A Comprehensive Guide to Use Rockwool To Grow Cannabis 

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Added 22 July 2023

As cannabis growers, we’re always looking for the best everything for our plants, including the growing medium. And in search of the best-growing medium, you are likely to come across rockwool, a revolutionary growing medium that is a favorite amongst gardening enthusiasts, not just cannabis growers. 

Rockwool is made from lava rock and is an excellent water and air retainer, and it is one of the best growing mediums you can use to grow cannabis plants. But there’s a lot more to cannabis than meets the eye, including specific steps to prepare it, safety measures, and much more. Learn all there is to know about rockwool for growing cannabis, whether in soil or hydroponically, in this article. 

What is Rockwool?

What is Rockwool?

Rockwool, also known as Stonewool, is one of the best soilless growing mediums you can use for your cannabis plants, especially if you are using a hydroponic setup. Rockwool is spun from molten volcanic rock and closely resembles the fibrous makeup of fiberglass. More on how it’s made later. 

The reason rockwool is so popular in cannabis gardens is that it has excellent water retention while retaining aeration, which is a recipe for incredible root growth and thereby plant growth. Plus, it is sturdy, so it can hold cannabis plants with ease. It is also sterile and lightweight.

Even though it competes with other soilless mediums, like coco coir, floral foam, clay pebbles, perlite, and gravel, rockwool is often the first choice thanks to these features. On a side note, it does have a naturally high pH, so you do need to prep it before use. 

Even purchasing rockwool is easy — it is relatively cheap and can be bought at almost every gardening or hydroponics store in a variety of shapes and sizes. You can purchase them in the form of cubes, chunks, or slabs. Just make sure the rockwool product you purchase is hydroponic-grade and not industrial. 

How is Rockwool Made? 

How is Rockwool Made? 

Back in the day when scientists were observing volcanoes, they noticed that sometimes strong winds whipped lava into fibers. Experts later tried recreating this process in a lab, and rockwool was born. 

Essentially, rockwool is created from basalt rock and slag from the steel industry. These materials are heated at 2,700 degrees Fahrenheit, and the molten lava is then spun aggressively, where the lava is whipped into thin strands. These strands are then treated with a special oil to hold them together. 

After the oil bath, these fibers are pressed with the help of rollers and turned into a dense mat of rockwool. These mats are then heated to activate the binding solution, and rockwool is ready. 

The production of rockwool is fascinating, considering the amount of heat and spinning that goes into producing these fine threads of rockwool. But despite its interesting history and process, rockwool is not eco-friendly. Producing so much heat in a furnace is an energy-intensive process, making rockwool a not-so-eco-friendly growing medium for cannabis growers. 

Benefits of Using Rockwool to Grow Cannabis

Benefits of Using Rockwool to Grow Cannabis

We already glanced over some benefits of rockwool, but let’s take a closer look at them. Here are some of the biggest benefits of using rockwool for growing cannabis hydroponically.

It Drains Well and is Airy

Perhaps rockwool’s biggest advantage is its excellent water drainage. Even if you soak it in water, the fibers allow the water to drain out efficiently. This is a terrific benefit because one of the biggest grower errors that kill cannabis plants is overwatering, which can not only drown the roots but also lead to fungal problems in the root zone. 

It has Excellent Water Retention Abilities 

Rockwool's extraordinary water retention is thanks to its fibrous structure that develops a network of small pores with the ability to hold water. When you soak rockwool with water, it holds the moisture evenly through the medium, thereby providing a consistent water supply to the roots of the plant. As a result, the medium doesn’t become too dry even if you skip watering for a few days. Of course, this depends on the stage of growth your plant is in, but rockwool is simply perfect in terms of water retention. 

For you as a grower, it’s incredibly important to maintain consistent levels of moisture throughout the plant’s life cycle, as a lack of consistency can affect plant growth leading to wilting and issues with nutrient uptake. In such cases, rockwool may come in handy. 

