# Growing in rockwool (stonewool cubes and slabs)

> Rockwool is an inert, unbuffered mineral fibre that holds a lot of water and air and gives precise control over the root zone. Pre-soak at pH about 5.5, run pH 5.5–6.0 and EC 1.8–3.0 mS/cm in flower, and manage it by volumetric water content and slab EC. It is the native medium of crop steering, and it punishes neglect quickly because nothing buffers mistakes.

- Canonical: https://genetixbay.com/knowledge/systems/rockwool.html
- Category: Systems · Level: Advanced
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Any stage · Systems: Rockwool
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**Do I need to pre-soak rockwool?** Rockwool does need pre-soaking, because fresh stonewool is slightly alkaline and releases [alkalinity](https://genetixbay.com/knowledge/glossary/alkalinity.md) on first wetting. Soak it for 12–24 hours in solution at pH about 5.5 and EC 0.5–1.0 [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md), then let it drain to [field capacity](https://genetixbay.com/knowledge/glossary/field-capacity.md) before planting. Skipping the soak and starting [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) at high pH is a common mistake.

**What pH for rockwool?** Rockwool runs best at an input pH of 5.5–6.0, at the lower end of the accepted 5.5–6.5 soilless range, after a pre-soak at about pH 5.5. Because rockwool is inert and unbuffered, it does not absorb pH errors the way soil does, so a wrong pH shows up within a day rather than over weeks.

**How do I know when to water rockwool slabs?** Water rockwool slabs by measuring them, not by feel: install a moisture and EC probe in a representative block or slab, or weigh it. Irrigate in shots of 2–6% of substrate volume, aiming for daily runoff that keeps slab EC within input plus 1.5 mS/cm. [Overnight dryback](https://genetixbay.com/knowledge/glossary/dryback.md) ranges from 10–15% VWC points for a vegetative steer to 20–35% for generative.

**Rockwool vs coco for cannabis?** Rockwool gives the most precise root-zone control of any medium and is where crop steering developed, but it is inert and unbuffered, so mistakes show within a day. Coco is more forgiving, though it needs buffering for calcium and magnesium. Rockwool runs at pH 5.5–6.0 against coco's 5.8–6.2, and rockwool is not compostable while coco can be rinsed and reused.

**Is rockwool dust dangerous?** Rockwool fibres and dust irritate skin, eyes and airways, so handle dry stonewool with care. Wet the material before cutting it, and wear gloves, long sleeves and a dust mask rated FFP2 or N95 when working with dry wool. Rockwool also isn't compostable, so dispose of used slabs responsibly rather than mixing them into garden soil.

**Short answer.** Rockwool is spun mineral fibre: sterile, inert, unbuffered, with high water-holding and air capacity. Pre-soak at **pH ~5.5**, run **pH 5.5–6.0**, input **EC 1.2–1.8 mS/cm in veg and 1.8–3.0 in flower** (Jardin default bands), and manage it by **VWC (volumetric [water content](https://genetixbay.com/knowledge/glossary/moisture-content.md)) and slab EC** [3]. It gives the most precise root-zone control of any medium and is where crop steering was developed, but it has no buffer: a mistake shows within a day.

## How it works

Fresh rockwool is slightly alkaline (it releases some alkalinity on first wetting), which is why growers pre-soak at pH ~5.5 [1][2]. After that it contributes nothing chemically. Its water content after drainage is high (product-dependent) with a stable air fraction, so roots can sit at high moisture without suffocating, as long as the slab drains.

Every irrigation replaces part of the slab solution. If you water little, slab EC climbs (deliberately, in generative steering); if you water a lot, slab EC approaches input EC.

## Numbers that matter

| Parameter | Target | Notes |
|---|---|---|
| Pre-soak | pH 5.5, EC 0.5–1.0, 12–24 h | Then let drain to field capacity |
| Input pH | 5.5–6.0 | Jardin rockwool band [3] |
| [Input EC](https://genetixbay.com/knowledge/glossary/input-ec.md), veg | 1.2–2.0 mS/cm | |
| Input EC, flower | 1.8–3.0 mS/cm | Steering range 1.8–3.2 [3] |
| Slab EC | Input +0 to +1.5 | Above input +1.5 Jardin triggers a flush shot [3] |
| Overnight dryback | 10–15 % (vegetative) to 20–35 % (generative) VWC points | See [crop steering](https://genetixbay.com/knowledge/systems/drain-to-waste-crop-steering.md) |
| Root-zone temperature | 18–23 °C (64–73 °F) | Slabs on cold floors stall roots |

## How to run it

1. Cube (2.5–4 cm) for seeds/clones → block (10 cm) → slab or larger block. Don't let small cubes dry out; don't keep them soaking either.
2. Put a block on a slab only once roots show through the block bottom; irrigate the block, not the slab, until roots enter.
3. Install a VWC/EC probe in a representative block or slab. Without one, weigh.
4. Irrigate in shots of 2–6 % of substrate volume; aim for a daily runoff that keeps slab EC in your target band.
5. Dispose of slabs responsibly; rockwool is not compostable.

## Failure modes

| Problem | Sign | Fix |
|---|---|---|
| Slab EC runaway | [Burnt tips](https://genetixbay.com/knowledge/glossary/nutrient-burn.md), slab EC ≫ input | More runoff shots or a flush shot |
| Water-logged | Slow growth, [brown roots](https://genetixbay.com/knowledge/glossary/root-rot.md), algae | Fewer/smaller shots, check drainage slit, raise slab off floor |
| Dry spots | Probe reads fine but plants wilt | Probe placement; drip emitter clogged |

## Jardin notes

Jardin medium "Rockwool" (with any drip method): dryback + slab EC loop; VWC/EC probe strongly recommended; P3 dryback matters more than in coco. Time-to-harm on pump failure: 24–48 h. Automation level 3 [3].

## Safety

Fibres irritate skin, eyes and airways. Wet the material before cutting, wear gloves, long sleeves and a dust mask (FFP2/N95) when handling dry wool.

## Common mistakes

- **Flushing before harvest.** Pre-harvest flushing is a *Myth* that costs you yield and resin: it starves the plant while it packs on its final weight, [trichomes](https://genetixbay.com/knowledge/glossary/trichome.md) and resin. Feed through to harvest at stage strength. The only valid flush is a [root-zone flush](https://genetixbay.com/knowledge/glossary/root-zone-flush.md) to clear [salt build-up](https://genetixbay.com/knowledge/glossary/salt-build-up.md), at any stage (see [the flush debate](https://genetixbay.com/knowledge/harvest-cure/flush-debate.md)). In rockwool, a flush shot when slab EC runs above input + 1.5 is that root-zone flush.
- Using it without measurement, like soil.
- Not pre-soaking and starting seedlings at high pH.
- Keeping propagation cubes soaked.
- Copying slab EC numbers from vegetable greenhouses without adjusting for cannabis.

## Evidence

Material behaviour is *Established* horticultural knowledge [1][2]. The pH band (soilless 5.5–6.5, run 5.5–6.0 in rockwool) is accepted practice from extension and grower consensus. Cannabis-specific EC and dryback bands are *Grower practice*; Jardin's ranges are flagged

## Sources

1. Raviv M, Lieth JH, Bar-Tal A (eds), 2019. Soilless Culture: Theory and Practice, 2nd ed. Elsevier (mineral wool chapter). https://doi.org/10.1016/C2015-0-01807-5
2. Resh HM, 2022. Hydroponic Food Production, 8th ed. CRC Press. https://doi.org/10.1201/9781003133254
3. GenetixBay Jardin engineering notes, 2026 (system profiles and failure margins).