Choosing the right growing medium for indoor cannabis cultivation is no simple matter.
Growers need a substrate that has high porosity and proper consistency, retains a moderate amount of water between irrigations, drains and dries rapidly, is free from pests and pathogens, and strikes a good balance between physical and chemical properties.
Growing media may consist of organic components derived from natural matter—such as peat, coir, wood, bark, compost or biochar—or inorganic materials like perlite, expanded clay, pumice, rockwool or vermiculate. And every substrate has its strengths and weaknesses.
In our previous blog, we discussed the importance of the growing medium and the different factors to consider in selecting one. In this blog post, we’ll examine the most used growing media for cannabis, their pros and cons, and how they impact plant growth.
Coconut coir
Coir-based substrates are among the most popular choices for soilless cannabis production. Made of coconut fiber, coir-based growing media come in several structural options:
- Loose pure coir
- Loose coir and perlite mixes
- Expandable coir blocks made from various formulations of coir pith, fiber and husk chips
- Preformed coir products that come in block or pyramidal structures
- Grow bags
- Grow slabs
Pros
One of the reasons why coco coir is so popular is that it is difficult to overwater. Its particle size and structure aerate the root zone well, which is important for both root respiration and disease prevention, because insufficient oxygen among the roots increases the risk of pathogens.
Coir is also renowned for its hydrophilic properties. Its absorbent nature allows it to retain a high volume of water, and it can be easily rewetted, reducing the need for wetting agents. Coir remains sturdy, maintains its volume during cultivation, and breaks down at a slow rate.
Cons
Coco coir is naturally rich in sodium, potassium and chloride, so it may be high in salt if not washed or buffered. However, salts are relatively soluble and leachable in coir.
Rockwool
Rockwool is composed of basalt rocks or other minerals that have been melted at high temperatures and spun into fibers.
Pros
Rockwool is a sturdy, durable and lightweight substrate. With a bulk density of 50‒100 grams per liter (g/L), it is easy to handle and use. Its total porosity is 91–95%, making it capable of holding water when saturated and air when dry. It is also highly consistent, which allows for uniform drainage. As an inert material, it has zero cation exchange capacity (CEC), so it won’t alter the ratio of nutrients in fertilizer solutions. Thanks to its high-heat manufacturing process, it is also a sterile substrate, typically free of phytopathogenic microorganisms.
Cons
Rockwool is hydrophobic, so it may require the application of a wetting agent—and it may need it multiple times, as wetting agents often wash out over time. Rockwool can also dry out too fast at the end of the season. Despite its porosity, rockwool restricts the root environment, so water flow to the root zone may be hindered, even when there is high water content.
Rockwool can also negatively affect fertilization. It has a high pH and a low buffering capacity, making it susceptible to pH shifts. Growers using rockwool need to adjust the nutrient solution and continuously monitor pH levels to ensure nutrients remain available for plant absorption. Its zero CEC also means that nutrients can leach easily.
Finally, rockwool’s durability makes it an environmental liability. It is nonbiodegradable, so rockwool will end up in the landfill at the end of its use.
Peat
Peat is partially decomposed plant material produced under low-oxygen conditions. Its characteristics vary based on the plant species it is derived from and the climate in which they grew. For instance, sphagnum moss has a spongy, fibrous texture with high porosity and water-holding capacity, and low pH, while sedge-based peats are darker, more decomposed, have a higher CEC and contain more plant nutrients.[1]
Peat can be mixed with other components to create a better substrate with improved aeration, reduced hydrophobicity and better CEC. Table 1 lists the most common components used in peat-based growing media for cannabis, their additional volume, and their physical and/or chemical contributions.
Component
Addl volume
Physical and/or chemical contributions
Perlite
10–35%
Aeration, physical stability, drier mix
Vermiculite
5–15%
Aeration, increased wettability, increased CEC
Wood fiber
20–40%
Aeration, better gas exchange
Coir pith
5–25%
Aeration, physical wettability
Coir chunks
5–25%
Aeration, physical stability
Processed bark
5–30%
Aeration, physical stability
Compost
0–10%
Nutrient change, biodiversity
Calcined clay
0–5%
Increase CEC and rate of mineral
Sand, azomite and/or volcanic rock
0–5%
Increase rate of mineral
Table 1. Most common components used in combination with peat-based growing media. Table source: Nemati, M. Reza, Jean-Pierre Fortin, Joseph Craig, and Shaye Donald. 2021. “Growing Mediums for Medical Cannabis Production in North America.” Agronomy 11: 1366. https://doi.org/10.3390/agronomy11071366.
Pros
Peat has a high buffering capacity for water, pH and nutrients.
Cons
Peat is acidic, hydrophobic and low in nutrients. Additives must be used to make it suitable for plants.
