The Complete Guide to Stone Wool Growing Media for Indoor Cannabis Cultivation

30 June 2022

Here’s everything you need to know about rock wool—what it is, how it’s used in indoor, greenhouse and hydroponic systems for cannabis cultivation, and how growers utilize stone wool products for crop steering with minimal resources lost.

Stone wool, also known as wool or mineral wool, is a type of soilless, inert growing media ideally suited to indoor cultivation of all plants, from cannabis to vegetables and floriculture.

Grodan invented and debuted stone wool in 1969, and in the decades since then, our company has led the way in advancing indoor gardening technology. We love to share our knowledge on best practices for rapid root development, and how to achieve consistent, efficient production and maximum yields with minimal resource inputs.

There’s a reason stone wool is the preferred choice as a substrate, thanks to its compatibility with both soil-based and hydroponic cultivation systems for irrigation, lighting, automated nutrient delivery and other top precision growing techniques.

The standardized size and shape of each Grodan stone wool product make it easy for cultivators to build a graduated system that grows with seedlings through maturity and harvest. And after the growing cycle is complete, stone wool is recyclable in more and more places around the world.

What Is Grodan Stone Wool Made Of?

The main component of this inert substrate is in its name—stone wool is made from sustainably sourced basalt and chalk rock. Here’s a quick rundown of how stone is transformed into an airy “wool” that is ideal for root development and is 1/10 of the weight of a bag of potting soil:

First, the raw stone material is superheated to almost 3,000 degrees Fahrenheit. The molten rock is injected with air and spun into a fibrous, yet light consistency resembling spun sugar. The material is treated with a hydrophilic binder to facilitate liquid absorption, which ensures even distribution of water and nutrients upon use.

The stone wool is then cut into a graduated series of sizes and shapes—from small-sized starter plugs to larger Gro-blocks and slabs - each designed for different stages of crop production. The finished products are wrapped in a special film that blocks UV light and limits the growth of algae on the substrate surface.

The result is a unique, uniform and inert growing medium that is sterilized of pathogens, including mold or mildew, with plenty of space for roots to grow and access moisture, nutrients and oxygen from well-distributed irrigation. Clean stone wool is designed to retain water as well as air, while also promoting healthy drainage from top to bottom - unlike soil-based media that are prone to compaction and certain soilless media that can become hydrophobic (water-repelling) if allowed to dry out.

A-OK Starter Plug close up with ZENLEAF

A-OK Starter Plug close up via ZENLEAF

What Size Stone Wool Plugs, Cubes, Blocks and Slabs Should I Use?

There are a variety of stone wool products that can be used individually or as part of a complete indoor cultivation system that optimizes efficient growth with minimal inputs throughout the plant life cycle. There are options for hydroponic systems as well as growing in containers.

Grodan Starter Plugs™ are designed for starting seedlings and rooting stem cuttings. The Plugs fit into Gro-Blocks, Gro-Cubes and the Grodan Gro-Slab™ once plants are ready for a larger-sized growing medium. Grodan’s Grow-Chunks and Cress Plate are also good choices for propagation.

Stone wool Gro-Blocks are the next-largest product size to starter plugs and can be combined with the stone wool Grow-Cubes as well as Gro-Slabs across different phases of growth or for various irrigation methodologies.

Grodan Hugo™ Gro-blocks are designed for transplanting cuttings into a final block configuration that will carry plant growth through harvest. They have special channels at the base to promote full drainage and uniform planting holes that seat individual plants at the optimal depth to encourage swift root development.

Gro-Slabs are the largest-size stone wool product and can support well-established plants, as well as younger plants started in smaller blocks or Starter Plugs. The Gro-Slabs are designed for crop steering and precision growing, with vertically oriented stone wool fibers that promote efficient drainage as well as extensive population of the root network.

 

Check out the full line of Grodan stone wool products

How Does Stone Wool Differ From Other Growing Media?

The main difference between stone wool, coco coir, peat and soil growing media is that stone wool is mineral-based substrate, not carbon-containing organic matter. It’s made of natural stone, not coconut husks, bog-sourced peat moss or the composted wood byproducts found in most potting soils.

Because stone wool is heated to such extreme temperatures (about 3,000 degrees Fahrenheit) during the manufacturing process, it’s a hygienic, clean growing media free of pathogens. It’s also fully compatible with hydroponic and automated grow systems, as well as techniques like drying back that rely on precise moisture timing and control.

Stone wool is the best growing medium for indoor cultivation environments where every aspect of the plants’ lighting, nutrition and hydration are controlled by the grower. Stone wool products are designed for precision growing and are fully compatible with the sensors and automation tools utilized in a data-driven cultivation strategy, as well as techniques like root zone management and crop steering.

A-OK Starter Plugs with AtlantiCann Medical Inc.

A-OK Starter Plugs via AtlantiCann Medical Inc.

Is Stone Wool More Sanitary Than Soil, Peat, or Coco Coir?

Another reason Grodan products are perennially popular with cannabis cultivators is that stone wool is a hygienic growing medium.

Due to the mineral-based, inert nature of the substrate, stone wool is a clean, sanitary growing medium that is much less likely to be contaminated by fungi, yeasts and bacteria as well as insects and microbes that feed and live on carbon-based organic matter in coco coir and soils.

That not only enhances crop quality, it also reduces the need for pesticide use as well as the risk of heavy metal leaching. In addition to the naturally hygienic qualities of stone wool, Grodan also wraps its products in a protective film that discourages algae growth and makes for easier handling by cultivation staff.

Is Stone Wool the same thing as Rock Wool or Rockwell?

