Last week we went to plant a serviceberry tree at the Montclair Residence. Once we arrived we encountered two apparent problems. The first was that there was a bag of green waste (grass clippings, tree branches, some dried leaves) left on the curb although the city green waste collection ended on June 12th, and won’t recommence until September 25. Since green waste is not permitted in the city food waste collection, it would unfortunately have to be thrown in the trash. Our second problem was that we had no mulch for our new tree, which would have to survive a hot and sunny weekend unprotected.

From these two issues came one simple, ecological solution: use the green yard waste as mulch!
As plants grow, they absorb nutrients from the soil and incorporate carbon from the atmosphere. When we remove it and throw it away, we are effectively removing nutrients from the system. Instead, we want to add them back into the system, either by composting, or more directly, as mulch. Think of a forest floor, there is no bare soil to be seen and all of the leaves fall and decompose in place, creating rich humus layers that protect and feed the soil, nothing is lost and everything is reincorporated. Mulching is the process of covering soil with organic material, where it decomposes in place. Mulch has several benefits: it improves soil quality by adding organic matter and a source of nutrients; it covers and protects bare soil, retaining moisture and storing heat; it can prevent the growth of unwanted plants; it creates habitat for insects and resources for microorganisms, increasing soil biodiversity; and finally it helps with carbon sequestration by storing carbon in the soil and not as a greenhouse gas! Bare soil erodes and loses fertility over time, and is also an open invitation to colonization by weeds. This concept in its simplest form is also known as ‘grasscycling’ (l’herbicyclage), which consists of leaving lawn clippings on the ground instead of picking them up.

Virtually anything can be used as mulch, but here are a few tips. First, in general, green, fresh material (fresh leaves, food scraps, grass clippings…) is higher in nitrogen, and will decompose faster; and woody or brown material (including straw, cardboard, dry leaves) is higher in carbon and will take longer to break down. This will determine where you should use it, because the ratio of carbon and nitrogen will affect the microbial processes happening in the soil. Generally, vegetables and herbaceous plants are more nitrogen-demanding so will appreciate the green mulch. Trees and woody shrubs require higher carbon ratios with more woody or brown mulch. Chopping up branches into smaller pieces will speed up the decomposition process. Certain types of wood, like cedar, have properties that make them extremely slow to break down, so are more often used for paths or the surface of a playground.

For weeds: weeds are mostly plants that we haven’t found a purpose for yet, and usually grow quickly in inhospitable places. These traits mean they can be a really effective mulch source, especially since many have deep roots that can draw up nutrients from deep underground. If you are worried about spreading weeds, make sure to use only the parts that cannot reproduce (avoid roots, flowers, and fruits), or leave roots face up to dry them out in the sun. If there are harmful invasive species (ie. ragweed), it may be best to avoid the risk and put them in the trash or make sure they are composted very well at high enough temperatures to kill the seeds.

Throwing away yard waste means removing fertility from your yard and breaking the natural, ecological cycles of decomposition. Mulching is not only a way to use material that would otherwise be considered waste, it is also extremely beneficial for plants and the soil microbiome, and reduces the need for pesticides and excessive watering. It doesn’t even require a compost pile – just let your soil fauna do the work!

Ella Martin (summer student)