Description
Grodan rockwool blocks in 3" / 7.5cm are the first pot-on from a propagation cube. Spun basalt rock, inert and pH neutral, with a pre-formed hole so a rooted cube drops in without the roots being handled at all.
This is the size most growers use to take a cutting from rooted to transplant-ready. Big enough to build a real root ball, small enough that it will not sit soaking wet while the plant is still small.
Choose your hole size
- 2.5cm small hole – for 25mm propagation plugs and smaller starter cubes.
- 3.6cm large hole – for the 36mm Grodan propagation cube and similar sized plugs.
Why growers use them
- Clean and inert – no pests, no pathogens, no background nutrient. You control the whole feed.
- Air to water balance – holds moisture while keeping oxygen at the root zone, so overwatering is harder to do.
- Uniform structure – even root distribution and even EC through the block.
- Works with everything – hand watering, dripper systems, flood and drain, NFT.
Before you use them
Rockwool leaves the factory with a naturally high pH, so it has to be conditioned before anything goes in it. Soak for 24 hours in a weak nutrient solution under 1.0 EC, adjusted to pH 5.5. A small amount of root stimulant in the soak water lifts the EC and gets roots moving faster. Let it drain until damp rather than dripping. Skip this and you will see the first week of growth stall.
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" (this product) | 7.5cm | First pot-on | 2.5cm or 3.6cm hole |
| Grodan Improved GR4 3" | 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 Rockwool Blocks 4" – The next size up when the roots reach the sides.
- Grodan Improved GR4 3" – Same size, newer wool structure and advanced wetting agent. Roots in faster and holds firmer.
- Clonex Rooting Gel – Used a step earlier, at the cut, before the cutting goes in a cube.
Frequently asked questions
Small hole or large hole?
Match it to what you are transplanting. 25mm plugs need the 2.5cm hole, 36mm cubes need the 3.6cm hole. A cube rattling around in an oversized hole dries out at the join and stalls.
Do I have to soak them first?
Rockwool leaves the factory with a naturally high pH, so it has to be conditioned before anything goes in it. Soak for 24 hours in a weak nutrient solution under 1.0 EC, adjusted to pH 5.5. A small amount of root stimulant in the soak water lifts the EC and gets roots moving faster. Let it drain until damp rather than dripping. Skip this and you will see the first week of growth stall.
When do I move up to a 4 inch?
When roots are showing at the sides and base, usually two to three weeks. Move into a 4" block, or jump straight to a 6" Hugo if you are running one plant to a block through to harvest.
What pH and EC should I run?
pH 5.5 to 5.8 throughout, and EC stepped up gently from around 1.0. A reliable meter matters more in rockwool than in soil because there is no buffer. See pH & EC Meters and Base Nutrients.
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Rockwool · Propagation Media · Growing Media · Base Nutrients · Dripper Systems
