Description
Grodan rockwool propagation cubes are where a rockwool grow starts. Spun basalt rock, pre-holed, pH neutral and completely inert, so a seed or a fresh cutting gets air and moisture in the right balance and nothing else competing with it.
Sold as a full tray of cubes. Once roots show through the sides, the whole cube drops straight into the hole in a Grodan block without ever touching the roots.
Sizes and what they fit
- 36mm (1.5") cube, 77 per tray – 3.6 x 3.6 x 4cm. Drops into any Grodan block with the large 3.6cm hole, and into the 36mm hole in the 6" Hugo Block.
- 25mm (1") cube, 150 per tray – more cuttings per tray, suits the small 2.5cm hole blocks.
Upgrade the tray they sit in
The 77-cube 36mm sheet is sized exactly for the Grodan Gro-Smart Tray Insert 78 Cells. Pop the sheet straight into the 78-cell side. It is a reusable, dishwasher-safe Dutch tray you sanitise and run cycle after cycle, and the rounded mesh cell walls lift each cube clear of standing water so young roots get airflow instead of sitting wet. The 150-cube 25mm sheet is a size down from those cells and will not seat as snugly.
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 (this product) | 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" | 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
- Clonex Rooting Gel – Seals the cut and feeds the wound site. The standard first step before a cutting goes into a cube.
- Grodan Gro-Smart Tray Insert 78 Cells – Reusable mesh tray sized for the 77-cube sheet, better airflow than the shipping tray.
- Grodan Rockwool Blocks 3" – The next rung up. Rooted cubes drop straight into the hole.
- Grodan 6" Hugo Block – If you are skipping the middle sizes, the 36mm cube goes directly into the Hugo.
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.
Which cube size should I buy?
If you are potting on into Grodan large-hole blocks or a 6" Hugo, take the 36mm. If you are running small-hole blocks or want the most cuttings per tray, take the 25mm. Both are shown in the size chart above.
When do I move them on?
When white roots are visible through the sides or base of the cube, usually 7 to 14 days on a cutting. Move into a 3" block or 4" block, or straight into a 6" Hugo. Do not wait for roots to circle and matt.
What do I feed at this stage?
Very little. Keep EC under 1.0 and pH at 5.5 to 5.8. Browse Propagation Nutrients & Cloning Products for the mild propagation feeds and root stimulants that suit cubes.
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Rockwool · Propagation Media · Propagators · Propagation Nutrients & Cloning · Growing Media
