Growing Mediums For Cannabis: How to Use Grow Cubes to Grow Cannabis?

Grow cubes are terrific mediums for hydroponic cannabis plants. They offer benefits that your plant won’t get from the soil. For instance, hydroponic grow cubes bring an ideal balance of oxygen, nutrients, and water for your cannabis plant. This is why they are often the preferred choice for many cannabis growers of all skill levels. 

Mostly, grow cubes are made of rockwool — it’s the most popular type, and hence, rockwool is what we will focus on. Grow cubes, also known as rockwool cubes, are ideal growing mediums for cannabis plants.

But what is rockwool? It is a fascinating compound that smart minds in labs have developed. Essentially, it is a fiber — like fiberglass — that is prepared from melting and spinning basalt rock. Think of it like cotton candy, but instead of sugar, it is made of basalt that has undergone extreme conditions. 

Since these grow cubes have a unique structure, it makes them excellent for hydroponic plants. Their structure offers your cannabis plant’s roots an ideal environment, so starting from a seedling to the end of harvest, grow cubes come in handy. 

What are the Benefits of Using Grow Cubes to Grow Cannabis?

If you’re wondering why grow cubes are so popular, it’s because they offer several benefits to hydroponic cultivation. Here are the benefits of using grow cubes for growing cannabis:

• Water Retention and Aeration

The porous nature of Rockwool enables it to absorb water and also retain it, thereby reducing the risk of dehydration and stress in cannabis plants. Its fibers can hold moisture without reaching saturation, ensuring a water supply to the roots.

This capacity for water retention proves useful when the weather is too hot and you need to water the plants frequently. Adequate moisture is incredibly important for the root zone as it helps with the uptake of nutrients and transports them within the plant. This way, you can rest assured that your plants don’t have problems taking in nutrients that are necessary for them to grow big. 

Any growing medium you use should be well-aerated. Rockwool excels in this department as it also facilitates aeration of the root zone. Its porous structure allows air to permeate through the growing medium, ensuring that roots get enough oxygen.

Oxygen is essential for root respiration — a process that generates energy for absorption and metabolic activities within the plant. Without it, the root zone will simply suffocate. Rockwool takes care of this problem, thanks to its characteristics. 

Another major advantage is that rockwool prevents waterlogging due to its ability to drain water at ease. If the growing medium you use doesn’t drain water properly, the roots will struggle with many diseases including root rot and suffocation. However, as rockwool drains a lot of water well, the roots will be safe. 

That said, you shouldn’t overwater as rockwool’s ability to retain water can have opposite effects. This means that if you overwater, it will hold a lot of that water and the roots will suffer. Thus, be careful not to overwater the plants regardless of the grow medium you use. 

• Resistance to Pests and Diseases

Rockwool is produced in regulated conditions with high temperatures ensuring that it is free from any harmful pathogens, pests, or weed seeds. This sterility guarantees that it remains free from microorganisms, thereby reducing the likelihood of soil-borne diseases and pest invasions that could harm your cannabis plants.

Using rockwool as a growing medium, you don’t have to rely too much on chemical pesticides or fungicides, promoting a friendly and sustainable approach to managing pests and diseases.

• Clean Environment

Adding to the previous point, its sterility reduces the spread of diseases as many pests and diseases spread when the grow room is unclean. Soil-based mediums tend to carry many pathogens that can spread to the plants easily if you don’t take any steps to heat it and kill the pathogens. With Rockwool, though, there’s little chance of diseases spreading to the plants as it is sterile already. This is particularly useful for growers who keep their plants very close to each other due to limited space, making it very difficult to clean the grow room often.

• Offers More Control

Having control over the growing conditions is an advantage of using rockwool. Its inert nature enables you to customize your nutrient solutions, adjust pH levels, and manage the growth of the plants with accuracy. For instance, since rockwool lacks nutrients, you can tailor the nutrient solutions to suit the requirements of your plants as they need different ratios of nutrients at various growth stages. Most importantly, rockwool does not interact with any of the nutrients that you provide, so you can expect consistent results with all the plants you’re growing. 

Compared to soil-based mediums, rockwool allows you to adjust the pH more accurately, which enhances nutrient absorption and reduces the likelihood of nutrient imbalances or toxicities. Therefore, rockwool is an excellent choice for hydroponic and soilless cultivation systems where meticulous control over growing conditions is crucial for achieving success.

• Consistency and Uniformity

The uniform nature of rockwool cubes can help you grow cannabis more efficiently. Rockwool cubes are produced under controlled conditions. So much so that the cubes are all uniform in size, shape, and density. Each cube is crafted to provide the perfect setting for plant roots to thrive. Not that consistency will help with plant development, but you can judge the growth of all plants based on this factor. If you grow the same strains, you can also expect consistent growth across all plants. 

