3 tips for selecting growing media for cannabis

Features - Production

Three professors discuss substrate considerations that can improve cultivation.

December 19, 2019

Clockwise from top center: Fine-grade peat, coarse-grade peat, pine tree substrate, perlite, course/chunky coconut husk, coconut coir
Photo courtesy of Brian E. Jackson

Some growers have a misconception that cultivating cannabis is markedly different than producing other crops. But that often isn’t the case, says Bill MacDonald, 25-year floriculture industry veteran, and professor and coordinator of Niagara College’s commercial cannabis production program. For example, he says his methods for growing and researching chrysanthemums and cannabis are very similar.

Rony E. Chamoun, academic coordinator and industry liaison manager of McGill University’s Commercial Cannabis diploma program, will soon start teaching about the nascent industry following work in the food market and at a wellness company that processes cannabis. In his experience and visits with growers, he’s found that the crop faces similar issues and has comparable requirements as other crops to grow it well. For instance, powdery mildew can infect it, and supplemental lighting can help influence vegetative growth and flowering.

One way cannabis separates itself from many other plants, though, is that it tends to be more resilient, MacDonald says. “You can beat this plant up quite a bit, and it’ll come back, whereas you do that with poinsettia, your crop’s done,” he says.

Like with any plant, growing media provides cannabis with water and nutrients. And if they follow these few substrate tips, cultivators might be granted a little mercy from this crop.

1. Work with reputable businesses and organizations.

For more than 15 years, Brian E. Jackson, associate professor and director of the Horticultural Substrates Laboratory at North Carolina State University, has worked with numerous crops such as ornamentals and produce, with an emphasis on growing media. With this experience, he is intensely familiar with the market.

Whether growing cannabis or any other crop, Jackson says growers should prioritize working with a reputable vendor, rather than picking a product and then choosing a supplier. Any commercial soil company in North America that provides service to multiple regions or growers can help growers find the right product.

Similarly, MacDonald says that if cultivators need information on growing media, they can often reach out to extensions that have experience with other crops.

2. Rethink using mineral soil.

“Cultivating containerized cannabis mineral soil is not the best approach for crop success and management,” Jackson says. This is in part due to issues of drainage, contamination, variability, weight and herbicide persistence. One of the main reasons growers shouldn’t use it, he says, is because it could contain heavy metals.

While Jackson says it may be “cheap” and “convenient” for a grower to collect mineral soil themselves, it can also be dangerous, as it could contain heavy metals. “It’s easy to cut corners on the front end, but the penalty can be great,” he says.

The fact that cannabis is consumed makes the issue more serious. “If you’re growing pansies or petunias, yeah, you don’t want heavy metals in your [growing media], but you’re not eating pansies or petunias,” Jackson says.

MacDonald agrees, and notes that Health Canada requires cannabis cultivators to test for heavy metals. “I would be very concerned [with the use of mineral soil] because cannabis can be a bioaccumulator,” he says. This means the plant can easily take up chemicals.

A peat-perlite soilless mix
Photo courtesy of Bill MacDonald

3. Weigh your options.

Many of the companies that grow cannabis use soilless container media or hydroponics, Chamoun says.

One of the soilless options that some Canadian cannabis cultivators use is rockwool, he says, adding that “The rockwool soilless substrate is considered to be a preferred substrate due to its capabilities in delivering good aeration and possessing water retention properties.”

Chamoun recently a cannabis cultivators in Quebec, IsoCanMed, that uses aeroponic systems. A relatively new development used in growing controlled-environment produce, aeroponics systems mist small amounts of water — about 40% less than indoor hydroponic systems — onto plants’ roots and recirculate the water, Chamoun says. He adds that IsoCanMed trialed this water-saving method for about six months before taking it to market. McGill’s cannabis diploma program, which starts in June 2020, plans to take field visits to aeroponic facilities, among others.

MacDonald has cultivated cannabis in peat-perlite soilless mixes and rockwool as a hydroponic medium. He says he has seen little consensus in the cannabis industry between various substrates and techniques, and so far, many of these decisions have fallen on individual growers.

“If you’re going to grow on a large-scale, I’d be heading towards the rockwool,” MacDonald says. “I just see how well it works in the greenhouse vegetable industry. And yeah, so it seems you’ve got a lot of control that way in a greenhouse, but I’ve also seen soilless media and good product out of that.”

Jackson and his colleagues have compared peat-perlite blends and coconut-perlite blends. While he says both mixes are “excellent,” the researchers didn’t see a benefit to one over the other. He adds that many cannabis growers use mixes of coconut and perlite (the latter of which is not toxic, he assures naysayers) because they have been on the market for a long time.

In general, cannabis cultivators can take a page from ornamental and produce growers in adhering to precision growth methods, Jackson says. Cannabis cultivators often try to flower their crop as quickly as possible, but they need to be able to control production by slowing down or speeding up plant growth based on weather and climatic factors.

“Growing cannabis is not like growing tomatoes,” Jackson says. However, some of the same principles largely apply if cultivators aim to achieve consistent product and yields. The ornamental industry has proven, he says, that controlling the system and the numerous inputs of precision growing can help ensure long-term cultivation and business success.