Rockwool Slabs




  • The ultimate hydroponic growth media
  • Unparalleled nutrient and pH control
  • Used for intensive commercial agriculture
  • Ideal for re-circulating or run to waste systems
  • Fastest potential of growth of any media
  • Large water buffer relative to volume
  • Easy to use
  • Easy to monitor EC and pH conditions

If you are truly serious about growing with hydroponics, then there should be no shadow of a doubt that the growth media that you should be using is rockwool. Grodan Rockwool slabs are a tried and tested growth media, used throughout the world in large commercial greenhouse settings, where getting the most out of your plants is essential! They offer the grower fine and precise control over the E.C and pH in the root zone, allowing for rapid growth and huge flower sets when properly controlled!

Grodan have been at the forefront of rockwool slab technology since its conception and are one of the leading suppliers of rockwool around the globe. They supply huge commercial greenhouse installations worldwide, being the media of choice for most of the industry. For massive commercial greenhouse ventures like this, maximising yields per square meter and overall profits from a crop is a serious business, and Grodan rockwool slabs are used for exactly this reason.

Ideal for using in either a run-to-waste, or re-circulating drip system the rockwool slabs come in either a 1m or 1.2m length, offering a variety of options for the spacing of your plants in your hydroponic grow room.


Rockwool is the result of harvesting and processing one of Earth’s most abundant sources, volcanic Basalt rock. In the making of rockwool for horticultural practices, the Basalt rock is mixed with chalk and then heated up to 1600 degrees Celsius, where it becomes molten. The molten mix is then spun inside a large spinning chamber, pulling the molten lava into sperate and very fine fibrous strands. It’s a very similar process to making candy floss from sugar, but of course, not quite as nice to eat.

Despite small traces of residual lime/chalk, for all intents and purposes the process of super heating the basalt rock and spinning into rockwool leaves it an entirely inert substance, both chemically and biologically. It results in one of the most efficient growing media, giving the grower a much greater control over variables like E.C, pH and moisture content in the rootzone. This gives the grower the option to steer a plant’s growth to a much greater degree than any other media.


Compared to other media like peat-based soil mixes and coco, rockwool has a good few distinct advantages. Most of them boiling down to the physical nature of the rockwool itself. Firstly, it is comprised mostly of empty space. In fact, roughly 95% of the volume of a rockwool is thin air, only 5% of it is the spun, basalt stone material. From that 95% of space, roughly 80% can be filled up with water, the remaining 15% being left as pockets of air. This 80% of space for the water is called the water holding capacity of a media, and rockwool holds the crown in this out of all growth media.

So, it has a much greater water holding capacity, meaning the plant can get the same amount of water from a much smaller root zone space. Typically, peat mixes have around 15% water holding capacity of their total volume, so in a 10L pot, the plant has roughly 1.5L of water potentially available to play with between each watering. So, to achieve the same available 1.5L of water between each watering with rockwool, you only need a volume of just under 2 litres for your root zone. Further to that, the physical act of taking this water from the substrate per volume also costs the plant relatively less energy to do so than with soil, leaving extra energy for flower production.

Rockwool also has an extremely low Cation Exchange Capacity (CEC). This basically means the rate at which it holds onto nutrients in the substrate is quite low. Compared to peat mixes with a high CEC (that hold nutrients for a long time) rockwool easily exchanges nutrients with the nutrient solution every watering, allowing much greater control for the grower over exactly what is happening in the rootzone. Coupled with being a stable, inert substrate, from the get go, there is no other media that gives you more control over the environment you give the roots.


Grodan rockwool slabs are designed to be used in either a run to waste, or re-circulating drip system. They are compatible with either high, or low-pressure drip systems, to be used with a mineral nutrient feed. They come in 1m or 1.3 metre lengths, giving different options for spacing your plants in a grow room to your liking.

Before you first plant out into Grodan rockwool slabs you need to give them a quick soaking in some pH adjusted water to make sure that the residual chalk/limestone is neutralised, and the pH is stabilised. All this requires is a quick soak in water that has been pH adjusted to roughly 5.0-5.5, for roughly 24 hours. After soaking drain the water away, quickly irrigate the slabs through with a nutrient solution set to the relevant level, for the plants you are transplanting. Usually, an E.C of 1.2-1.4 and a pH of 5.6-5.8 is a good ball park to begin with.

The frequency of the irrigations very much depends on the entire climate of your room, and how quickly your plants are drinking said water. After initially planting on to the slabs, they may not need watering again for another 3+ days, depending on how long they take to sufficiently root into the slab. Once they have rooted you will need to increase the frequency of the irrigations and E.C as the plants needs become greater and greater. Checking the amount of run off you get by time is one way to do this. When start seeing less and less runoff from each irrigation, you know you need to increase the length or frequency of irrigations, depending on circumstance.

Each irrigation you should aim for roughly 10% runoff. You can measure this by either volume of the water, or the time of the irrigation. For example, if you drip feed 2L of solution, you want 200ml as runoff. Or if you are irrigating for 3 minutes (180 seconds), you want the runoff to be dripping for 10% of that time, roughly 18 seconds. Checking the E.C and pH of the run-off on each irrigation will indicate how you need to respond with feed levels for your next irrigation. Alternatively, you can use a syringe to extract water directly from the rootzone shortly after each irrigation, to get a more accurate reading of the E.C and pH in the rootzone.

Responding in changes to each irrigation in terms of volume of water and E.C and pH levels is crucial to success in rockwool. Ultimately you are in control of the plant a lot more when growing hydroponically with rockwool. If you are interested in using rockwool for your substrate or want some more advice on getting the most out of your grow, feel free to give us a ring in store. We are happy to help!


Wear long sleeve t-shirts and gloves when handling. The rockwool fibres can be a slight irritant to your skin.

Try to always propagate in smaller rockwool blocks if you are using rockwool slabs as your final growth media. Keeping the same rooting media throughout the plants entire life will make for a much more efficient root system

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Rockwool Slabs

Rockwool Slabs