Description
The Grodan Gro-Block Improved GR4 3" with hole is the current-generation 3 inch rockwool block. 3 x 3 x 2.5 inches, shrink-wrapped in strips, with a pre-formed hole for a propagation cube.
UK and EU stock.
What "Improved" actually changes
Grodan worked with Wageningen University in the Netherlands and with commercial growers in North America to re-engineer the wool structure and the wetting agent. Three things come out of it that you can feel in the room:
- Faster rooting-in – the optimised fibre structure lets roots take hold sooner after transplant.
- Even water and EC through the whole block – not just across the block but over its height, which is what makes crop steering predictable rather than a guess.
- Holds its shape – structure, firmness and rigidity stay put across the cycle instead of slumping once it has been saturated and dried a few times.
The advanced wetting agent and hydrophilic treatment are the reason it re-saturates so evenly, which in turn means less water and less nutrient wasted getting the block back to where you want it.
Wetting instructions
The Improved blocks have their own preparation procedure, which is not the same as the 24 hour soak used on standard rockwool. Grodan's wetting instructions for Improved Gro-Blocks (PDF). Follow those rather than the generic advice.
Improved GR4 or the standard 3 inch block?
| Improved GR4 3" (this product) | Standard Grodan 3" Block | |
|---|---|---|
| Size | 3 x 3 x 2.5" | 7.5cm |
| Hole | Pre-formed, one size | Choose 2.5cm or 3.6cm |
| Wetting agent | Grodan Advanced | Standard |
| Preparation | Follow Grodan's Improved wetting PDF | 24 hour soak at pH 5.5 |
| Best for | Crop steering, tight EC control | Straightforward propagation, hole choice |
Which rockwool size do I need?
Rockwool works as a ladder. You start small and pot on into the next size up, each time without disturbing the roots. Here is the full range we stock and where each one sits.
| Product | Size | Where it fits | Takes |
|---|---|---|---|
| Grodan Propagation Cubes | 25mm or 36mm | Seed and cutting | Bare cutting or seed |
| Cultiwool Loose Mini Cubes | 10mm, 90L or 200L | Loose fill, any pot | Fills pots and trays |
| Cultiwool Mapito | 80L bag | Loose fill, any pot | Fills pots and trays |
| Grodan Block 3" | 7.5cm | First pot-on | 2.5cm or 3.6cm hole |
| Grodan Improved GR4 3" (this product) | 3 x 3 x 2.5" | First pot-on | Pre-formed hole |
| Grodan Block 4" | 10cm | Veg | 2.5cm or 3.6cm hole |
| Grodan Improved GR10 4" | 4 x 4 x 3.1" | Veg | Pre-formed hole |
| Grodan 6" Hugo Block | 150 x 150 x 140mm | Veg to harvest | 36mm hole |
| Grodan 6" Hugo, dripper hole | 150 x 150 x 140mm | Veg to harvest | 36mm hole plus dripper hole |
| Grodan Improved Hugo 6" | 6 x 6 x 5.8" | Veg to harvest | Pre-formed hole |
| Cultiwool 8" Big Daddy Block | 20 x 20 x 19.5cm | Full cycle, large plants | Takes an 8" cap |
| Cultiwool 25cm X-Fibre Mini Slab | 25.2 x 19.5 x 7.5cm | Under a block, full cycle | One 6" or 8" block sits on top |
| Grodan Vital Slab 1m | 100 x 15 x 7.5cm | Multi-plant, full cycle | Three to four blocks |
| Grodan Vital Slab 1.3m | 130 x 15 x 7.5cm | Multi-plant, full cycle | Four to five blocks |
Sizes are nominal. Blocks are sold as singles or in strips depending on the line.
Goes well with
- Grodan Propagation Cubes – The rung below. Root the cutting here first, then drop the cube into this block.
- Grodan Improved GR10 4" – The next size up in the same Improved range, so the wool behaves the same way.
- Grodan Improved Hugo 6" – Where the ladder ends if you stay on Improved throughout.
- Clonex Rooting Gel – Used a step earlier, at the cut, before the cutting goes in a cube.
Frequently asked questions
Do I prepare these the same as normal rockwool?
No. The Improved range has its own wetting procedure because of the advanced wetting agent. Use Grodan's Improved Gro-Block instructions, not the generic 24 hour soak.
What does GR4 mean?
It is Grodan's code for the small Improved block. GR4 is the 3 inch, GR10 is the 4 inch. The number is the Grodan range code, not the size in centimetres.
Is it worth paying more than the standard block?
If you are crop steering and watching EC and water content closely, yes, because the even distribution through the block is what you are paying for. If you are simply growing cuttings on to a transplant size, the standard 3" block does that job and lets you pick your hole size.
What feed should I run?
pH 5.5 to 5.8 and EC stepped gently up from around 1.0 at this stage. See Base Nutrients and pH & EC Meters.
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Rockwool · Propagation Media · Growing Media · Base Nutrients · Dripper Systems
