What’s the Difference Between Stone Wool, Coco, Peat and Soil Growing Media?

The four most commonly used types of growing media for cannabis cultivation include coco, peat, potting soil mixes and stone wool (aka rock wool).

Each has its benefits and tradeoffs—and not all are as sustainable as they claim. Understanding these substrates and the differences between types of cannabis growing methods are key to maintaining a cost-effective and successful cultivation business.

Coco, sometimes referred to as coir, is made from the processed husks of coconuts.

It’s favored by growers for its ability to soak up and retain moisture, its buffering qualities, and because it can be easily mixed with other substrate types. Coco can come in preformed blocks in different sizes for various stages of plant growth, or loose, like potting soil. It’s typically economical for commercial purposes and can be procured from a variety of sources, mostly in India and Sri Lanka.

Coco growing media is made of coconut husks that have been flushed with fresh water to remove the salts. While coco is a natural substrate, coco processing is required to make it usable, which often comes with ecological impacts ranging from deforestation to the release of high-saline wastewater back into the environment. If it isn’t properly treated by the manufacturer, studies show that coco can also include high levels of salts, pectin, pectosan, fat, tannins, polyphenols and even pathogens that may adversely affect the root zones and water transference of various crops, including cannabis. In addition to solvents and other byproducts leaching into natural waterways, coir processing can also adversely affect irrigation lines and water recirculation efforts, reducing the efficient use of freshwater resources.

Peat is a form of preserved and partially decomposed plant matter that accumulates naturally in bogs and wetlands around the world. Peat occurs where acidic water with low oxygen concentrations causes biomass to compress and decay at a slow rate.

Peat is a precursor to coal and has unique properties: It’s a flammable material used as an alternative to firewood in home heating and power-generation in a number of countries. It is favored by cultivators for its ability to retain moisture (however, when allowed to dry out, it can become hydrophobic or water-repelling). Depending on its age and how it is processed, it can impart nutrients and beneficial microorganisms to soil.

Similar to coco, peat has a loose structure that gives plant roots room to grow and creates pockets of air that lend themselves to hydroponic (aka soilless) growing methods. Also like coco, peat is often marketed as a sustainable substrate because it’s a naturally occurring, organic resource. However, sourcing peat requires the mining of fragile ecosystems and releases carbon captured and held for many thousands of years by the peatland. That’s especially true of black peat varieties that are sourced from deeper within the bog, where the biomass has had longer to decompose and where it’s a more integral part of the bog’s complex ecology.

Soil is what might first leap to mind for many people when they think about plant growing media. Most soil growing media are made up not of the dirt you’d find in, say, a forest, prairie or tilled field, but from a formulated mixture of organic and inorganic components such as peat, coco, compost, wood byproducts, sand, vermiculite and perlite.

Potting soil is an agricultural mainstay for a reason—it offers a balance of moisture retention and nutrients, and is heavy enough to keep plants rooted and supported from the seed stage through growth and harvest.

Because of its diverse ingredient makeup, soil mixes combine the beneficial attributes of peat and coco substrates while also carrying the same concerns about sustainable sourcing.

Soil is also disadvantageous for hydroponic growing applications due to particulate runoff that can clog water reclamation systems and add to operating costs over time. However, it is widely available and its makeup can be manipulated for greater or lesser moisture retention depending on the needs of any given crop.

Stone wool stands out from other types of cannabis growing media because it is inert (giving it a near-zero cation exchange capacity), and therefore does not interfere with the chemistry of any nutrient solutions introduced by the grower. Unlike soil, coco, and peat, stone wool is not “alive” and doesn’t bond to other compounds. Because of this, stone wool is sterile and clean. Growers, therefore, don’t have to add extra agrichemicals at the beginning of the grow cycle to ensure the crop will be free of pests and viruses.

Because stone wool product formats include solid cubes, blocks and slabs, as well as a bulk form designed for containers, it provides an excellent support base for cannabis plants, even tall and lanky sativas. And because of the fibrous structure of the mineral substrate (hence the wool moniker), stone wool provides plenty of room for oxygen around the roots, even when employing flood systems. These unique qualities allow for precision growing techniques, giving each cannabis plant exactly what they need—no more and no less.