The rapid legalization of marijuana for medical or recreational use in the U.S. raises questions about how to aerobically compost this waste effectively. To understand this process we should first look at what makes composting this plant so unique.
Cannabis as a Leafy Green
First of all, we can notice that cannabis overall is a leafy green. Therefore, cannabis composting is similar to composting other types of green waste. Cannabis leaf itself has a relatively low carbon to nitrogen (C:N) ratio (e.g., about 15:1). However, by adding in fibrous stem waste, the C:N ratio is close to ideal for composting (e.g. about 30:1).
Major Parts of the Cannabis Plant Often Available for Composting
Some common components of cannabis plant waste product include:
- Fan leaves. low in cannabinoids and therefore typically a waste product. High nitrogen, low carbon.
- Sugar leaf/flower waste – the residuals from processing the female cannabis flowers. These wastes may not be allowed to be composted in some states. High nitrogen, low carbon.
- Stem – the fibrous support structures of the cannabis plant. High carbon, low nitrogen
Grinding Cannabis Waste
Since the grinding of all cannabis waste is often a requirement for state regulations, many aspiring composters of cannabis will need to contend with these grinding requirements. This is particularly a challenge for those generators who produce a small amount of cannabis waste on a daily basis.
Cannabis stems are perhaps the most challenging cannabis feedstock to grind. If the stems must be included in the compost mix, the grinding of stems is critical for effective composting. The stems should be shredded to a fiber length of ideally 1” or less.
Unshredded Cannabis Stems Shredded Cannabis Stem (using a Chipper/Shredder)
Two of the most commonly available tools for grinding cannabis waste include:
- Slow-speed grinders
Chipper/shredders are high-speed shredders that tend to throw chopped materials and generate loud noises while grinding. These shredders are also tend to clog when shredding green or wet waste. Therefore, these grinders typically are slow and laborious for cannabis waste. However, for generators of small amounts of cannabis waste, this may be one of the few affordable types of shredders available.
A more ideal grinding technology for cannabis is a slow-speed grinder. These grinders excel at grinding of wet feedstocks and material that is difficult to shred such as fibrous stems. Plus the particle size is often smaller, which facilitates fast composting and creates a more attractive and consistent looking compost. The downside with slow-speed grinders is cost. They tend to relatively expensive, compared to garden-style chipper/shredders.
Soil/Root Balls/Growing Media
The remaining cannabis feedstocks often include root balls and growing media such as coir or peat moss. These are all fine contributors to a compost recipe and represent a good source of carbon and porosity.
There are also often inorganic materials such as soil, vermiculite, and rock wool. These constituents do not biologically decompose and therefore do not directly contribute to a biological composting process but may contribute (or inhibit) porosity depending upon density, moisture content and particle size.
The 50%-50% Rule
Many state regulations for the disposal of cannabis waste require the cannabis to be mixed at least 50%-50% by volume with non-cannabis waste in order to render the cannabis waste unusable. While this rule requires composters to source non-cannabis waste, these non-cannabis feedstocks may also help produce a balanced recipe.
Common sources of carbon-rich bulking agent include: wood chips, leaves, chopped straw, horse manure and bedding, shredded cardboard. Common sources of nitrogen-rich feedstocks include: shredded green-waste or grass clippings.
Our Compost Calculator Results for Cannabis Residuals
The follow is a screenshot from GMT’s Compost Calculator software illustrating a typical starter compost recipe for cannabis waste by itself. This recipe assumes a ratio of 3 parts fan leaves to 1 parts chopped stem by volume.
Next, we built at a starter recipe assuming a 50%-50% mix requirement with non-cannabis waste. This recipe is: 3 parts fan leaf, 1 part shredded stem, 2 parts wood chip, 2 parts grass clippings. Overall, this is a 5:3 recipe by volume, 5 parts “greens” (cannabis leaves and grass clippings) to 3 parts “browns” (shredded stems or wood chip).
In conclusion, composting cannabis is not tremendously different than composting other types of green-waste. However, state regulations for grinding and blending 50-50 with non-cannabis waste create challenges for the aspiring composter, particularly at a small-scale.
If you would like assistance your cannabis composting project, please contact us at [email protected].
Read more about our cannabis composting clients here!
Written by Van Calvez, System Designer, Engineer, Sales & Customer Service