Cannabis Growing Mediums: Understanding the Science Behind Substrates
Take a deep dive into the science behind cannabis growing media (or substrates).
Like building a home’s foundation, the growing medium we choose will become the building blocks for a healthy cannabis crop. Fortunately, there is a lot of good information from qualified growers and researchers we can draw on to choose the best cannabis growing medium.
Some, but not all, growers use pre-mixed soilless growing media in their cannabis production instead of mixing their growing medium from several substrates. The most common combinations of those who prefer to mix and match include coco coir, compost, or adding field soil to a peat-based growing medium.
A word of caution if you use soil. Field soils aren’t sterile – they may contain disease pathogens and insects. If you encounter issues, they’re harder to correct than with soilless mixes like peat and coco coir. Finally, field soils often contain at least some clay, which quickly hardens in the greenhouse growing environment.
Peat Moss Based Products
Germination and growing mixes with names like Bacto, Pro-Mix, and Berger have been around for some time. Peat moss is one of the best substrates for container-grown cannabis as a host of ornamental greenhouse plants. When perlite and vermiculite are added to peat moss, it retains water and drains well (i.e., it has high porosity).
Peat moss has a low pH and electrical conductivity (EC) and a high cation-exchange capacity (CEC), which is a nutrient-holding capacity of cationic (positively charged) nutrients. It also has a high pH buffering capacity, resulting in a more consistent pH readings over time.
The peat-based growing media you purchase may also contain mycorrhizae and other nutrient additives. Mycorrhizae is the one component most growers seek in a growing media. If there are any downsides to peat-based products, it’s the fact that they need to be stored at room temperature and, for the most part, aren’t reusable.
People worry about the sustainability of peat moss, but it is carefully managed and harvested in Canada and other parts of the world. Canada has tapped into 3% of over 3 million acres of peat, and they also have sustainable practices in place to mitigate and replace the peat bogs that are mined.
Close on the heels of peat in popularity among cannabis connoisseurs is the fibers of coconuts currently used in container, hydroponic, and cannabis field production.
Coco coir is, in fact, the number one alternative to peat moss, with the demand increasing every year.
Coco coir is derived from the fibers of coconuts and harvested from coconut trees in Sri Lanka and other tropical and subtropical climates. It’s considered a renewable resource, which is appealing to many growers looking for sustainability. It hydrates quickly, retains water well, and has an excellent cation exchange capacity.
Coco coir is recommended for newbies because it’s easier to manage nutrients, pH, and EC than other mediums. Should things go a little haywire, you can more easily correct the issue with coco coir. For instance, you can quickly fix salt buildup or over-fertilization by flushing out the media with fresh water.
Seasoned hydroponic growers also prefer coco coir because fewer nutrients leach out, saving on fertilizer costs and our underground aquifers.
Coco coir is versatile. You can combine coco with other mediums or use it alone. In addition to the aforementioned applications, cultivators often use coco coir for seedling production, cuttings, and, alas, for growing mature cannabis plants!
Since it doesn’t provide good aeration on its own, growers may wish to add perlite to the medium. It also requires the supplemental feeding of calcium and magnesium, i.e., Cal-Mag, into your feeding regime.
Stonewool and Hydroponics
Stonewool is an inorganic material that comes to us from the construction industry, where it is still used for insulation. Like perlite and coco coir, it’s an inert material containing no nutrients. This makes Stonewool especially popular with hydroponic growers.
Stonewool gives growers control of nutrients and pH. It provides a primarily sterile environment with little risk of disease and insect infestation. Nutrient delivery and pH need to be closely monitored when growing with Stonewool. It is also possible to start seeds in Stonewool cubes where they can remain without suffering transplant shock until they are fully mature.
The dust from Stonewool can irritate the eyes and skin. Heavy metals and other contaminants have been found in cannabis harvests from various sources, including Stonewool and other growing media and pesticides. Regulations vary from state to state on acceptable levels of contaminants in cannabis. Check with your state and local regulating agencies on current practices and limitations.
Wood and Bark Substrates
Wood and bark have been gaining popularity in the past few years. Researchers at the University of North Carolina (and others) are conducting extensive research into these increasingly popular substrates.
Wood substrates used in commercial horticulture usually come from managed forests of loblolly pine (Pinus taeda). The wood used as a growing media can be manufactured into various sizes and shapes.
Like peat moss, wood has a high pH buffering capacity and high porosity. The pH range for wood fiber is between 4.5 to 6.0. It also has a low cation exchange capacity (CEC) and good drainage and hydraulic properties. More research needs to be done on using wood substrates either as a stand-alone product or, more likely, as a mix with other substrates.
Making the Best Choice
It all comes down to making the best choices for your cannabis cultivation. The old standbys like peat and Rockwool offer an excellent selection for growing a superior crop of cannabis. Other products, like coco coir and wood substrates, are drawing interest and enthusiasm from cannabis growers.
All mediums are only as good as their ability to deliver nutrients efficiently to the plant. Carefully and consistently monitor pH, nutrients, and moisture levels to grow your best crop of cannabis.