How To Plant Weed Seeds In Rockwool Cubes

Hydroponics is all about getting bigger, more potent buds, faster than a typical soil grow. We take a closer look at cannabis cultivation with rockwool. How To Start From Seed With Rockwool Cubes In this lesson, we will learn how to start your own seed with Rockwool Cubes. Below is a detailed guide to success with Rockwool, and a step-by-step Rockwool is the hydroponic substrate of choice for growing cannabis. Click here to get the basics down with this step-by-step guide from Zamnesia.

Rockwool For Hydroponic Marijuana Growing

Rockwool is a great medium for growing marijuana. You can take your strain of choice through the whole cannabis life cycle with rockwool. Growers of all levels, using a variety of hydroponic setups are harvesting heavier yields a whole lot sooner. It’s about time you tried rockwool too.

Cannabinoids, terpenes, phytochemicals, organic cultivation

Rockwool harnesses the power of natural stone to produce a fluffy and aerated growing medium ideal for cannabis growing. This material drains well, supports root development, and arrives in a cuboid shape that is easy to transplant into hydroponic or soil-based systems.

In this guide, you’ll learn everything you need to know about this robust material. Find out why to use Rockwool in both hydroponic and soil systems, how it works during every stage of the growing cycle, and much more.

Contents:

  1. What is rockwool?
  2. Why use rockwool to grow cannabis?
  3. How to use rockwool to grow cannabis
  4. Top tips for using rockwool
  5. Should you use rockwool for growing cannabis?

What Is Rockwool?

Rockwool is a branded form of mineral wool, a material used primarily as thermal and acoustic insulation in the construction of homes, hospitals, recording studios, and even stadiums.

The Danish founders of Rockwool obtained a licence in 1935 to produce stone wool in Scandinavia, Germany, and New Jersey. Over almost a century, the Rockwool company has continually refined their manufacturing process to create a more efficient and effective product.

As well as serving as a crucial building material, Rockwool has helped to facilitate the growth of hundreds of thousands, if not millions, of cannabis plants. The substance brings a host of benefits to the grow room and garden.

For one, Rockwool doesn’t contain any organic material. This means it’s immune to decomposition at the hands of mould and fungi. Rockwool is also free from heavy metals. These elements, such as lead and chromium, are highly toxic to plants, cause yellow leaves and prevent photosynthesis, and can even kill plants altogether.

How Is Rockwool Made?

The inception of mineral wool came about when observers at a volcano watched strong winds whip lava into woolly fibres.

A combination of basalt rock, fast spinning actions, and extremely high temperatures are used to make Rockwool. The process starts with a combination of basalt rock and slag recycled from the steel industry.

As a form of solidified lava, basalt rock forms underground when rock melts and then rapidly cools down. Whole basalt rocks, and briquettes made from basalt particles, are melted into lava alongside steel slag within a furnace. To replicate the extreme environment of a volcano, furnaces are cranked up to 1500°C.

Upon mimicking a volcano, the factory machinery then needs to mimic the wind. The lava enters a spinning machine that whips it into thin strands of stone wool. The addition of some binding solution oil helps to hold the strands together. Another machine layers several sheets of wool on top of each other, then a conveyor belt passes them through a series of rollers that compress the wool into a dense mat and improve its structure.

Finally, the dense sheets enter a large oven. The heat activates the binding solution and enables the wool fibres to hold their shape.

Can Rockwool Be Composted?

Yes! Rockwool contains basalt, which works great as a soil amendment in general. Once it becomes weathered over time, it releases nutrients such as phosphorus into the soil.

This nutrient supports the growth of every plant cell, aids in energy transfer and photosynthesis, and plays an especially important role during the flowering phase. As a bonus, basalt rock dust also helps to absorb large quantities of carbon dioxide from the atmosphere.

Shred your used Rockwool cubes into fine strips before adding them to your compost pile to speed up the rate at which they release minerals into the soil.

Is Rockwool Biodegradable?

Technically, no. Because Rockwool contains no organic material, microbes such as fungi and bacteria won’t break it down in the same way they would food scraps, leaves, and twigs.

