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) 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)being reviewed 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, 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 |
| Root-zone temperature | 18–23 °C (64–73 °F) | Slabs on cold floors stall roots |
How to run it
- 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.
- Put a block on a slab only once roots show through the block bottom; irrigate the block, not the slab, until roots enter.
- Install a VWC/EC probe in a representative block or slab. Without one, weigh.
- Irrigate in shots of 2–6 % of substrate volume; aim for a daily runoff that keeps slab EC in your target band.
- Dispose of slabs responsibly; rockwool is not compostable.
Failure modes
| Problem | Sign | Fix |
|---|---|---|
| Slab EC runaway | Burnt tips, slab EC ≫ input | More runoff shots or a flush shot |
| Water-logged | Slow growth, brown roots, 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 and resin. Feed through to harvest at stage strength. The only valid flush is a root-zone flush to clear salt build-up, at any stage (see the flush debate). 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 flaggedbeing reviewed