Stone wool, rock wool and rockwell are all terms that have been used to describe the same product over the years. Like soft drinks and water fountains, which term growers use to describe these products can vary by geographic location— rock wool tends to be the more popular term in the United States, while stone wool is preferred in Europe. Rockwell is a colloquialism that came from how rock wool sometimes sounds depending on various regional accents around the world.

Whether you call it stone wool, rock wool or rockwell, all these terms stem from the appearance of the mineral base materials once they’ve been super-heated and injected with air, which creates a fibrous texture not unlike natural wool. One reason we at Grodan prefer to use the term “stone wool” regardless of geographic location is that Rockwool, Grodan’s parent company, also makes similar products used for other purposes like insulation. To keep things as simple as possible for cultivators looking for the best cannabis growing medium, we refer to our substrate as “stone wool” to avoid confusion.

AOK Starter Plug

Starting Seeds in Stone Wool

One reason that Grodan stone wool products have been so popular with cultivators since 1969 is the efficiency and ease of use of this growing medium for hydroponic systems , from germination through harvest. Stone wool streamlines seed starting, cutting propagation and maintenance of mother plants to ensure a consistent production schedule.

Popping seeds is as simple as prepping the stone wool substrate of choice to the preferred pH level, inserting the seeds, irrigating and waiting for shoots and roots to emerge.

The stone wool substrate supports the rapid establishment of small seedlings and cuttings by ensuring adequate uptake of nutrients and water during the crucial early rooting stages. Commercial cannabis cultivators  also appreciate the standardized, graded sizes and automation compatibility that Grodan stone wool products afford over the lifespan of the plant, reducing labor overhead and facilitating data collection to optimize facility productivity.

Rooting Clones in Stone Wool

For cultivators conducting clonal propagation, planting and rooting with stone wool further enhances the consistency of growth characteristics from the mother plant to the final harvest.

The standard size and shape of Grodan’s Starter Plugs and Grow-Cubes help cultivators achieve uniform growth across a batch of clones and ensure rapid, healthy root development.

Cultivators can further streamline their production schedule efficiency and use of resource inputs with tools such as the Grodan GroSens MultiSensor System, which offers real-time analytics for water content, electrical conductivity, and temperature within the stone wool substrate to ensure consistency across a crop.

Image of Grodan Improved Gro-Slabs and Uni-Slabs

Grodan Improved Gro-Slabs with GroSens

Vegetative Development With Stone Wool

When seedlings or clones have rooted through, they are ready for transplanting into Gro-Blocks, a Gro-Slab or Grodan Loose Media designed for containers. This crucial stage in efficient cannabis cultivation  is when root zone management can be used preemptively to avoid plant shock and stunted growth from overwatering, underlighting or inconsistent temperature within the substrate.

How cultivators approach root zone management  depends very much on the type of growing media they’re utilizing.

Root zone management is achieved by using precise measurements of each plant’s individual climate conditions and a clear understanding of how the substrate behaves to achieve tightly choreographed vegetative growth.

Because stone wool is an inert, highly consistent substance, it doesn’t have a tendency to retain moisture or nutrients unpredictably like different production batches of coco coir, peat or soil can. Instead, it is designed to retain moisture and nutrients through drip irrigation cycles as well as flush out easily for cultivators using the drying back method or making pH adjustments.

Crop Steering With Stone Wool

Using stone wool for clones and seedlings sets the stage for continued healthy crop development in other ways too. The uniquely robust, fibrous consistency of stone wool gives roots plenty of room to develop, ensuring a faster and more thorough rooting-in process as the plants grow during the vegetative phase.

Because the cannabis plants have increased access to water, nutrients and oxygen, their uptake is improved even when the cultivator is introducing artificial drought conditions. That’s especially true when growers use data-driven crop steering techniques to ensure optimal delivery of nutrients and hydration well into the generative (or flowering) growth phase.

e-Gro Growth Tracker screen at Harmony Roots for grodan101

e-Gro Growth Tracker screen at Harmony Roots

Precision Growing and Irrigation With Stone Wool

Stone wool also offers singular benefits compared to other substrates for the important transition to flower development after plants have bulked up and established a healthy root zone in the vegetative phase.

Finding the desired balance between vegetative and generative growth requires a smart cannabis irrigation strategy  tailored to the cannabis cultivar’s unique genetics and growth habits, the cultivator’s goals for the end product, and how the root zone was established and managed during previous growth stages.

Gro-Blocks and Hugo Block products make it simple to calculate irrigation needs due to their uniform size and dry weight. Monitoring and measuring the weight of a stone wool Gro-Block regularly throughout the irrigation process is one way cultivators can determine the timing and allotment of daily water to introduce. It also makes it easier to get a sense of the stone wool substrate’s water retention and drainage cadence.

For example, for Hugo Blocks, Grodan recommends smaller water volumes introduced on a more frequent cadence during the vegetative growth phase, while the reverse is true during the generative phase. Weighing Gro-Blocks also helps to determine runoff, and re-saturation rates can be dialed into the production balance that the cultivator wishes to achieve through hydroponic systems. Weight monitoring can also assist with troubleshooting as cultivators finesse their drip or flood allotment.

Can Stone Wool Be Recycled?

Stone wool can be recycled into a compost ingredient for reuse in floriculture and agriculture, an ingredient for brickmaking and as a component for new stone wool products. Presently 45% of Grodan products sold are recycled after use, and the company is researching additional ways to reuse every component of stone wool Plugs, Cubes, Blocks and Slabs, including the protective wraps.

Because Grodan products already contain at least 30% recycled content at the outset and because the company offers access to recycling solutions for 70% of its volume sold across European Union member countries as well as in Canada, Grodan has been granted the EU Ecolabel for environmental excellence.

Grodan supports growers overseeing operations of all sizes with data-driven cultivation tools and sustainability-minded crop steering techniques for indoor, greenhouse and outdoor environments. How can we help you?