Reduced Watering Frequency

Thanks to its water retention abilities, rockwool allows cannabis plants to have access to moisture for more extended periods between watering sessions. Unlike soil that could dry fairly quickly, rockwool extends the time needed between watering cycles. 

This advantage is specifically useful in scenarios where you can't water the plants frequently. So, if you’re using rockwool, you can even head out for 2-3 days and come back without worrying about killing the plants. This way, you can reduce the frequency of watering and save time and effort while ensuring the plants remain adequately hydrated.

Oxygen Availability

Despite its water-retaining properties, rockwool also ensures sufficient oxygen availability to the cannabis roots. The fibrous structure develops air pockets within the medium, permitting air to penetrate the origin area. Adequate oxygen is vital for root respiration, where roots absorb oxygen and release carbon dioxide, facilitating nutrient uptake and energy production.

In situations where the plants are likely to be deprived of oxygen, rockwool can be a great help as its aeration properties help prevent such issues and promote healthier root growth. This balance of water and oxygen in the root zone is crucial for robust and vigorous cannabis plant development

Nutrient Distribution

Rockwool's capacity to retain water plays an important role in circulating nutrients to plants. When you apply nutrients to your rockwool, the water-retaining fibers help disperse the nutrients evenly throughout the root zone. This guarantees that the roots have access to essential elements, creating a more efficient nutrient uptake process.

In addition, proper nutrient distribution is vital for avoiding nutrient deficiencies or toxicities, both of which can negatively impact cannabis growth and overall plant health. The controlled delivery of nutrients in rockwool allows you to manage the nutrient levels more effectively and tailor the feeding regimen to the specific needs of your plants at different growth stages.

It’s an Ideal Choice for Germinating Cannabis Seeds

Rockwool is also a terrific medium for germinating cannabis seeds. Once you saturate rockwool cubes, they provide all the three elements required for the seeds to sprout, which are warmth, darkness, and moisture. 

Rockwool is Easy to Grow Into

Often, many cannabis plants face problems when the growing medium is too compacted or hard since the roots can’t expand properly, but rockwool is safe from this problem. This medium is fibrous, so the roots don’t have any trouble expanding their network through the fibers of rockwool. 

It’s Easy to Transplant

Once your cannabis seedling is ready to be moved into its container, rockwool makes it quite easy to transplant it. You don’t need to break the medium apart or risk getting your space getting messy, which is often the case with soil. Just pick up the rockwool and transplant it into the container of your choice. 

It’s Sterile

Rockwool is inorganic, and hence, it is completely sterile. Even the production process, which involves extremely high temperatures, ensures nothing in the medium can survive except the fibers. And thanks to its sterile nature, rockwool can go a long way in ensuring your plant’s roots are protected against harmful microbes. 

You can Compost It

Rockwool takes a long time to decompose, but you can still shred it down into fine particles and add them to your compost pit. Soon, the basalt rock minerals will decompose into the compost, adding a nutritional boost to the pile. 

Disadvantages of Rockwool for Growing Cannabis 

Disadvantages of Rockwool for Growing Cannabis 

Rockwool isn’t exempt from downsides, despite all the benefits. Here are some disadvantages of using rockwool for growing cannabis. 

It’s Not Biodegradable 

If you want to make your cannabis operation as eco-friendly as possible, then rockwool wouldn’t be the best option. Unless you’re willing to use rockwool in your compost or go through the extent of shredding them, your discarded rockwool will sadly end up sitting in a landfill somewhere for many years. 

Manufacturing Rockwool Isn’t Eco-friendly, Either

Additionally, rockwool production is not eco-friendly. Rockwool is produced from basalt when it’s heated at extreme temperatures and then spinning it into fibers, which rely on a lot of coal energy to melt the rock. Moreover, basalt is non-renewable. Also, some rockwool products could contain additives and binders that aren’t eco-friendly either. 

Considering all this, rockwool may not be the most sustainable or eco-friendly choice for cannabis cultivation when compared to some other growing mediums like coco coir, perlite, or peat moss, which are renewable or have lower environmental impacts.

