remove top growth every couple of days.

Sheet Mulching Benefits

As this is a system very similar to No Dig the same benefits accrue.

It is close to a natural process .. we are copying nature.

Weed control is significantly enhanced

Moisture retention AND drainage are improved.

Significant labour savings

Reduced costs … cardboard and organic materials are often being recycled.

Improved soil structure and soil biodiversity.

Sheet Mulching Disadvantages

I see few disadvantages.

Cats love the deep layer of organic matter for what cats often do! That’s a major disadvantage but I find that laying a few twiggy branches over the surface means they can’t find enough space in which to operate! Galvanised wire netting works just as well and can be added and removed very easily.

A few perennial weeds try to keep going. But if you have them they wail be less vigorous and easier to control than in many other systems.

You need to add organic material every year. But many gardeners do that anyway .. and they have the hassle of digging it in. Plus, in year two much less organic material is needed. I only add 1-2 inches in year two. The reason more is added in year one is to get rid of the bulk of weeds.