When you think of warm, fluffy wool, rocks are probably the furthest thing from your mind. But alongside the many eccentric inventions of the 70s came rockwool: an impressive human-made substance made by melting and spinning rocks into wool and capturing some of the benefits of both.
It’s a long-time staple in hydroponic gardening that has attracted plenty of criticism due to its unsustainable and potentially harmful attributes. Nonetheless, Rockwool is still widely used and has an important place in hydroponic history. So, whether you choose these tough cubes as your growing medium or opt for one of the many modern alternatives, they’re worth understanding!
What are Rockwool Cubes?
Rockwool cubes have been a popular hydroponic and aquaponic growing medium for decades. These petite cubes are favored for their excellent water-to-air ratio and availability in a wide range of sizes to suit any system. They are particularly useful for germinating seeds and cloning (or propagating) new plants from cuttings.
What are Rockwool Cubes Made of?
Rockwool is made from basalt rock and chalk. The ingredients are melted at an extremely high temperature to become lava. Manufacturers then put the lava into a rapidly rotating chamber to spin it into thin fibers reminiscent of fiberglass or cotton candy.
Next, the fibers are mixed with a binding agent and pressed into blocks. From there, they can be cut into cubes and slabs, or ground down into granules. Slabs are commonly used as home insulation, and granules are useful for filling planters, but for hydroponics and aquaponics, cubes are the go-to choice.
The Pros and Cons of Rockwool Cubes
Are rockwool cubes the right growing medium for your hydroponic garden? It depends. Rockwool grow cubes offer many benefits to hydroponic plant roots, but they also have a large environmental footprint and could be a health concern.
As with any growing medium, you can weigh the advantages and disadvantages for yourself:
The Pros of Rockwool Cubes
Rockwool has been a popular hydroponic and aquaponic growing medium since it was developed in Denmark in the 1970s. The benefits of rockwool include that it is:
- Sterile: rockwool comes free of weeds, pests, and pathogens, so you don’t have to worry about any unwanted guests damaging your plants.
- Light Weight but Stable: being light weight makes it easy to handle and move around, but once it absorbs water it’s still heavy enough to solidly anchor your plants in the system.
- Water Retentive: rockwool can hold a lot of water, keeping plant roots well hydrated.
- Well-Draining: it also holds 18-25% air, keeping roots well oxygenated and preventing root rot.
- Easy to Transplant: rockwool will hold its shape, making it easy to pick up and move plants from one area or system to another.
- Durable and Reusable: rockwool doesn’t break down, so can be used repeatedly until it becomes clogged with roots. You will need to sterilize it by steaming or boiling it between uses to prevent transmitting diseases to your next crop.
The Cons of Rockwool Cubes
With so many hydroponic growing mediums to choose from, the downside of rockwool grow cubes outweigh the benefits for many gardeners. You should be aware of all the cons before you use rockwool in your garden. Rockwool is:
- A Potential Health Hazard: because it is made of rocks, the fibers and dust can be hazardous to any humans who breathe them in. It can also be irritating to your skin and eyes. Wearing a dust mask, goggles, and gloves can help protect you against this.
- Not a Sustainable Product: the production of rockwool uses non-renewable resources, chalk and rock. It also demands a significant amount of energy and transforms the ingredients into a hybrid material that will never break back down into its components.
- Bulky: rockwool cannot be compressed for transport like another popular growing medium, coco coir, so this adds to transportation costs.
- High pH: the high (more basic) pH of rockwool requires you to adjust your nutrient solutions to compensate, and you’ll need to carefully monitor the pH levels of your system for the fluctuations that it’s prone to. This can add a lot of extra work to your gardening schedule.
- Not Biodegradable: because it is made of rock, rockwool will not degrade when discarded in a landfill. It can, however, be reused at least a few times, then broken down and incorporated into soil mixtures.
Alternatives to Rockwool Cubes
If you’d rather avoid using rockwool cubes in your garden, there are plenty of alternatives to choose from, including:
- Coco coir
- Peat moss
- Expanded clay pellets
- Perlite
- Growstones
- Rice Hulls
- Pumice
- Sand
- Gravel
- Wood fiber
- Brick shards
- Vermiculite
How to Choose the Right Rockwool Cubes for Your System
Choosing the right sized rockwool grow cubes for your hydroponic or aquaponic system is important to ensure you give your plants the right amount of insulation and room to grow. The wrong size can clog up your system and cause stunted growth by crowding your plants.
When shopping for rockwool cubes, here’s what to look for:
Factors to Consider
When choosing rockwool grow cubes for your system, there are a few things to keep in mind:
- The size of your system – you’ll need to make sure the size of the cube you use fits your system. In general, smaller cubes are best for smaller systems, and larger cubes for larger ones.
- What you’re growing – smaller plants like herbs can be grown in smaller cubes because they require less space between them, whereas larger plants like tomatoes will need more room to grow and therefore do best in larger cubes.
- The style of your hydroponic system – ebb and flow systems work best with smaller rockwool cubes, whereas drip systems work best with medium or large cubes.
- Your climate – if you live in a warmer climate, larger rockwool cubes will provide your plants with the insulation they need to keep cool. You can also choose rockwool with higher levels of insulation to aid with this.
