Description
Grodan rockwool blocks in 4" / 10cm are the veg-stage block. Rooted cuttings and seedlings transplant into them and grow on to a proper transplantable size before they go into their final pots or into a hydroponic system.
Spun basalt rock, pH neutral and inert, with a pre-formed hole so the cube below drops in and the roots are never touched.
Choose your hole size
- 2.5cm small hole – for 25mm propagation plugs.
- 3.6cm large hole – for the 36mm Grodan propagation cube.
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" | 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" (this product) | 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 Rockwool Blocks 3" – The rung below, if you want a smaller first pot-on before this one.
- Grodan 6" Hugo Block – The next size up, and the one most growers finish in.
- Grodan Improved GR10 4" – Same size, newer wool structure and advanced wetting agent.
- FloraFlex 4" Flora Cap v2.0 – Sits on top of a 4" block to spread the feed evenly and block light, which keeps algae off the wool.
Frequently asked questions
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.
Can I grow a plant to harvest in a 4 inch block?
For small plants and short cycles, yes. For anything that will carry weight you want a 6" Hugo, an 8" Big Daddy or a mini slab under it. The size chart above shows where each one fits.
How much run-off should I aim for?
10 to 30 percent per feed. Rockwool is unforgiving about salt build-up, so run-off is how you keep the EC in the block honest. Browse Grow Room Irrigation Systems for the kit to automate it.
Do they work in flood and drain?
Yes. Blocks sit well in flood and drain systems and in IWS systems. Make sure the tray drains fully between floods so the block is not permanently saturated.
Browse more
Rockwool · Growing Media · Propagation Media · Flood & Drain Systems · Dripper Systems