However, healthy soil doesn’t just contain organic substances. Vital minerals and elements required by plants are inert by nature. Some of these are locked away inside of Rockwool and will eventually end up inside your cannabis plants, where they’ll enable and support key physiological functions.

Tossed whole onto a compost pile, Rockwool will take an extremely long time to break down—hence why many growers see it as bad for the environment. You should aim to shred up your used cubes as finely as possible to accelerate this process.

Is Rockwool Reusable?

We don’t recommend using the same Rockwool cube to start a new seed. Although the material itself prevents microbial growth, small pieces of roots and organic matter can end up trapped between the fibres, which can encourage the growth of detrimental fungi.

The structural integrity of a Rockwool cube will also deteriorate after a single round of growth. As the thick and strong main roots develop, they’ll make holes in the cube, in turn making it less able to accommodate another set of roots in the future.

To put your used Rockwool cubes to use, we recommend simply shredding them and tossing them onto your compost pile to recycle the nutrients within.

However, if you’re trying to grow cannabis as cheaply as possible, you can probably squeeze another plant or two out of the same Rockwool cube. First, you’ll need to submerge it in boiling water to kill off lurking pathogens dislodged from the previous root system.

Second, consider wrapping a rubber band or soft garden tie around the middle of the cube to ensure it doesn’t break in half during the next grow.

Why Use Rockwool To Grow Cannabis?

There are many benefits to growing cannabis with Rockwool, especially when doing so hydroponically.

Pros of Rockwool for Growing Cannabis

Rockwool offers a long list of benefits to cannabis growers. Not only does it prevent pathogens from multiplying, but it helps seeds germinate and facilitates good drainage.

Rockwool cubes are sterile and inorganic. After being exposed to extreme heat, Rockwool arrives at a grower’s doorstep clean and ready for use. Because it doesn’t contain any organic matter, Rockwool helps to protect the root zone against damaging microbes.

Although they take a long time to break down, growers can shred their used blocks down as finely as possible and throw them onto the compost pile. Over time, key elements within basalt rock will leach into the compost and add a nutritional kick to your future soil.

Rockwool cubes become saturated with water when you soak them, but it quickly drains away. Each cube contains many small spaces between the fibres that help to effectively drain water. Overwatering kills many grows before they even get started. Damp conditions can foster fungal pathogens that quickly destroy seedlings. Although Rockwool cubes are largely safe from microbes, excess water can still suffocate roots and hamper growth. Good aeration stops this from happening.

Great for germination

Rockwool cubes provide a haven for seeds during germination. By keeping them sufficiently saturated, they provide the three key demands required for successful germination: darkness, moisture, and warmth.

Sometimes seeds struggle to become established when sown in compact and hard soils. However, they establish themselves quickly and easily within Rockwool cubes. The young roots easily pass through the gaps between the fibres.

Once your seedling looks ready for a new home, you’ll find it incredibly easy to move it directly into a pot full of soil or a hydroponic net pot. Unlike transplanting from soil, you don’t need to break apart the initial growing medium or deal with any mess. Simply pick up the cube and slot it into its destination.

Rockwool cubes are sterile and inorganic. After being exposed to extreme heat, Rockwool arrives at a grower’s doorstep clean and ready for use. Because it doesn’t contain any organic matter, Rockwool helps to protect the root zone against damaging microbes.

Although they take a long time to break down, growers can shred their used blocks down as finely as possible and throw them onto the compost pile. Over time, key elements within basalt rock will leach into the compost and add a nutritional kick to your future soil.

Rockwool cubes become saturated with water when you soak them, but it quickly drains away. Each cube contains many small spaces between the fibres that help to effectively drain water.

Overwatering kills many grows before they even get started. Damp conditions can foster fungal pathogens that quickly destroy seedlings. Although Rockwool cubes are largely safe from microbes, excess water can still suffocate roots and hamper growth. Good aeration stops this from happening.

Rockwool cubes provide a haven for seeds during germination. By keeping them sufficiently saturated, they provide the three key demands required for successful germination: darkness, moisture, and warmth.