Rockwool Cube Sizes
Rockwool cubes come in a range of sizes suited to different uses and hydroponic systems. They’re generally sized as:
- Small (around 1” x 1” x 1 ½”) – best for small hydroponics systems and growing small plants like herbs and lettuce.
- Medium (around 1 ½” x 1 ½” x 1 ½”) – best for drip irrigation and growing larger plants like tomatoes and peppers.
- Large (around 2” x 2” x 1 ½”) – best for plants that need plenty of space, such as cucumber and squash.
- Extra-Large (3” to 4” square) – best for big hydroponic systems and growing very large plants that need lots of space.
You might also see rockwool cubes classified as mini, briquette, and large.
How to Prepare Rockwool Cubes for Use
Rockwool cubes arrive completely dry, so you’ll need to prepare them for use by soaking them thoroughly. It’s also important to neutralize their naturally high pH during this process. The high pH comes from lime that is deposited between the fibers during manufacturing – lime neutralizes acidity, raising the pH of the rockwool to a level unsuitable for growing plants. By soaking your rockwool cubes, you can both hydrate them and lower the pH to between 5.5 and 7.0, the ideal range for most plants.
Preparing your rockwool cubes will take a few days, so plan accordingly when starting a new garden or adding new cubes to your system. You’ll need to:
- Fill a bucket with water – pure, distilled water is best because it’s completely neutral, but you can also use tap water.
- Add your growing nutrient solution.
- Adjust the pH until you reach the pH you want for the plants you plant to grow.
- Soak the cubes in the water for 24 hours. They will float at first, then sink to the bottom as they absorb water.
- Remove the cubes and test the pH of the runoff. It should be between 5.5 and 7.0. If not, return the cubes to the water and soak for another 24 hours, then test again.
- Add your rockwool cubes to your hydroponic system and run it without plants in it. Monitor the pH until it stays stable, which indicates that they are ready to use.
How to Use Rockwood Cubes to Sprout Seeds
Once you’ve prepared your rockwool grow cubes by soaking them, you’re ready to use them to germinate seeds in your hydroponic or aquaponic system. To do this:
- Insert a seed into the hole in the rockwool cube. Press it into the bottom of the hole using a toothpick or similar tool.
- Pinch the hole closed.
- Set the cube in a tray or directly into your hydroponic system. Cover it with a humidity dome or grow tent to create a moist environment. You should keep your cube damp and maintain a temperature between 70 and 80 degrees Fahrenheit until the seed sprouts.
- Remove the humidity dome and move your cube under grow lights when your seed sprouts.
- Watch its growth. Your seedling is ready to transplant when it has grown 2-3” high r, if you sprouted it in your main system, leave it where it is and watch it grow to maturity!
How to Use Rockwood Cubes to Clone Cuttings
Cloning or propagation from cuttings is a popular way to grow new plants in a hydroponic system. To do this:
- Water the mother plant the night before you plan to take the cutting.
- Take a cutting from the mother plant. Choose a healthy section with fresh new growth, and cut it as close to the main stem as possible. Your cutting should be 3-4” long.
- Dip the cutting in rooting hormone if you wish. This can help improve your chance of success by stimulating root growth, but it is not necessary.
- Plant the cutting in a rockwool cube. Ensure it does not poke out the bottom.
- Fill a tray halfway with perlite or vermiculite and place the rockwool cube on top. Alternatively, you can place the rockwool cube directly into your main hydroponic system.
- Cover the plant with a humidity dome or tent to maintain a humid environment. You should maintain a temperature close to 80 degrees Fahrenheit.
- When roots emerge, raise the humidity dome slightly to allow more air flow. Gradually raise it more each day to expose the plant to the outside environment slowly.
- Remove the humidity dome after a few days of this.
- Watch for roots. Your clone is ready to transplant when you see roots poking out the bottom of the rockwool cube. If you started it in your main system, you can leave it where it is and watch it grow to maturity!
Quick Tips for Using Rockwool Cubes Successfully
Rockwool cubes can be an effective hydroponic growing medium if you prepare and handle them well. For the best results when growing in rockwool cubes, always remember to:
- Prepare the rockwool cubes well to ensure they are fully hydrated and have a suitable pH.
- Wear protective gear to protect your skin, lungs, and eyes from dust and fibers.
- Handle wet cubes gently so you don’t destroy the internal structure that provides benefits to plants. If you need to remove water from your rockwool cubes, it’s better to gently shake them than to squeeze them.
- Provide adequate nutrition to your plants. Rockwool is inert, so won’t provide your plants with any of the nutrients they need to grow.
- Sterilize any rockwool cubes you plan to reuse. You can steam or boil them to prevent mold, bacteria, and disease spreading to your new plants.
- Dispose of spent rockwool cubes responsibly. They will not break down in your compost or landfill, so it’s best to reuse them as many times as possible then break them into pieces you can incorporate into your garden.
Join the Ponics Life Club
If you’d like access to more helpful hydroponic guides, join the Ponics Life Club! We’ll notify you every time a new guide is available, as well as keep you in the loop about resources, webinars, and exclusive products just for club members!
Sign up on the blog today.
