Grodan Rockwool Blocks

Size: 3" Block 1" Hole
Sale price£0.45


The original hydroponic growing medium and still the most widely used in commercial applications. Rockwool blocks are an inexpensive and mess free solution, perfect for NFT systems. They are an inert substrate with great water and air holding properties which allow you full control over your plants nutrition.

Rockwool Blocks come in either 3, 4 or 6 inch square versions, with either small or large holes. The smaller size hole is used to transplant the 25mm Rockwool Propagation Cubes and the large size hole is used for the 36mm Rockwool Propagation Cubes, Jiffy's and Root Riots. The size of block you need sometimes does not necessarily make a significant difference, but there are of course more suitable applications than others depending on the system that you are using. Please feel free to give us a call to see what would be most appropriate for you.

Using rockwool as a hydroponic substrate gives you fantastic advantages over traditional growing media for your indoor garden. Firstly, the water holding capacity is much greater, meaning liter for liter it can hold substantially more water than coco coir or soil. It also releases this water much easier than soil or coco does, meaning the plant does not use as much energy obtaining the water from the substrate. This means you can use a lot smaller volume of media, and your plant has more energy available for vegetative or generative growth above ground. Maximum efficiency in almost every way!

As rockwool is inert and contains no nutrients you have complete control over what your plants are fed. This also means that you need to be a little more careful with your pH and EC levels as whatever you give your plants is what they will get! There is no buffer or ‘forgiveness’ you would get from soil or coco. Fortunately you can also monitor these levels with far greater accuracy than with other mediums. Being a stable and inert substrate the EC and pH level found in your hydroponic system tank will also be that which is in the rockwool block. Some people even choose to stick a syringe into the block itself and extract a sample of water! It is that easy!

How To Use

Before you begin using these blocks it is important to treat and prepare them. All you need to do is soak them in a low pH (pH 4.5-5) solution, some growers choose to do this for up to 24 hours but many will do a quicker soak. This is to neutralize the residual lime from the manufacturing process and ensure your start off with a more stable and natural pH level for growing in. After this pH bath, soak them again in some premixed nutrient solution to your required EC and pH levels and you are good to go!

After you have soaked your cubes, they will be heavily saturated with water. You need to drain some of this water so you do not overwhelm a young root system when you transplant. DO NOT be tempted to squeeze the water out of the blocks. This will damage the fibrous structure of the block and potentially ruin its performance. Preferably leave them to drain naturally but if you are in a hurry you can gently whip/flick them to remove the excess water. Aim to reduce the water content by roughly 25% for best results.


Always ensure your plants are well rooted within the block before transplanting to your hydroponic system. For best results root your plants outside of the hydroponic system and off of any flat surfaces to ensure optimal air flow around them to help air prune the developing roots. This can be achieved by placing them in a Garland Tray or Saucer with Clay Pebbles in the bottom. This also ensures they are not sitting in water. Leave the plastic sleeves on the blocks during this stage to help prevent light getting to the roots but remember to remove them when you plant out!

If moving to an NFT system ensure the blocks are placed the right way round! Underneath the block you will notice ‘channels’ running along them. These should be positioned in the same way as the water is flowing on your system.

Where possible, feel the weight of each block after each watering. As a general rule do not irrigate again until it feels at least 50% of its starting weight.

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