Sometimes seeds struggle to become established when sown in compact and hard soils. However, they establish themselves quickly and easily within Rockwool cubes. The young roots easily pass through the gaps between the fibres.

Once your seedling looks ready for a new home, you’ll find it incredibly easy to move it directly into a pot full of soil or a hydroponic net pot. Unlike transplanting from soil, you don’t need to break apart the initial growing medium or deal with any mess. Simply pick up the cube and slot it into its destination.

See also  Seedman Weed Seeds

Cons of Rockwool for Growing Cannabis

Just like every other form of growing media, Rockwool cubes also carry some unique disadvantages.

Many cannabis growers and businesses are making the switch to more sustainable and environmentally friendly products and methods. The idea of used Rockwool cubes sitting in a landfill site repels some cultivators. Because many growers still don’t make their own compost, and don’t want to go to the effort of shredding up their cubes, this is exactly where many Rockwool cubes sadly end up.

Rockwool cubes are naturally alkaline. Because cannabis plants prefer slightly acidic soil, growers have to stabilise the pH of their cubes before sowing seeds (you’ll find out how to do this below).

The process of making Rockwool cubes also takes a toll on the environment. In addition to the endless quarrying of basalt rock, it takes a considerable amount of coal energy to heat furnaces and power the machines needed to process the whipped-up lava wool.

Rockwool also poses a potential health hazard to growers. New Rockwool cubes can contain a lot of loose fibres and dust. These particles can end up in the air, and even on your skin and in your eyes, mouth, and lungs. Much like asbestos, tiny fibres can build up in the lungs over time if you work with new Rockwool cubes every day.

These small particles are classed as “fine mineral fibres”, which the Environmental Protection Agency (EPA) in the United States has identified as probable human carcinogens [1] .

Many cannabis growers and businesses are making the switch to more sustainable and environmentally friendly products and methods. The idea of used Rockwool cubes sitting in a landfill site repels some cultivators. Because many growers still don’t make their own compost, and don’t want to go to the effort of shredding up their cubes, this is exactly where many Rockwool cubes sadly end up.

Rockwool cubes are naturally alkaline. Because cannabis plants prefer slightly acidic soil, growers have to stabilise the pH of their cubes before sowing seeds (you’ll find out how to do this below).

The process of making Rockwool cubes also takes a toll on the environment. In addition to the endless quarrying of basalt rock, it takes a considerable amount of coal energy to heat furnaces and power the machines needed to process the whipped-up lava wool.

Rockwool also poses a potential health hazard to growers. New Rockwool cubes can contain a lot of loose fibres and dust. These particles can end up in the air, and even on your skin and in your eyes, mouth, and lungs. Much like asbestos, tiny fibres can build up in the lungs over time if you work with new Rockwool cubes every day.

These small particles are classed as “fine mineral fibres”, which the Environmental Protection Agency (EPA) in the United States has identified as probable human carcinogens [1] .

How To Use Rockwool To Grow Cannabis

Now that you know everything about Rockwool cubes, including their pros and cons, how should you go about using them when growing weed?

Stabilise the pH and Feed Your Blocks

Before you pop your seeds into the cubes, you’ll need to adjust the cubes’ pH to make them less alkaline. This step also gives you a chance to infuse nutrients into your cubes to give your seedlings the best start to life possible.

For this key step you’ll need:

  • pH-down product (optional) (optional)

1. Fill up a shallow bowl with water and test the pH. Add your pH-down product until your meter shows an acidic reading of 5.5.

2. Submerge your blocks into the solution and leave them to soak for 24 hours. After a while, the alkalinity of the blocks will raise the pH of the solution. Return to the blocks once every few hours to test the solution. Re-apply the pH-down product as needed to maintain a reading of 5.5.

3. After 24 hours have passed, fill up another bowl with 1 litre of water. Add your organic nutrient solution, fungi, and bacteria to the bowl in the quantities stated on the instructions. Soak the blocks in the solution for another two hours.

4. Now your blocks have the correct pH, are loaded with nutrients, and harbour beneficial bacteria and fungi that will fuse to the roots and help them thrive later on in the growing cycle.

