Indoor sheet mulch

Something we tried out in the fall last year (2012) was to incorporate a sheet mulch bed into our greenhouse. This bed is contained in a large box (5' x 2.5' x 3') which is then covered with old windows that hinge up acting as a coldframe within the greenhouse.

We first put in some leftover firebricks and larger stones we had removed from parts of our outside beds during the spring. There were two thoughts in my head that I hoped to achieve with the bricks. Most of them were put down at the sun facing side of the box to collect heat and act as some form of thermal mass. So far it seemed to have worked, the bottom of the box on the inside is consistently 2-3 degrees warmer over night than the middle. The other thing I hoped to achieve is a small air gap or pathways for air to flow through to aid decomposition.   I will have to see later this year how that went.

Old bricks add thermal mass and allow for air flow

So now the box is ready to start the actual sheet mulch process.

For the sheet mulch (which I also like to call compost cake) we layered several different materials into the box starting with wood and branches. We even threw in some whole pieces of firewood that started to have fungi growing on them to provide long term nutrients. So in effect, the bottom of the sheet mulch bed is like a Hugelkultur bed. 

Sticks and stones

The next layer consisted out of woody plants with some green material on them. Most of that was alder branches that I cut back the day before. Alders are nitrogen fixers and their leafs and stalks introduce more nitrogen into the bed than other shrubs or tree material.

Lots of Goldenrod (Solidago canadensis)

The next layer was comprised out of plant matter like Goldenrod and other weeds that we had pulled from the garden and areas where they needed to be cut. We also added yarrow which is a very good accelorator for the decomposition process.

The next layer was grass clippings, topped with some topsoil, topped off with more grass clippings. We have a wild meadow that we bushhog once a year in fall. We then gathered up as much of this cut grass we could and used it for sheet mulching. Quite a good bit went into the box in the greenhouse. As you can see, there is no such thing as too much mulch. 

At this point, it might be important to add that grass cuttings and other weeds are not to be left out of a compost or sheet mulch bed for fear of spreading the weeds. A good-going compost heap will reach temperatures that will kill most of the seeds and in a sheet mulch bed the top mulch will smother most weeds. So it is better to include the weeds and with them their nutrients.

Grass - Soil - Grass layer

The next layer on top of all the grass was a thick layer of newspapers. We tried to leave the papers and flyers together and folded so the the layer is about 1/4 to 1/2 an inch thick. Then we watered. A lot of water to soak everything. The box is not sealed so that water and air can flow out and in respectively. As you can see, the water makes a nice way out and soaked the floor in our greenhouse. But what fun is it when you can't make a mess?

No such thing as too much mulch

...and then some paper....

.... a lot of paper...

A watering mess....yay!

On top of the soaked newspaper we mulched heavily using bulrushes from our swampy parts of the property. We found that bulrushes make excellent mulch and we don't even chop the rushes up. Of course you can do that and it might make for a neater look of the bed, but I didn't have much time or the appropriate tools to chop the plants.  Bulrushes are used to waterlogged soil and no rushes will germinate in your bed if it is not waterlogged.

Job's done!!!

After all this is done, all you have to do is wait for spring, so far the theory. I will let you know how the growing season will work out.