• Easy To Transplant

Rockwool cubes are user-friendly and practical for germinating seeds or propagating cuttings. The seeds sprout quickly, and form root systems within the cubes, making it easier to transfer into larger containers or hydroponic setups. This helps minimize shock during transplantation and helps the roots establish themselves quickly, resulting in better growth and earlier harvests.

• Versatile

Rockwool is versatile and compatible with many ways of growing cannabis, including hydroponics, aeroponics, and traditional soil-based cultivation. It can be used in pots, integrated into a drip system, or incorporated into recirculating systems like flood tables. This adaptability allows you to customize your growing environment exactly how you want it. 

• Reusability

Rockwool can be reused for several growing cycles by sterilizing and maintaining it. In fact, some growers use it up to 6 times to get the best out of it. We will discuss this in detail in the next few sections. 

What are the Disadvantages of Using Grow Cubes to Grow Cannabis?

Although rockwool offers benefits for cannabis cultivation, there are a few drawbacks that growers should take into account before selecting it as a growing medium. Here are some of the disadvantages associated with using rockwool:

• Respiratory Irritation

The lightweight nature of rockwool fibers makes them airborne when handled improperly, increasing the risk of inhalation. Medical experts are still figuring out exactly how these fibers can be dangerous, but caution is recommended. 

Breathing in these fibers could irritate your respiratory system. Those suffering from respiratory issues already can experience coughing, wheezing, difficulty breathing, and throat discomfort. If you’re continuously exposed to the fibers, it might even be possible to develop asthma or bronchitis, although it’s rare. 

• Skin Irritation

Direct contact with rockwool fibers might lead to skin irritation for those with sensitive skin or existing skin problems. The rough texture of rockwool fibers could provoke redness, itching, and inflammation once they come in contact with your skin. Handling rockwool without protection like gloves and long sleeves could lead to skin abrasions or dermatitis if the fibers get stuck in the skin.

Therefore, you should take things seriously and reduce your exposure to those fibers, and minimize health risks. Always wear protective gear including gloves, long sleeves, long pants, and closed-toe shoes to shield the skin from direct fiber contact. It does seem like a lot but you should do it if you don’t want unnecessary health issues. 

Also, don’t touch your face, eyes, or mouth with hands that may have been in contact with rockwool fibers. This precaution helps avoid ingestion or inhalation risks. Finally, don’t forget to thoroughly wash your hands and any exposed skin using soap and water to eliminate any remaining fibers and minimize irritation.

• Pre-Treatment Required

Rockwool requires some pre-treatment before you can use it to ensure optimal growing conditions and prevent potential nutrient imbalances. This is done to remove any salts that may have accumulated during manufacturing. If you don’t flush out these slats, they can be harmful to the plant roots. Soaking the rockwool beforehand helps to remove any salts from the material, reducing the chances of salt accumulation in the root area. 

• pH

Rockwool tends to have an elevated pH level usually falling within 7 to 8. Yes, we mentioned that its pH stability is an advantage, but you need to get this pH down initially to make it compatible with growing cannabis. After that, you won't have any problems with the pH of rockwool itself unless you use nutrient solutions that aren’t pH-balanced. 

Soaking and rinsing rockwool helps to decrease its pH level to a range for cultivating cannabis, typically around 5.5 to 6.5. This adjustment in pH ensures that the roots can take in nutrients and grow as usual without any imbalances or deficiencies in the nutrients. 

As you can understand, preparing rockwool for cultivation requires soaking that takes up some time, and adds an extra task to the whole process. Growers need to plan and set aside some time for this preparation before they can move their seedlings or clones into rockwool.

• Not Environmental Friendly

Rockwool is crafted from renewable resources like basalt rock. Although it can be recycled and reused for growing cycles, its production process consumes natural resources that may have some negative impact on the environment. Due to this, some growers may opt for other sustainable alternatives.

For this very reason, disposing of used rockwool poses challenges. If you don’t shred it, it can sit in the garbage fills for years. This is why it’s recommended that you either recycle it or shred it completely before disposing it. 

• Very Limited Microbial Activity

Over the years, many people have shifted to soilless mediums to improve the growth of their plants. However, most soilless mediums still offer some advantages that soil offers. For instance, coco coir can also encourage microbial activity when you mix it with compost. 

In contrast, rockwool lacks the microbial activity that promotes soil health and plant resilience. One advantage is that rockwool doesn’t promote soil-borne diseases, but it also means that you must add extra supplements to support the plants. 

• Expensive

Compared to other growing mediums, rockwool is a bit expensive. For example, you can get a lot of soil and compost for cheap in your local nursery store; however, with rockwool, the initial investment in rockwool cubes or slabs and the fact that you have to spend time adjusting the pH initially could be challenging for those on a very strict budget. 

How to Recycle and Reuse Rockwool to Grow Cannabis?