Germination

Rockwool makes germinating cannabis seeds super easy. Moreover, it works well for hydroponic growers who want to use cuttings.

Cuttings (Hydroponics)

Hydroponic growers often start things off with clones to speed up the growing process. Many soil growers use this method as well. Fortunately, cuttings root extremely well in Rockwool cubes. Use the technique below to get a head start.

1. Take a cutting: Follow the correct procedure to take a cutting from your mother plant.

2. Nourish your cutting: Dip the cut tip of your clone into rooting gel to expose it to rooting hormones, nutrients, and trace elements.

3. Put your clones into cubes: Use a skewer to create a sizeable hole in the top of each Rockwool cube. Inset the stem of your cutting into the hole, gently pushing it so it reaches the bottom half of the block.

4. Water, light, and humidity: Place your cuttings in a tray and place a plastic covering over them to increase the humidity. Use a spray bottle to add moisture to the bottom of the tray. Hang up some high-quality LED lights over the top of the dome on a cycle of 18 hours on and 6 hours off.

Seeds (Soil)

Soil growers often choose to start the process with seeds. But the process below is effective regardless of whether you grow in soil or hydro.

  1. Insert your seeds: Use a wooden skewer to create a small hole in the centre of each soaked block. Place your seed into the hole and use the skewer to gently nudge your seed into the centre of the cube.
  2. Create a humidity trap: Seedlings germinate faster and more successfully in environments with high humidity. If you own a mini greenhouse, just place your cubes into trays and situate them on the shelves. Alternatively, just grab a relatively deep bowl or tray and a roll of cling film. Place your blocks into the container and cover with cling film to trap humidity as the water content escapes from the blocks.
  3. Set up your lights: Set up a high-quality LED grow light above your mini greenhouse, or around 10cm above your bowl or tray. Turn on the lights one day into the germination process. It’ll guide your seedlings to the surface and prevent them from turning leggy. Set your timer to 16 hours on and 8 hours off to help photoperiod seeds get used to their vegetative cycle.
  4. Be patient: Germination can take anywhere from two days to one week depending on genetics and other factors. Check in on your seeds once in a while, but understand that they need time to make the magic happen.

Vegetative Phase

After seedlings have developed their first few sets of true leaves, they enter the vegetative phase. By this time, you’ll notice roots starting to protrude out of the Rockwool blocks. This means they’re ready for a bigger home!

Hydroponics

When your seedlings/cuttings have outgrown their blocks, you’ll need to move them into your hydroponic setup. Before we explain how to do this, here are the best and most suitable systems to use alongside Rockwool blocks.

  • Drip irrigation: As one of the most popular types of hydroponics, these simple systems use low-flow irrigation to save on water. Nutrient solution from the main reservoir is slowly dripped into separate containers to provide food and moisture to individual plants.
  • Deep water culture: In DWC systems, plants have their roots constantly suspended in a nutrient solution. Growers need to place an airstone in the tank to continuously provide the roots with oxygen.
  • Nutrient film technique:NFT uses a shallow stream of nutrient solution that constantly recirculates over the plant’s root system. Through the use of a pump, the nutrient solution leaves the main reservoir tank, flows through the roots in a separate channel, then reenters the main reservoir.

Once you’ve chosen your preferred technique, use the steps below to kick off the vegetative stage:

  1. Grab some net pots: Pace your Rockwool clones into hydroponic net pots. These containers will keep your clones supported while allowing their roots to grow through the gaps and into the tank below.
  2. Apply clay pebbles: Place clay pebbles around and on top of your Rockwool cubes. They’ll work to block out the light and prevent algae from forming, while allowing good aeration.
  3. Start up your system: Flick the switches to your pumps, add your hydroponic nutrients to the reservoir, and get your system circulating.
  4. Turn on the light: Position your light 10cm above the canopy and keep elevating it as your plants develop through the seedling stage into the vegetative stage. During vegetation, maintain a light cycle of at least 18 hours on and 6 hours off.