Phenolic foam
Like rockwool, phenolic foam is lightweight—its bulk density is about 15–25 g/L—and sterile, providing a clean, pathogen-free substrate. It is also available in various configurations and sizes.
Pros
Phenolic foam is homogenous and consistent, with high air porosity and drainage suitable for cannabis. It has excellent aeration and retains its air space, even after repeated waterings. It also responds to fluctuations in the root environment quickly and, with no CEC or buffering capacity, does not react to or cause changes to nutrient ratios.
Cons
Phenolic foam is fragile when handled and has a moderate to low water-holding capacity. Moreover, it is nonbiodegradable and will become waste.
Perlite
Perlite is made of volcanic rock that has been crushed and heated rapidly to a temperature of 1,800oF (~982oC). The result is a substrate that expands to form a white, lightweight aggregate with high pore space.
Pros
Perlite’s lightweight nature prevents compaction, allowing the roots to access oxygen easily. Its porous structure also ensures proper water drainage, reducing the risk of waterlogging and root diseases.
Perlite is non-toxic and chemically inert, meaning that it has no CEC or nutrients, and it maintains a neutral pH. It is also stable and can be reused after sterilization through multiple crop cycles.
Cons
While perlite can hold up to four times its weight in water, its water-holding capacity is fairly low: Water is only retained on the surface and in the pores between the particles.
Vermiculite
Vermiculite is a silicate material processed like perlite. Heating tremendously expands the particles, creating a very porous lattice structure. It is available in several grades for different phases of the plant life cycle. Fine grades are used for seed germination; coarser grades are for developing roots, as the fine grades are too small to hold enough air and water, and are susceptible to compaction.
Pros
The main benefit of vermiculite is water retention. It absorbs and holds moisture, which provides not only consistent hydration but also nutrients from fertilizer solutions, promoting healthy root development.
Cons
Vermiculite’s CEC is relatively high at 2–2.5 milliequivalents per 100 cubic centimeters of growing media, and its pH varies from slightly to very alkaline, depending on the source. It is not stable long term, and heat destroys it, so it cannot be steam-sterilized for reuse. It is also relatively expensive, which is a main reason fewer growers use it.
Table 2. Comparison of four popular growing substrates by several physical properties adjusted for a height of 14 cm. Table source: Nemati, M. Reza, Jean-Pierre Fortin, Joseph Craig, and Shaye Donald. 2021. “Growing Mediums for Medical Cannabis Production in North America.” Agronomy 11: 1366. https://doi.org/10.3390/agronomy11071366.
Effect of growing media on plant growth
While each growing medium has its pros and cons, some substrates may be better suited for cannabis cultivation.
One study looked at the differences in growth of two cannabis genotypes in three different substrates: peat (PM), peat substituted with 30% green fiber (G30) and coco coir fiber (CC).[2]
Cannabis performed better in the PM and G30 than in CC; plant height was 20% and 12% higher, respectively, than when grown in CC. Plants grown in PM and G30 also had a higher leaf SPAD chlorophyll index and higher nitrogen content in the leaves.
However, only one genotype (Kanada) yielded significantly more flowers, with Kanada grown in PM yielding the highest. The other genotype (0.2x) saw no difference in flower yields, regardless of the substrate. Nor were there significant differences in cannabinoid levels among the growing media and genotypes.
While the growing media did not have a direct impact on bud yield and cannabinoids, the researchers reiterated that a healthy root system is the key factor for high biomass, and the G30 substrate resulted in the greatest root length density (Figure 1).
Figure 1. Root scan images of cannabis genotype 0.2x grown in three different substrates. Root length density were highest for the substrate consisting of peat and 30% green fiber. Image source: See footnote 2.
Best practices for using growing media
Whichever growing medium is used, it should be tested weekly throughout the entire crop cycle. Conducting a pre-plant media analysis will indicate potential nutrient deficiencies, pH imbalances or excessive soluble salts. This step is particularly important for growers who mix their own media.
Understanding the properties of growing media is crucial to making informed decisions about the best substrate for cannabis production. Selecting a substrate wisely ensures that the roots have an optimal environment, contributing to bigger yields and healthier plants.
Emerald Harvest Team
[1] Will, Elizabeth, and James E. Faust. 2010. “Growing Media for Greenhouse Production.” The University of Tennessee Agricultural Extension Service. https://trace.tennessee.edu/utk_agexcomhort/28/.
[2] Burgel, Lisa, Jens Hartung, and Simone Graeff-Hönninger. 2020. “Impact of Different Growing Substrates on Growth, Yield and Cannabinoid Content of Two Cannabis sativa L. Genotypes in a Pot Culture.” Horticulturae 6(4): 62. https://doi.org/10.3390/horticulturae6040062.
