Rockwool

What rockwool is

Rockwool — stone wool — is molten basalt and limestone spun into fibres and compressed into cubes, blocks or slabs. Grodan's listings describe it exactly that way, and note that Grodan is part of the Rockwool group. It arrives sterile: Speedgrow states its Multigrow stone wool is free of diseases, insects and weeds, and Grodan says the same of its own substrates.

Inert, with no buffering capacity

This is what separates stone wool from coir. Rockwool holds nothing back and gives nothing up — it does not bind calcium out of your solution and does not release potassium into it, so what you mix is what reaches the root. The trade-off is that it has no pH buffering either. Fresh stone wool is not pH neutral and must be conditioned before planting: Growool states its granulated product needs flushing with pH-adjusted water at 5.5 before use, and that the solution should sit at 5.5 too. For a medium that carries its own charge, see coco coir and coco peat.

Our rockwool range, by size

Propagation — 38mm and 40mm

Grodan wrapped 40x40mm cubes come in 12-packs and 30-packs, or as MM40/40 in 2250-cube cartons. Speedgrow's 40x40x40mm cubes come in 8-packs, and its 84-cell plug tray uses 38x40mm plugs with the hole already formed. For volume work, Grodan's SBS 36/77 carton runs 18 trays of 77 cubes.

Transplant blocks — 75mm to 150mm

Grodan's Delta 4G is a 75x75x60mm block in with-hole and no-hole versions, 384 to a carton, plus a 6-pack for smaller runs. Delta 10 steps up to 100x100x100mm — Grodan calls it the four-inch cube, grooved at the base for drainage — and Hugo cubes are 150x150x142mm, 48 to a carton.

Slabs and loose fill

The Grodan Crop Box slab is 370x550x150mm, four to a pack, sized for a 45L crop box crate. Each is wrapped in UV-resistant plastic and built with a horizontal fibre structure, which moves water slowly and suits less frequent watering or hot conditions; Grodan rates it as a two-year slab. Growool also sells a loose 13.6kg bag making up to 122L.

How to choose the right rockwool

1. Start at 40mm. Seeds and cuttings go into a 40mm cube or a 38mm plug tray. A larger block holds too much water around a small root mass.
2. Step up when roots show. Grodan's instruction is to plant on into a larger transplanting cube once roots emerge through the sides — 75mm first, then 100mm.
3. Finish in a big block or on a slab. Grodan states blocks can be placed on top of a slab for a larger root volume.
4. Match the pack to the run. The 6, 8, 12 and 30 packs are home quantities; the 144, 384, 1386 and 2250-cube cartons are commercial.
5. Decide hole or no hole. With-hole cubes take a cutting or a smaller plug straight in; no-hole cubes suit direct sowing or being cut to fit.

Preparing and handling rockwool

Soak before you plant. Fill a tray with pH-adjusted water — Growool specifies 5.5 — and let the cubes take it up for about an hour, then drain before sowing. Grodan's SBS instructions match: make up 3–4L of solution with a root accelerator and pH down, sit the tray in it for an hour, drain five minutes, then place the seed. Afterwards keep it damp rather than waterlogged — stone wool holds a lot of water, and Growool notes it needs watering rarely.

Slabs need preparing too, and Grodan spells out the method: cut holes in the plastic to match your cube size, fill the slab with water, flip it and cut drainage slits across the width of the base, then flip it back and stand your planted cubes on the cut-outs.

Wear gloves and a mask with dry rockwool. Growool states plainly that the fibres irritate skin and, if breathed in, the lungs. It is the one rule that applies to every product here.

New to inert media? The store's guide to pH in a hydroponic system is worth reading first — rockwool leaves no margin for a drifting reservoir. Propagation mediums covers plugs and trays, and the wider grow mediums range has the alternatives.