If you’re growing in soil, you can transplant the Rockwool cubes directly into your new growing medium. Soil growers have a variety of choices when it comes to what to do next. You can transplant into plastic or fabric pots, place your plants into greenhouses or polytunnels, or plant them directly into garden beds.

Some growers like to keep repotting their plants as they grow. However, for the sake of simplicity, we recommend transplanting into a fabric pot and keeping your plants there until harvest. This reduces future transplant stress and keeps plants growing quickly.

Use the steps below to transplant into soil:

  1. Prepare your potting mix: Grab a high-quality potting soil (or your own mature compost) and mix with some organic nutrients, mycorrhizal fungi, and rhizobacteria.
  2. Fill your pots: Load up your enhanced mix into your pots. Compact the soil down and soak with water.
  3. Transplant: Make a hole in the centre of the growing medium large enough for the Rockwool cube and the very bottom of the stem.
  4. Fill it in and water: Replace the dug-up soil and cover up the cube. Water your plant to help it settle into its new home.
  5. Lighting: If you’re growing indoors, maintain a light cycle of at least 18 hours on and 6 hours off during the entire vegetative phase.
  6. Feeding: Feed your plant with veg nutrients high in nitrogen and use the mix every time you water your plants.
  7. Watering: To avoid overwatering, only water your plants once the top 2–3 centimetres of the soil are completely dry.
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Flowering Phase

The flowering phase is very similar between soil and hydroponic grows. Here are the main factors to consider.

Give your plant one last prune before it enters the flowering stage. This will allow light to penetrate the canopy so each bud site can thrive.

If you’re growing a photoperiod plant, you’ll need to switch your light cycle to 12 hours on and 12 hours off to force them to flower. If you’re growing outdoors, nature will take care of this for you. Keep autoflowering varieties on a light cycle of 18 hours on and 6 hours off if growing indoors.

Cannabis plants have different nutritional demands during the flowering stage. Clean out your hydroponic nutrient reservoir and fill it back up with a flowering formula. If you’re growing in soil, start watering your plants with bloom nutrients to ensure big, healthy buds.

Flush your soil or hydro plants one week before harvest. Restrict nutrients and only provide water to remove excess salts from the soil and allow plants to use up the last of their supplies. This will result in tastier and smoother buds.

After harvest, reclaim your cubes! If you plan on reusing them, submerge them in boiling water and give them a clean. If not, shred them up as finely as possible and dump them on the compost pile.

Top Tips for Using Rockwool

Here are some final tips to reference so you can master using Rockwool to grow cannabis.

1. Don’t Overwater

Try and refrain from overwatering your Rockwool cubes during the seedling stage. They drain quite fast, and many growers get the impression that they’re underwatering. However, the inside of the cube remains nice and moist and will keep your seedlings happy without suffocating them.

2. Don’t Squeeze Your Cubes

Some growers attempt to squeeze their cubes like a sponge to remove excess water. Don’t do this! They drain fine on their own, and squeezing them will damage the cube and the root system of your seedlings.

3. Give Them Room

Don’t place your Rockwool cubes too close together when raising seedlings. The material allows for water to drain and for air to flow through. Don’t lose this advantage by overcrowding your trays.

Should You Use Rockwool for Growing Cannabis?

As you can see, Rockwool cubes offer growers plenty of benefits, from improved aeration to pathogen prevention. However, more environmentally concerned growers choose to avoid them because of how they’re made, and what they’re made of.

If you chose to reap the rewards of Rockwool cubes, why not start a compost pile? You’ll create your supply of soil and keep used cubes out of the landfill site. A win-win.

How To Start From Seed With Rockwool Cubes

In this lesson, we will learn how to start your own seed with Rockwool Cubes. Below is a detailed guide to success with Rockwool, and a step-by-step video tutorial can be found at the bottom of the page.

Preparation

Items you will need:

Rockwool Cubes have a PH of roughly 7.8. This is pretty alkaline, yet our plants prefer to grow in a slightly more acidic environment (between 5.5 – 6.5). In order to prepare our Rockwell cubes for the seeds, we need to soak them in some PH adjusted water, that way they have everything the seeds need to germinate and sprout; water and a slightly acidic environment.

