We here with Yarducopia normally build 100 sf sheet mulch (or ‘Lasagna’) style gardens for our participants. Sometimes we do other things, and sometimes our builds don’t exactly follow this recipe, but sheet mulching is one of the best ways to turn old lawn into a new, productive garden quickly. Here is a photo essay of the basic steps:
First mark out the outlines of the garden. For a rectangular garden just place markers at the corners, a more shapely garden, like this one, might require a lime or chalk outline. The lawn under the garden-to-be can be limed, poked full of holes, or otherwise prepared at this time as needed, or not.
Next, we generally dig a small, 6″ by 6″ trench around the outline of the garden. This is one reason to prefer a square garden! Less trench to dig :) This keeps weeds and grass from running back into the garden as quickly. Sometimes we skip this step if the vegetation looks well behaved – i.e. it isn’t the type that tends to spread by root runners. The removed sod can be laid on top of the grass where the garden will go.
Now we lay down cardboard to stop weeds and grass from getting up into the new bed from below! Wet each stage of the build well – ideally one would wet the ground, cardboard and each layer on up to wrung out sponge levels. I have built a number of dry beds though, and they will just require a lot of water later. If you are building in the rainy fall for next year, maybe the sky will do the watering for you after the build. We pull or cut staples from the cardboard to minimize heavy metals and pull off plastic tape that won’t decompose anyway, and overlap all pieces of cardboard by 6 inches in each layer, with at least 2 (even better is 3) layers. Big cardboard can be found at bike shops, from cabinets and appliances, and between shipping skips. We use about 10+ bike boxes for 100sf.
Next come the organic mulch layers to break down in place and provide all the carbon and nitrogen the plants and soil microbes need to be happy! We generally start with a truck-load (about 2 cy) of stable bedding – sawdust and horse manure. This gives us a nice thick (about a foot or so) layer that will decay quickly to much less height! It’s hard to know the exact carbon to nitrogen ratios of the starting materials (what exactly is the urine/manure/bedding ratio in each load anyway?), but things are pretty forgiving. If plants don’t grow well, probably you are short on nitrogen, and one can always fertilize with fish emulsion or similar to help out the bed.
On top of this go other layers of whatever is available – more nitrogen rich: spent grain from the Midnight Sun Brewery parking lot, bags of coffee grounds from a local cafe, half finished compost, grass clippings; to more carbon rich: dried grass clippings, dried leaves. Then a thin layer of compost (usually composted horse manure in our case because that’s what we found a free source of!) or soil to plant into.
On top of this goes a thick layer of straw or dried leaves (or both) mulch to keep weeds
And that’s it! Some things, like carrots, don’t grow super great in a first year sheet mulch bed, but most things love it, and by the second year it has broken down into a deep, chocolate-brown, rich soil!