Cannabis cultivation choices are complicated. Which genetics do we cultivate for high-grade flower, which for concentrate, and which for our nursery business? What PPFD do we want to maintain, and at what week and stage of growth? How do we deliver water, with what nutrients and at what rate? What leaf temperatures do we need, and at what costs and risks?

There are so many questions, and the answers are all interconnected. It’s very challenging to definitively say this is the best light, genotype, fertigation strategy or cultivation media without considering the challenges, benefits and facility impact of each one.

In this White Paper Wednesday, we unpack an article on cannabis cultivation media, explore popular substrate choices, discuss opportunities and challenges and illustrate how the media you choose impacts your racking decisions and overall facility design.

Breaking It Down: Top Media Choices Among Real Cultivators

When it comes to substrates, growers have hundreds of options to choose from. There’s rockwool, soil, coco coir, clay pebbles, foam, peat, pure carbon, aeroponics, ebb and flow, deep water culture and even aquaponics, and it can be hard to feel confident in any one of them.

As Nemati et al. point out in their white paper, much more research needs to be done before the industry can agree on the “best” grow medium. For now, growers can look at specific case studies for helpful information, as results are directly connected to media, fertigation, genetics and standard operating procedures (SOPs) rather than scientific generalizations.

To simplify the process, we collected the four most popular media choices according to our customer base and decided to do a quick comparison.

Most Popular: Rockwool

Rockwool is made from basalt rocks or other minerals, which are liquefied at a high temperature and then spun into fibers. The fibers are then layered and compressed to form a sturdy yet lightweight substrate.

THE GOOD

  • With good aeration and water holding capacity, rockwool can hold more nutrient solution and air than any other medium.
  • Precision control/monitoring is possible. Drybacks and VMC control are easiest in this media.
  • Rockwool is usually free of phytopathogenic microorganisms due to the use of heat, which renders the material sterile and safe for growing plants.
  • It has a low nutrient retention capacity (cation exchange capacity is about zero), making it easy to flush.
  • Rockwool exhibits 91 to 95% total porosity that can be occupied with water when saturated or with air when dried [12,21].

THE BAD

  • Rockwool is not environmentally friendly, so dealing with waste can be challenging. However, there are some companies that specialize in collecting spent rockwool to use in soaking up ocean oil spills.
  • It needs a wetting agent and is sensitive to dry out, which can lead to channeling and reduced use value if you cannot control temperatures in the room.
  • It may encourage algae growth that robs oxygen and compromises plant health.

THE UGLY

  • Rockwool is radioactive in large quantities.
  • It needs sophisticated fertigation to capitalize.
  • And it’s itchy!

RACKING CONSIDERATIONS

Rockwool is lightweight, so there’s less worry about your substrate weighing down your grow trays (a critical consideration when you have multiple tiers). It also enables you to play around with plant density and plant spacing. Additionally, rockwool is used for growing smaller plants, so less tier to tier spacing is needed.

A Close Runner-Up: Coconut Coir

To start the process, coconut husks are soaked in fresh or saltwater to soften them. If the husks are soaked in tidal waters, they’ll absorb salt that needs to be flushed out later in the manufacturing process.

After the husks are softened, the coir is mechanically removed from the husks and dried. Then the coco is chemically treated to remove unwanted elements, sterilized and often charged with calcium to prevent the binding of other nutrients. Then it is baled or otherwise packaged for sale.

THE GOOD

  • Coco efficiently  retains water and has a high wettability.
  • The air-to-water ratio is as good as it gets.
  • Until it starts binding nitrogen as it decomposes, coco coir is recyclable and reusable.
  • Thanks to its high wettability, it’s difficult to overwater coco coir.

THE BAD

  • Contaminants are possible. For example, coco sources have been the source of failed heavy metal testing in cannabis certificates of analysis (COAs).
  • Managing the nutrient content of coco can be challenging and lead to lockout and compromised plant health.
  • Unwashed and unbuffered coir is usually rich in sodium, potassium and chloride and, therefore, may cause a risk of calcium and magnesium deficiency during a lag period.

THE UGLY

  • Sourcing can be a big challenge. Sri Lanka is the number one supplier, and other countries do contribute. However, there’s no baseline for consistent production around the globe.

RACKING CONSIDERATIONS

While coco is often heavier than rockwool, it is also considered a lightweight media. It can be messy to deal with, causing pathogen vectors if SOPs are not followed. Coco can also clog drains and lead to anaerobic water conditions that breed pathogens.

A Good Go-To: Soil

The basic components of living soils are peat moss, coir pith, coir chunks, naturally aged bark, wood fiber, composts and natural additives. A variety of inoculants can also be used in living soils: compost, bacterial inoculants, and mycorrhizae. Most indoor cultivators will choose to build custom blends that allow for proper air-to-water ratios, cation exchange, nutrient balance and physical plant support.

THE GOOD

  • Soil cultivation might be the best choice for the environment as it can be 100% recyclable.
  • It is often associated with high-quality flower products.
  • Soil is very customizable. You can combine various organic materials to find the mixture you prefer.
  • Soil uses nutrient cycling as its feeding method, allowing the plants to eat thousands of times a day.
  • It ​​breeds beneficial microorganisms that work like warriors to protect plant health.

THE BAD

  • It’s easy to mess up a soil mix, and failure can be catastrophic and time-consuming to rectify.
  • Soil composition will change over time.
  • Long-term maintenance can be a challenge.
  • It’s difficult to quickly make the soil’s nutrients available to the plant.

THE UGLY

  • Custom soil mixes need to account for local product availability and potential inconsistency.
  • There is no “quick fix” if mistakes are made.
  • When the soil goes bad, you could lose a whole garden.

RACKING CONSIDERATIONS

Soil benefits from combined cubic media, which can add a lot of weight to your racks. For example, a 2-ft-deep bed on a 4 ft x 8 ft tray will weigh about 2,880 – 3,360 lbs. (1,200 – 1,400 per cubic yard). Soil cultivators often find it hard to go multi-tier, and they usually need extra support when considering any racking option. If you want to be successful long term, using a soil substrate ill add to the capital expenditure of your racking choice.

The Rise of Deep Water Culture (DWC)

Also called “bucket growing,” deep water culture refers to cultivation in aerated water. In short, your cannabis roots are submerged in a mineral solution, where they receive constant access to water, oxygen and nutrients.

THE GOOD

  • DWC can lead to amazing root growth.
  • Integrated pest management is easier because there is minimal grow medium.
  • It’s easy to flush and change the pH of DWC systems.
  • The roots can immediately absorb nutrients in the water.

THE BAD

  • Getting plants established in the system can be difficult.
  • You MUST maintain water temperatures, or biofilms can develop.
  • Shared water can mean shared pathogens, like fusarium.
  • To grow successfully, you need to understand flow volumes and how to maintain oxygen in the water as roots develop.

THE UGLY

  • Failures can be messy and time-consuming to correct.
  • If pathogens develop, they can spread rapidly across your plants.

RACKING CONSIDERATIONS

Water is heavy, with one gallon weighing in at 8.33 lbs. This can make it challenging to add additional tiers and requires racking providers to consider additional supports that add cost to a racking buildout. Additionally, integrating your plumbing can be challenging whether you’re working on one tier or two.

The Bottomline

According to a 2015 white paper, there are 956 different cultivation media — at least. Each medium has its pros and cons, and there isn’t necessarily a “best” way to grow. At the end of the day, growers must look at their experience, cultivation style and management techniques to begin testing and narrowing down the right substrates for their plants.