It is Naturally Alkaline 

Rockwool doesn’t have a neutral pH; naturally, it has high pH, but cannabis prefers a slightly acidic growing medium. So, you need to treat rockwool before using it in your cannabis garden. More on this below. 

It Can Harm You

Rockwool contains loose fibers, which can irritate your skin, eyes, lungs, and mouth. In fact, extended exposure to rockwool dust without protection can lead to fiber buildup in the lungs, just like asbestos. And according to the Environmental Protection Agency (EPA), rockwool fiber is classified as a probable human carcinogen

pH Management

Typically, rockwool is alkaline, which is higher than what you’d prefer to grow cannabis. Thus, you must carefully adjust the pH of the nutrient solution to make it more acidic and within the optimal range for cannabis plants. If you don’t do this correctly, the plants can experience nutrient imbalances as some essential nutrients become less available to the plants at higher pH levels. Monitor the pH levels regularly to ensure that the plants receive the nutrients they need for healthy growth.

Additionally, as the rockwool ages and undergoes repeated use in multiple growing cycles, it can become more difficult to adjust and stabilize the pH. This is because of an accumulation of mineral deposits and changes in the chemical properties over time, which can affect its pH-buffering capacity.

Initial Prepping

Properly preparing rockwool for use in cannabis cultivation can be time-consuming and requires attention to detail. Before planting, the rockwool cubes or slabs need to be soaked in water to remove any initial alkaline residues and to saturate the fibers with moisture. The water used for soaking must also have its pH adjusted to create an environment suitable for the plants. Ensuring the correct pH level and thorough soaking are critical to avoid shocking young cannabis seedlings or clones when they are transplanted into the rockwool medium.

Retention of Salts

Over time, mineral salts can accumulate in the rockwool medium, particularly as the water evaporates and leaves the salts behind. These accumulated salts can lead to increased electrical conductivity (EC) levels in the growing medium. High EC levels can interfere with nutrient absorption and lead to nutrient imbalances in the plants. When unchecked, excessive salt buildup can cause a "nutrient lockout," where certain nutrients become unavailable to the plants, resulting in deficiencies and poor growth.

Root Zone Saturation

Rockwool's water retention properties can be a double-edged sword. While it can hold water well, it can also become easily saturated if watered excessively. When the rockwool is overly wet, it limits the availability of oxygen in the root zone, leading to anaerobic conditions that can encourage the growth of harmful microorganisms, such as root rot pathogens. Proper watering practices and ensuring adequate drainage are essential to prevent waterlogging and maintain a healthy root environment.

How to Use Rockwool to Grow Cannabis?

How to Use Rockwool to Grow Cannabis?

Rockwool is relatively easy to use but it does require some preparation and you also need to take care of a few other aspects, depending on your cannabis operation setup. Let’s take a look at how to use rockwool below. 

Stabilizing the pH of the Rockwool

Once you get your hands on rockwool, the first step you need to do is to stabilize it by adjusting its pH to make them suitable for cannabis cultivation. You can also feed the blocks at this stage so they can sustain your cannabis seedlings. 

For this step, you need the following things:

  • pH-down solution 
  • pH meter 
  • Organic nutrients for seedlings 
  • Optional: mycorrhizal fungi or rhizobacteria 

Once you have all things ready, follow these steps to prepare your rockwool cubes for germination:

  1. Fill up a small, shallow container with tap water and test the pH
  2. Use the pH down solution to bring the ph to 5.5 
  3. Submerge the rockwool cubes into this water and let them sit there for at least one entire day 
  4. The blocks’ high pH will raise the water’s pH, too, so test the pH every few hours and re-apply pH down products until a 5.5 pH reading is maintained 
  5. Then, fill up another container with water and add in your organic nutrients as per the manufacturer’s recommendations 
  6. Soak the rockwool cubes in nutrient solution for a couple of hours 

Your rockwool cubes are ready. 

