Rx Green Technologies Coco vs. Rockwool: Which Media is Right For Your Cannabis Grow?

Cultivating cannabis is both an art and a science, especially at the commercial level. One of the most important decisions any growing team has to make is which substrate (also commonly known as growing media) they want to use for their plants. Determining the ideal substrate requires weighing numerous variables, including end-harvest goals, ambient moisture in the local environment, indoor vs. outdoor vs. greenhouse, and other factors. 

Among the most popular cannabis growing media are coco coir, rockwool, and soil. Each has its own unique set of attributes, which we’ll cover in this exclusive Rx Green Technologies blog. We conducted a study where we compared our Ready-To-Use coco grow bags against rockwool cubes, and the results are compelling. Across hundreds of the same strain planted in our RTUs, with a rockwool control group, the RTUs delivered an average yield 40% higher than the control group while consuming 60% less water. Make sure to read the whole article to get all the details.

An example of bagged, Ready-To-Use coco coir for cannabis planting

Why is Coco Coir Popular with Cannabis Growers?

Coco coir, or simply coco, is the sustainable byproduct of coconut husks. Coco coir is produced by threshing the husks into a fibrous pulp that ultimately provides excellent drainage, aeration, and fertigation opportunities for cannabis cultivation. Coco is one of, if not the most, popular substrates for growing cannabis for these reasons,but also because it’s affordable and available in a variety of different formats, such as compressed coco, loose coco, or our Ready-To-Use pre-hydrated grow bags. Perlite is one of the most common additives to coco as it further improves drainage; depending on your grow’s location, it is typically recommended that your coco coir has at least some percentage of perlite incorporated. Coco’s flexibility allows for customizing your media – depending on preference and environment – to optimize your operation for better results from the start. 

Additionally, coco coir must go through a process called buffering, which is essential for growing healthy cannabis plants. Buffering ensures the coco is thoroughly washed to remove excess salts; if this step is not done correctly, it can create downstream challenges in managing EC and pH levels. We pre-buffer all of our coco and conduct extensive quality-control tests at every stage of production – something not all manufacturers can claim.

If you want to go even deeper on coco, check out our blog here to learn more.

An example of rockwool cultivation cubes laid out on a tray for planting

What is Stonewool and How do Cannabis Growers Use It?

Stonewool, or rockwool, is another common cannabis growing medium that’s made from basalt rock and other naturally occurring minerals, which are melted down at high temperatures and spun into fibrous cubes or slabs. You might also encounter this kind of product referred to as mineral wool. Even though it was originally engineered to be used as insulation, it has become widely used in horticulture thanks to its inert, sterile, and uniform structure. It’s also a common material for soundproofing and fire protective barriers due to its noncombustible properties. Rockwool is also highly porous, meaning it retains water while also maintaining some degree of aeration, which can help roots receive necessary moisture and oxygen. Because it is pH-neutral and free of nutrients, growers have complete control over feeding programs; however, it does require more fine-tuning of EC and pH as a result.

While rockwool is relatively popular in cannabis cultivation settings for its water retention capabilities and sterility, it does come with several hangups. One major issue many growers struggle with is rockwool’s non-biodegradable nature, which creates significant waste since it cannot be easily recycled or composted. This problem is especially acute given how many plants a cultivation facility grows and harvests in a given year. 

Rockwool also requires extra careful management of nutrient concentrations since it has little to no buffering capacity, meaning plants can quickly suffer from nutrient imbalances if not monitored closely. Additionally, handling dry rockwool often releases harmful fibers that can irritate grow technician’s skin, eyes, and lungs, which requires protective gear to use properly. Finally, its costs of deployment are often higher in the long run compared to more sustainable or coco-based alternatives, and as we’ll show, final yields can be significantly lower when compared to using coco in certain cases.

Rx Green’s RTUs were compared side-by-side against rockwool

Rx Green Technologies’ RTU Bags vs. Rockwool Cubes

We decided to put rockwool to the test against our Ready-To-Use coco grow bags to see which media performed better in real-world settings. Our test, which was conducted at a licensed cannabis grow facility in Massachusetts, pitted 450 total plants of the same strain (226 in our RTUs and 224 in rockwool cubes) against one another. The results were clear: average total end weight for the plants grown in Rx Green’s RTUs was nearly 40% higher than their rockwool counterparts, along with a 60% reduction in total water usage due to the superb moisture retention properties of the coco used. Skeptical? Here’s how we did it.

Analysis Methods Explained

• Total plant weight was collected for each individual plant at the time of harvest, and these values were used to perform a t-test analysis of the difference between Rx Green Technologies’ coco-filled RTUs and rockwool growing substrates. Because of this, the t-test analysis only indicated if the testing variable was likely responsible for the biomass increase or decrease observed. 
• Grams per square foot values were projected at the time of harvest based on historical data for drying and trimming rates.
• Total cannabis values include all usable material collected, whether used for extractions or flower products. Yield comparisons highlighted in green indicate a difference in which Rx Green outperformed rockwool, while red-highlighted cells indicate a difference in which Rx Green did not outperform rockwool.
• In the Figure 1 Rx Green RTU Coco Test – Yields bar chart comparison, the test group was harvested as top cuts only, while the control group had some wet trim bud weight. This is why you will see 100% less bud weight in the test group, but 125% more top cut weight.
• Fertilizer feed usage per plant was recorded throughout the trial to determine the difference in the number of gallons utilized during the trial. In the comparison table, cells highlighted in green indicate cases where Rx Green coco required fewer gallons than rockwool. It is assumed that the percent cost difference would be roughly equal to the difference in water usage.

Yield Comparison

Figure 1. Bar graph displaying the harvested yield metrics from each experimental group.

Table 1. Yield comparisons between the test group that was planted in Rx Green coco and the control group that was planted in rockwool.

Table 2. T-test analysis of the difference in total plant weight.

Mature cannabis plants grown in Rx Green Technologies’ RTU bags

Summary of Testing: Higher Yields, Less Water Used

The above study firmly concludes that cannabis plants grown in our Ready-To-Use coco bags produced overall higher yields with less water usage on average compared to those grown in rockwool. Additional findings concluded that plants grown in Rx Green’s RTUs resulted in:

• Higher average plant weight
• Higher average total cannabis weight
• Higher average top-cut weight
• Higher average wet bud weight

Why All of This Matters

The price per pound of cannabis has fallen substantially in most mature markets, and even in new markets, price compression tends to happen more quickly than many operators anticipate. That’s why squeezing every possible gram out of each plant is an essential goal for every grower to ensure that they’re getting the maximum possible value out of every harvest cycle in each room. Additionally, water usage is a significant cost driver in any grow, and reducing water consumption is both healthy for your bottom line as well as the environment. With the prospect of increasing yields and decreasing water usage by making a relatively straightforward change in your grow, it’s worth exploring. If you’d like to learn more, contact our cultivation experts today to get started.