Reusing rockwool that was previously used for cultivating cannabis can reduce waste and also help you save some money. Here’s how to do it:

• Clean and Sterilize 

Before recycling, remove any leftover plant material, roots, or other organic substances from the rockwool. Next, rinse it thoroughly to get rid of salts, nutrients, and any remaining debris.

To sterilize the rockwool and get rid of any pathogens or pests, soak it in a solution of hydrogen peroxide mixed with water. Or, use any disinfectant according to the instructions provided by the manufacturer.

• Drying

Once cleaned and sterilized, let the rockwool dry completely in a ventilated area. This step is important as you can handle dry rockwool easily compared to soggy cubes. 

• Break them down

Once the cubes are perfectly dry, it’s time to break them down. Use scissors or shears for this process. Breaking them down makes it easier to incorporate them into growing media. Make sure you wear gloves and masks so you don’t inhale the fibers. 

Next, mix the rockwool you have broken down into compost, perlite, or even coco coir to add some extra aeration. You can now use this new medium to grow plants. 

Although you can use this new medium to grow cannabis again, it’s best to rotate it and grow another type of plant to prevent the possibility of diseases. If you don’t want to add it to compost, you can simply sterilize and reuse it as is in hydroponic systems. If the cubes have degraded or have been damaged, it’s time to toss them and use new cubes. However, don’t toss them into recycling or compost binds just like that as it’s not environmentally friendly and won’t decompose fast. 

What are the Types of Grow Cubes to Grow Cannabis?

Visit your local hydroponics store and you might get confused with the various types available there, but in most cases, the differences boil down to one thing: dimension. 

Grow cubes come in various sizes, ranging from an inch to a few inches. For commercial growers, grow cubes are also sold as slabs or bricks, which you can later cut and use as per your requirement. 

Depending on your requirements and how much support your cannabis plant needs, you can pick a size accordingly. There is no proper rule of thumb here; if you are growing a big cannabis strain, pick a big cube or vice versa. 

Jiffy Pellets

You will see another type of grow cube made out of coco coir. Also known as jiffy pellets, they are perfect for germinating cannabis seeds. Like rockwool, they offer various benefits including water retention, aeration and root growth.

Keep in mind that you should only go for pellets meant specifically to grow cannabis as they are designed while keeping pH and water retention in mind. Use them just like you’d use rockwool. While some growers prefer germinating their seeds in paper towels and then placing the seeds in the pellets, you can place the seeds directly in the pellets. Make sure you sow the seeds at the recommended depth as it’s easy to plant them too deep or too shallow. 

Again, like rockwool, jiffy pellets are great at draining water but they retain a lot of it as well. Thus, you shouldn’t soak them wet. Instead, if it’s soaking wet (which it will be after you soak it in water), wait for it to dry out a bit and then plant the seeds as they need a moist environment rather than a saturated one. 

Begin by placing the pellets in a tray or container and add water until they expand fully. Once the water drains out and the pellets are moist, make an indentation in each pellet. Carefully plant the seeds at the recommended depth.

Next, regularly water the coir cubes to keep them consistently moist. Do not overwater at any cost or the seeds will simply rot. Once the seeds sprout in a few days, wait for them to develop into seedlings and you can transfer them to pots directly. There’s no need to remove the seedling from the jiffy pellet as it adjusts to the new soil or coco growing medium pretty well. 

Oasis Cubes

Oasis cubes are nothing but foam that’s typically used floral industry to arrange flowers. Like other grow mediums, they absorb plenty of water and offer support to the stems. They are also available in various shapes and sizes as pre-cut cubes or blocks. 

It’s pretty easy to germinate any type of seed, including cannabis, using oasis cubes. However, just like you prepare rockwool, you’ll have to prepare oasis cubes too before sowing the seeds. To start, you need to soak the cubes in plain water and wait until they are saturated. As oasis cubes are pH neutral, there’s no need to use pH-balanced water at this point. 

Next, wait for the water to drain off so the medium is moist and not soaking wet. Oasis cubes will generally have premade holes. Place the seeds in the holes and wait a few days for them to germinate. 

Once the seeds have sprouted, wait for them to develop a few leaves and a decent root system. Next, transplant the seedlings along with the cubes into the soil or a hydroponic system.

You can repeat the same process for clones as well. Again, the cubes must be moist when you take cuttings and plant them. Use rooting hormones if required. If your cubes don’t have holes, use a knife to jab some holes. Keep your cuttings in indirect sunlight or grow light with less light intensity and mist them regularly to maintain a humid environment. Once the clones develop roots, you can transplant them into another growing medium.

How to Use Grow Cubes to Grow Cannabis?

As mentioned earlier, to use rockwool grow cubes properly, you need to prepare them before using them. This is because grow cubes are inert and have a naturally high pH, so you need to soak them in a nutrient solution so they can absorb the nutrients and be ready to sustain a cannabis seedling. 