Germinate Seeds with Rockwool 

Germinating seeds with rockwool is super easy, so follow the given steps to give your cannabis plants the best start possible:

  1. Use a skewer or something similar to punch a hole in the center of the cube
  2. Place your cannabis seed into the hole and ensure it is as close to the center of the cube as possible 
  3. Since seeds germinate faster in high humidity, place your cubes in a mini-greenhouse or container and seal it with cling wrap. 
  4. Set up grow lights around 10 cm above the tray and turn them on on the second day of germination, and stick to a light cycle of 16/8 to prepare the seedlings for the vegetative stage 

Depending on the type and strain of seeds you use, the seeds can germinate between two to seven days. Meanwhile, be patient. 

Germinate Clone Cuttings with Rockwool

If you want to clone cannabis, you can still use rockwool since it is quite an effective cloning medium, too. Follow these steps.

  1. Use the right method of your choice to take cuttings from the mother cannabis plant 
  2. Dip the cut of the cutting into rooting hormone 
  3. Then, make an appropriate hole in the rockwool cube and stick the cutting into the hole — be gentle 
  4. Then, place your cutting on a tray and cover them; you can also spray the tray with water to increase humility at the bottom and set up grow lights at a light cycle of 18/6 

Your clones will germinate soon.

Vegetative Stage with Rockwool 

Once the seeds are ready for transplanting — they have grown a few sets of true leaves — your plant is ready for the vegetative stage. They are ready for the next phase of their lives.

If you are growing cannabis hydroponically, you need to move the seedlings with the cubes into your setup. Here is how to do that:

  1. Choose the right hydroponic setup for rockwool; the best ones are deep water culture, drip irrigation, and nutrient film technique 
  2. Place the cubes with seedlings into net pots, which will support the plant and allow the roots to expand through the gaps
  3. Add in some clay pebbles on and around the seedlings to block light from reaching the roots while improving aeration 
  4. Start your hydroponic system 

On the other hand, if you are growing cannabis in soil, you can still transplant rockwool seedlings into the soil. Follow these steps:

  1. Get your hands on high-quality soil and add in organic nutrients suitable for cannabis 
  2. Fill your container with this soil mix (pro tip: use fabric pots)
  3. Compact the soil and water it 
  4. Dig a hole in the center and place your rockwool cube in the hole 
  5. Cover up the cube with more soil and water it 
  6. Let the plant settle and take care of it by watering it on time 

Apart from this, you don’t need to take any special care of the rockwool cubes during the vegetative or flowering stages. Simply water your plant well, take care of it, prune it on time, and give it enough light and it will grow into a big, healthy cannabis plant. 

Some Tips for Using Rockwool to Grow Cannabis

Some Tips for Using Rockwool to Grow Cannabis

As mentioned earlier, using rockwool is easy but you need to keep certain things in mind. So, to help you improve your use of rockwool, here are some tips on using this excellent growing medium for cannabis plants. 

Always Pre-soak Rockwool

Perhaps the most important tip to keep in mind is to pre-soak your rockwool cube before use. Remember, rockwool is shipped dry and has 0% moisture content, which is why you need to initially moisten it so that it can sustain a seedling. 

You need to immerse it in pH 5.5 water for 24 hours to stabilize its pH, followed by soaking it in a nutrient-rich solution for a couple of hours. This extended soaking allows the moisture to penetrate into the tiny fibers. After soaking it, let the excess water drain out so that the air can re-enter the pores. 

Fertilize Your Rockwool Cubes

Speaking of soaking, it is also a good idea to fertilize the cubes when you are soaking them, as mentioned in the above steps. Once the cubes are fertilized, they will have all the crucial nutrients to support germination, but you must always fertilize your rockwool cubes; unlike soil, rockwool will not sustain your cannabis plant for long without nutrients. 

Never Squeeze Rockwool

In the growing circles, many growers believe that rockwool must be squeezed before use to remove excess water. Never do this! This is a myth that can have several consequences for your cannabis plant. 