Step 1: Hydrate And Stabilize The Rockwool Cubes

Get a bowl or some other container that is big enough to fill with water and have room left for your Rockwool cubes. Your average salad bowl will work fine for 3 Rockwool cubes, if you are planning on doing more than you will need a larger container.

Fill the container/bowl with water from your tap. You may also choose to use water filtered through a britta or reverse osmosis (R/O) water, I’ve had success with all 3 of them so whichever you have on hand will work fine.

Using either a PH test kit or a Ph meter, determine the Ph of the water. Water comes out pretty alkaline, usually around 7.4, so you will need to acidify it a little bit to bring that Ph down to the desired level. Aim for as close to a Ph of 5.5-6 as you can get.

To accomplish this, use either Ph down chemicals, or lime juice (as it’s acidic). Add these to the water in small increments (VERY SMALL), and test the water to see where the Ph is. Continue doing this until you have a Ph of 5.5-6.

Important: Do not let the PH of the water go below 5. A Ph this low will damage the fibers of the Rockwool Cube

Now that we have the Ph adjusted water, it’s time to stabilize and hydrate the Rockwool cubes in it. Insert the Rockwool Cubes into your container and let them soak for roughly 1 hour. Once the hour is up, the cubes will be big and fat with water. Take them out of the bowl of water and put them somewhere you don’t mind getting a little wet. Save the remaining water for step 3.

DO NOT SQUEEZE THEM TO DRAIN ANY WATER

Rockwool Cubes are designed to maintain the correct water to air ratio and squeezing them may damage their structure. Some of the marijuana forums advocate doing everything from squeezing them completely dry, all the way to flinging them around like paintbrushes in order to get excess water out. None of that is necessary, leave it as it is. It will stay moist for several days without needing to be watered this way as well.

On top of that, Rockwool is like asbestos, you don’t want to be squeezing it or breathing it or generally touching it any more than you need to. Here is a good article on some of the health concerns of Rockwool. I use it because it is what works best, but be cognizant to the fact that it is a potentially dangerous substance to be making contact with so don’t do anything more than you need to with it.

Here is what it should look like:

Step 2: Plant Your Seeds

Most Rockwool cubes come with holes in them, if yours did not, than create a hole in one side that is approximately a quarter inch (0.75 cm) deep.

They should look like this:

Take 1-2 seeds and insert them carefully into the holes. Use a toothpick or similar object to push them down to the bottom, as you want them to be at the bottom of that hole. Rip or push a piece of the Rockwool over the hole (you don’t have to fill it completely), so that the seed can germinate in a dark moist environment.

Now, if you can, place them in a tray with a dome on it. This will help create a little humidity in there which seedlings like. This is not mandatory, but it helps. Whichever you choose, take your cubes and put them in a cool dark place, and leave them alone. The temperature should be roughly 68 degrees F, though my house stays at about 72 and they do fine there. I usually place them above my refrigerator and just leave them for a day or two. My lettuce seedlings sprouted with a quickness the last time I tried, and by the 3rdday they had grown so tall that I had to take the plastic dome off of my container because they were bumping up against the ceiling.

Step 3: Leave Them Alone And Let Them Grow

If you put more than one seed in your cube (just in case one didn’t make it), than you probably have several seeds sprouting up in each cube at the end of ~3 days. Once the first true leaves emerge, we want to select for the strongest one (the one that grew the tallest), and cut off the tops of all other seeds that are growing next to it. Do not pluck them out, as you may uproot it’s neighbors. Simply cut it off as close to the hole as you can without messing with the stronger one that you plan on keeping alive.

Depending on how hot it is (and other factors) you may need to water your cubes 1-4 times a day. Use the Ph adjusted water when doing so (that’s why I had you save the leftovers from step 2). If you already threw that water out, go make another batch of Ph adjusted water and keep it in a separate bottle or container for watering. Note: Do not over water, in fact while some say to water 1-4 times a day, I did it only once a day when I got home after work.