Do not skip this step. It is essential, and here is how you can prepare your rockwool grow cubes:

• Prepare a nutrient solution of at least 1.5 mS/cm EC and 5.5 to 6 pH
• Soak the grow cubes in the nutrient solution, but avoid stacking them 
• Let them soak for about 24 hours. Some growers let it soak for 48 hours but that’s not required.  
• Then, remove them and use them 

Two points to remember here. It is recommended to use a slightly lower pH solution to balance out the high pH the grow cubes naturally have. You will also find some growers recommending squeezing them, but do not do this — this ruins the oxygen and water retention capabilities of the cubes. 

Germinating Cannabis Seeds or Cloning Cuttings

The best way to use grow cubes is for germinating your cannabis seeds or plant propagation. Grow cubes enable the ideal conditions for root development, which is a key process for germination. Essentially, grow cubes bring the right amount of moisture, enabling the enzymatic reactions in the seed. 

To start seed germination in grow cubes, use ones with premade holes in them. If your cubes don’t have holes, simply poke a small hole in the center with a pen or something similar — don’t use bare fingers. 

Then, prepare your cubes as mentioned above and place your cannabis seed in the hole. Then, continue watering the cubes every few days to keep it moist. These cubes have terrific water retention, so don’t overwater or the seeds will fail. 

On the other hand, if you are cloning cannabis, you can follow a similar approach. Take your cutting and drip it into the rooting hormone. Then, place your cutting into a prepared grow cube — put it at least an inch deep but don’t let it come out of the bottom. Consider using bigger grow cubes here.

Then, keep taking care of the clones and keep the cubes moist. Soon enough, your clone cuttings will start developing roots. 

Maintaining a Proper Supply of Water and Nutrients 

Once you start growing cannabis in grow cubes, you need to pay special attention to the nutrient solution to give it. For this, you first need to understand exactly what your plant needs along with the right techniques for irrigation. 

Ensure that your nutrient solution contains all the necessary macronutrients and micronutrients, depending on your plant’s growth stage. The nutrients should be in the correct ratio, too, since these cubes are completely inert. You are the master of nutrients here, not nature. 

It is also essential that you constantly monitor the pH of the runoff water. Rockwool cubes are susceptible to pH fluctuations, so if the runoff water shows a different pH than your nutrient solution, flush the cubes and calibrate your nutrient solution to prevent this fluctuation in the future. 

You also need to learn how to water these cubes properly. Your goal is to keep them moist, but not soaked. So, reconsider your watering schedule and water your plant accordingly. It is also a wise choice to keep humidity in check. 

Maintaining Your Plant With Grow Cubes 

Another important aspect you should always keep in mind is how to take care of your plant. There are two main factors you must factor in, such as the following. 

• Keep an Eye on the Roots 

The first thing you need to ensure is to monitor the root growth. Always keep an eye on your plant’s roots — they should look healthy without any signs of mold, popping, disease, etc. 

• Transplant With Care

Second, you need to learn how to transplant your cannabis growing in grow cubes. The first transplant will occur at the end of the seedling stage when the roots start growing out of the cube’s bottom. 

Here, you need to ensure you pick a size that’s suitable for your plant and be extra gentle while moving it. 

Tips for Vegetative and Flowering Stages

Depending on the growth stage of your plant, you’ll have to adopt different strategies. Here are some tips on both the vegetative and flowering stages. 

Vegetative Stage

• Aim for a low pH of around 5.5
• Ideally, around 60 to 70 ml of fertilizer should be enough for a single cannabis 
• Use the right NPK ratio here: 1:1:1

Flowering Stage

• During the first week, boost the pH to 5.8 to 5.9 and change the NPK ratio to 1:2:2 (NPK) — your plant doesn’t need so much nitrogen during bloom 
• During the second week, change the EC between 0.8 to 0.9 and the pH to 6.0
• Starting the fourth week, increase the EC to 1.2 and the pH between 6.0 to 6.2
• Change the EC to 1.6 during the seventh week and 1.8 during the eighth week
• During the final weeks of bloom, drop the EC down as much as you can and maintain a pH of 6.2

Summary: Growing Mediums For Cannabis: How to Use Grow Cubes to Grow Cannabis?

Cannabis grow cubes are a boon for many hydroponic growers. These rockwool cubes offer excellent water retention and aeration capabilities that only the highest quality soil can offer, but what’s more, these cubes also give you ultimate control over nutrient delivery.

Yes, they do have a few drawbacks, but most of these drawbacks are easy to work with. And remember to always wear gloves, glasses, and a mask when handling rockwool cubes. Use the tips mentioned above to cultivate cannabis using rockwool and you’ll be rewarded with a flourishing cannabis plant.