Rockwool is made of fine fibers, which get completely destroyed when you squeeze them. A squeezed rockwool will not benefit from excellent water retention and air porosity, and it won’t sustain your cannabis seedling properly. Squeezing rockwool is a recipe for disaster and must be avoided at all costs. 

Allow Sufficient Drainage

Your cannabis plant consumes water at a higher rate than nutrients, so there can be a little salt buildup in the rockwool cubes between irrigation cycles as mentioned earlier. This can create nutrient problems and even change the pH of the medium. 

When you irrigate your cannabis plant again, the water will take all the excess nutrients with it and replace them with fresh ones. So, it is crucial that you allow sufficient drainage of water. Ideally, you should water your plants until 25% of the water has drained out of the bottom. 

Don’t Overwater Your Plant

You need sufficient drainage, but never overwater your cannabis plant growing in rockwool. Overwatering can lead to fungal or algae growth within the medium, and it can also choke the roots, which can prevent them from developing properly. It can be nasty for your plant as it can slow its growth and, sometimes, even kill it. 

Again, stick to 25% runoff water and you should be fine. If there’s more runoff water, you are giving your plant a lot of water, so take a step back. On the other hand, if the runoff is very little, your rockwool cube can dry out and your plant can suffer from lack of water. 

Avoid Over-stacking

To know how tall your rockwool container can be, you need to take into account the porosity and structure of your rockwool. So, if you stack rockwool too high and irrigate it from the bottom, the water will not reach more than 5 to 6 inches, thanks to gravity. The top of the medium will be dry. 

So, avoid over-stacking rockwool unless you are using a hydroponic system like ebb and flow and irrigating the medium from the top, so the water can travel through the stack and moisten the entire height of rockwool evenly. 

Keep the Rockwool Foil On

When you purchase rockwool cubes, you’ll notice that they are covered in a plastic foil, which keeps the cubes in shape while keeping the light out, and it can help prevent fungus or algae. So, do not remove the foil right away. 

Avoid Reusing Rockwool 

While many growers will argue that rockwool can be recycled for the next cycle of plants, we strongly recommend against it. Used rockwool will have small organic matter, like pieces of roots, that can make the rockwool cube prone to fungi or mold. 

Plus, rockwool is made of delicate fibers, and it will be filled with holes made by the roots, so the same fibers will fail to retain water and air or accommodate a new set of roots. Instead, it’s best to shred rockwool and add it to the compost bin. 

But if you are out of options and still want to recycle rockwool, you can try growing another plant with the same cube. To do this, you need to submerge the rockwool cube in boiling water to eliminate any pathogens from within. Next, wrap a soft garden tie around the cube to support its shape. 

Do Not Toss Rockwool in the Bin 

As mentioned earlier, rockwool is not an organic material, so it does not biodegrade easily. So, if you toss them into the bin, they will end up in a landfill waiting to be eaten away very slowly. The best option here to decrease your cannabis operation’s carbon footprint is to use rockwool in compost. 

Used rockwool will still be loaded with nutrients, and compost can leach out these nutrients from compost, which can benefit your plant when you use the compost. And to accelerate the process of breakdown, shred up rockwool into tiny pieces before tossing it into the compost pile. 

Always Protect Yourself When Handling Rockwool

As mentioned above, rockwool fibers can be toxic for humans as they can irritate the skin, eyes, and lungs. So, it is best to use safety glasses, gloves, and a mask while handling rockwool. It won’t necessarily harm you, but there’s always a chance of it happening. 

Summary: A Comprehensive Guide to Use Rockwool To Grow Cannabis 

Rockwool is fantastic, isn’t it? So, why not use it? Use this article as a guidebook on using rockwool for growing cannabis, and remember to follow all the tips and steps mentioned above. It may be a bit confusing at first, but give yourself some time and you’ll become a pro at using rockwool. 

In any case, remember some crucial points. Always soak it and fertilize it, do not squeeze it, and protect yourself. Even with the bare minimum, rockwool can still surprise you with the results. It’s a very forgiving growing medium to use. Stay tuned to know more.


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