Some people claim they use a diluted nutrient solution to water their Rockwool cubes with during germination. Do not do this, as my experience has always been negative. Note the picture below, where I did an experiment by adding a very diluted grow nutrient to the Rockwool cube on the far left. It died within an hour or two, and the others went on to live happy lives. In my opinion, they do not need nutrients until they get into your hydro system.

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Do not add any nutrients to your Rockwool Cubes. The one on the left got nutrients, the other two did not.

Step 4: Transplant Into Their Final Destination

About 2-3 weeks after germinating, you are ready to transplant these babies into the hydroponic system of your choice. A good rule of thumb to go by is that you want to transplant them once the first roots begin poking out of the Rockwool cube. Don’t wait too long though, as eventually the roots will begin tangling around the cube since it is their only source of water. You want to catch them right as they pop out, so that when you transfer them into your hydro system the roots will grow down into the system, and not just try to feed off the Rockwool cube alone.

If you have any questions, suggestions, or comments, please leave them below and I will do my best to answer all of them.

Growing Cannabis In Rockwool

This guide covers the basics of growing cannabis in rockwool, from germination all the way to fabulous, fat flowers. Don’t be intimidated by this tried and true hydroponic substrate.

GROWING CANNABIS IN ROCKWOOL

Rockwool, stone wool, or mineral wool has been the hydroponic substrate of choice for a number of decades. Beginning its life in general hydroponic horticulture, it proved to be an excellent medium for growing all sorts of plants. It was only natural then for existing methods to translate over into the world of weed.

“Rockwool” is one of those trademarks that went on to become the generic term for this type of product. Just like Velcro or Aspirin, which are also trademarks, Rockwool is used to describe most products under this umbrella, and often drops the capital “R”—much to the chagrin of the trademark holders.

Rockwool was first produced commercially by the Danish company Rockwool in the late 1930s. It is an industrial method of mimicking a naturally occurring substance called Pele’s hair [1] , which is produced when wind blows through molten lava.

When it’s produced artificially, air is forced through a stream of molten basalt slag. The resulting “wool” is then pressed into different densities for different purposes. It can be used for sound/thermal insulation, and with its custom density, is ideal for growing plants hydroponically.

The modular nature of rockwool means never having to disturb the root systems of plants. The small cubes neatly fit into the next size cube up and so forth, with the final position usually being a large slab that contains a number of plants. Rockwool is highly absorbent and retains water very well. The loose structure presents virtually no resistance to roots, allowing for very efficient plants.

These top tips for growing in rockwool will make sure your indoor grow is successful.

1. PREPARATION

Rockwool needs to be stabilised and prepared before using it to grow cannabis. Untreated, it has a pH of 7.0, which is too alkaline for healthy cannabis growth, as there are no naturally occurring buffers like with soil. When the substrate is too alkaline, there is poor nutrient uptake, especially of essential compounds like calcium and magnesium.

Soak the rockwool for 24 hours in a nutrient solution corrected to a pH of 4.5 with an EC of 0.4–0.5. This range will make sure that young plants get off to a solid start with no nutrient deficiencies.

To check if the rockwool is stabilised at the correct pH, drench with a 5.5 pH nutrient solution and test the runoff water with a pH meter. If the runoff remains at 5.5, the substrate is ready for use; if it is still too high, re-soak for a few hours and then test again.

2. GERMINATION

Rockwool doesn’t require any specialised germination techniques, and the tried and true paper towel method works just fine. Put seeds between two layers of damp paper towel on a plate and cover with a second plate. Leave in a dark, warm place until seeds split and the taproot emerges.

Gently insert the sprouted seed in a prepared rockwool cube and place under propagation lights for early vegetation. The well-fed sprout will quickly appear, and the cotyledons will shed the seed husk and spread, soon to be followed by the first set of true leaves.

Once young plants are established in their small rockwool cubes, they can be transplanted into the next size of properly prepared cube. Being careful not to damage any visible roots, gently press the small cube into the larger one to ensure a tight fit with no air pockets. Plants are now ready for exposure to full artificial light in a grow room.

3. CLONING AND ROOTING CUTTINGS

The process of making cuttings and rooting them in rockwool is the same as with any other method like jiffies, peat moss, or coir.

