Growers have plenty to think about when deciding on the best substrate for cannabis cultivation. But where hobbyists and home growers have the luxury of being able to experiment with various options, for cultivators who grow recreational and medicinal cannabis, substrates mark a key business decision. From the quality of the grow medium and its ability to take in oxygen and nutrients, to its overall cost and environmental impact and more, multiple factors can influence choosing one cannabis substrate over the other. By understanding the pros and cons of each substrate, cannabis plant growth can be crop-steered even more effectively in the long run. Here are things to keep in mind when choosing between the two most popular hydroponic cannabis substrates in commercial production: coco and Rockwool.
Choosing the best cannabis substrate:
Coco vs Rockwool
Coco coir is an organic substrate made from coconut husk, with a naturally porous structure known for allowing effective air and water exchange. Growers who crop steer may prefer coir as their go-to cannabis substrate thanks to its higher cation exchange capacity – basically its ability to supply negative cations calcium, potassium, and magnesium for the plant to take in – making it less susceptible to quick changes in the electrical conductivity of the root zone. It’s also typically more forgiving, as plants growing in coir will start to demonstrate physical stress responses before the amount of available water gets too low. Compressed blocks make it easy to store on site, and as a natural substrate, coco itself is minimally processed and can be recycled for soil mixing, home gardens, and other outdoor uses. But because it is produced primarily overseas, shipping increases coir’s impact on the environment.
Lightweight with great airflow and long used in hydroponic growing, Rockwool is a substrate made from basaltic rock that’s been melted down and spun cotton candy-style into fibers before being shaped into grow cubes, blocks, and slabs. Growers appreciate Rockwool’s ability to retain water longer – its matric potential curve is more linear than coco’s, which means Rockwool can achieve nearly 0% water content before any plant stress becomes visible. In this way Rockwool is less forgiving than coir; that’s why bringing Rockwool on as a substrate after a facility’s irrigation and environmental parameters are dialed in is a generally good strategy. As a processed product it’s more taxing on the environment than coir is, with individual manufacturers providing options for recycling. Rockwool is also often shipped in huge form factors on pallets, which can lead to damage and impact its effectiveness as a cannabis substrate.