Monthly Archives: July 2012

Sheet Mulching

It has been said that Kailua Kona has poor soil. There are places where I have tried to dig a shovel full of soil only to find hard lava rock. While some people may have great soil around here it can certainly be hard to plant anything and see it thrive without serious attention being given to the soil profile. In this area of the world the problem is volcanic rock that has not had enough time to be covered by organic material. With this in mind you may begin scratching your head and wondering how weeds of every kind find a foothold and grow happily along any soil deposits they may happen to find. There is a lesson or two to be gleaned in there, but for now I want to share what we are doing about it in the ATV.

As you can see from the picture above there are lots of weeds in the ATV. We have many methods to dispose of these weeds, but the bulk of them are very labor intensive and will not prevent them from coming back. We do, however, hold a key that will not only remove the unsightly and unwanted weeds while slowing their return but will help to build the soil at the same time. This is the wonder of sheet mulching. The concept is simple: cover the weeds with a biodegradable but fairly persistent material and cover this in a deep mulch layer. The weeds will be smothered and be unable to grow through the thick medium as the mulch is not a very hospitable location for weeds to germinate, the mulch holds water instead of letting it runoff the areas you intend to plant, and the organic mulch and barrier will eventually decompose into soil…a win-win proposition!

In the past people used plastic and other materials that cover the ground and while they prevent weed regrowth they are not biodegradable. Add to this the cost factor and it is neither a sustainable option nor a cost effective one. What we do have that is free, biodegradable, and environmentally smart is cardboard. You might imagine that here in Kona we have a lot of cardboard, and if we do not repurpose it then it will cost us to have it hauled to a landfill somewhere; a proposition that does not make anyone happy. Instead we will use it as our base for the sheet mulching.

The weeds can be pretty tall, but this should be okay as far as laying the cardboard over it. As you can see in the above photo, simply lay the cardboard down and ensure you are overlapping the pieces so there are no gaps. This is a critical aspect to sheet mulching as missed gaps allow weeds to penetrate the mulch and just give them a nice protective coating so they can grow more efficiently. You will see that in the lower right corner of the photo below the gaps that were missed allowed weeds to penetrate.

Smaller pieces of cardboard may be needed in areas where you need to place the cardboard around plants you want to keep. It is an important point to make here that waxed cardboard should not be used as it does not readily break down. Taking plastic tape off of the boxes is also a great idea as it will not disintegrate.

The next step is to get your mulch and place it on top of your cardboard. We have a transfer station here that accepts green waste and then turns it into mulch. Anyone in the community can go pick it up for free. Check with your local community for similar situations. We use this by the wheelbarrow full to add on top of the cardboard 4-8 inches deep. You can see in the picture below how we have weeds in the background, cardboard that has been placed and is not yet covered, and then the mulched cardboard in the foreground.

Here is are a couple of different angle:









When the process is underway you will begin to see a big difference. The areas that have already been covered with both the cardboard and mulch contrast greatly with those that have yet to be worked with:

When the area is finished you will not only have dealt with your weed problems but you will also be building soil, reusing materials that would otherwise end up in the local landfill, and doing this in such a way as maintain sustainable practices without pesticides.