Place cuttings into properly prepared rockwool cubes (as above). The cubes should be damp, but not wet so as to avoid stem rot and slow striking speeds.

Keep cuttings in a high humidity environment like a domed tray and regularly mist lightly with water. Once cuttings strike, lower the humidity for a few days, then remove the cover to harden them off before transplanting.

4. GROWING IN ROCKWOOL

An automatic irrigation system is ideal for rockwool. The material is highly absorbent and distributes nutrient solutions evenly throughout the root zone before settling at the bottom. This moisture gradient is what makes rockwool such an ideal substrate for hydroponics.

Hand watering is okay, but be prepared for multiple visits to the grow room per day, and be careful not to overdo it.

Plants sit in a shallow film of nutrient solution 2–3mm deep. Rockwool should be at medium saturation levels in the very lower layers, and drier in the upper layers.

Lower roots feed from the film of nutrient solution, while upper roots are exposed to moist air and all the oxygen they can get for healthy transpiration. Rockwool is not supposed to be wet like a sponge, as this is the equivalent of submerging roots, which will kill the plant.

Rockwool can also be used to hold plants for DWC, and there is little salt buildup since at each irrigation, fresh solution forces old salts out and draws down more fresh air into the medium. At all stages, watch plants for any kinds of nutrient stresses and adjust nutrient solutions accordingly.

5. VEGETATION

The rockwool is stabilised at 5.5 pH and with an EC of 0.4–0.5. During the early stages of vegetation, when plants are still small, irrigate for one minute to apply 60–70ml of nutrient solution twice per day. Once when the lights come on, then again halfway through the light cycle. This facilitates root growth and prevents water logging.

Watch plants for changes in nutrient demand as they grow and increase the amount of solution per plant as they get bigger. Continue this regime until the first week of flowering.

6. FLOWERING

• Week 1

Swap to a flowering nutrient and gradually increase pH over a week from 5.5 to 5.8–5.9.

• Weeks 2 & 3

Increase the EC to 0.8–0.9 and the pH to 6.0. Increase the frequency of watering from twice a day to three times per day. Irrigate for two minutes when the lights come on, then for one minute six hours later, and for one minute prior to turning lights off.

• Weeks 4 & 5

Increase the EC to 1.0–1.2 and fluctuate the pH between 6.0 and 6.2 to ensure complete nutrient uptake. Increase frequency of watering to four times per day. Irrigate for two minutes at the beginning of the day, then for one minute at hours four and eight, then for two minutes again before the lights go off.

• Week 6

Increase EC to 1.4 and keep pH fluctuating between 6.0 and 6.2. Increase frequency of watering to six times per day. Irrigate for two minutes at the beginning and end of the day, and for one minute at regular intervals in between.

• Week 7

EC can be increased from 1.4 up to 1.6–1.7, and fluctuations in pH can rise to between 6.0 to 6.3. Keep the irrigation frequency the same as in week 6.

• Week 8

Continue as per week 7. Depending on specific strain demands, EC can be increased to 1.8.

• Weeks 9 & 10

Depending on strain, it is the last week of flowering for cannabis. Reduce EC to as low as possible and keep pH at 6.2. Keep irrigation frequency the same, but increase to two minutes at every watering.

During the final week is the time to thoroughly flush your cannabis with a flushing product to remove all excess salt buildup from the root zone. Flushing improves the final aroma and flavour of the flowers prior to harvest.

7. CAN ROCKWOOL BE RECYCLED?

Not really. Reusing rockwool again for another grow can introduce pH fluctuation problems that are difficult to control. Moreover, leftover organic matter like roots and leaf fragments decompose and cause further problems.

If you are keen to use your rockwool again, painstakingly remove all organic matter. Then, steam with a commercial steam cleaner or wall paper steamer at 100°C or hotter.

Generally, it is far easier to start new grows with fresh material and get rid of the old stuff. But where? Rockwool doesn’t biodegrade and can’t just go to the landfill, but the Rockwool brand offers technologies for recycling used material.