Rockwool

What rockwool is

Stone wool, sold here as rockwool, starts as molten basalt and limestone. The melt is spun into fibre and pressed into cubes, blocks or slabs. Grodan's listings describe it that way, and note that Grodan is part of the Rockwool group. It arrives sterile. Speedgrow states Multigrow stone wool is free of diseases, insects and weeds, and Grodan says the same of its substrates.

Inert, with no buffering capacity

Coir and stone wool part company here. Rockwool will not bind calcium out of a solution and will not release potassium into it, so whatever you mix is what reaches the root. The same chemistry means it has no pH buffer. Fresh stone wool is not pH neutral and has to be conditioned before planting: Growool states its granulated product needs flushing with pH-adjusted water at 5.5, 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

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

Transplant blocks — 75mm to 150mm

The Grodan Delta 4G is a 75x75x60mm block, offered with a hole or without, 384 to a carton, with a 6-pack for smaller runs. Delta 10 is the next size, 100x100x100mm. Grodan calls that the four-inch cube and grooves the base so it drains. Hugo cubes measure 150x150x142mm and ship 48 to a carton.

Slabs and loose fill

The Grodan Crop Box slab measures 370x550x150mm and comes four to a pack, sized for a 45L crop box crate. A UV-resistant plastic wrap and a horizontal fibre structure slow the water, so Grodan points this slab at less frequent watering or hot rooms and rates it as a two-year slab. Growool's loose fill is a 13.6kg bag that makes up to 122L.

How to choose the right rockwool

1. Start at 40mm. Put seed and cuttings in 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. Once roots come through the sides, Grodan tells you to plant on into a larger transplanting cube — 75mm first, then 100mm.
3. Finish in a big block or on a slab. Grodan states you can sit blocks on top of a slab when you want a larger root volume.
4. Match the pack to the run. Home quantities are the 6, 8, 12 and 30 packs; commercial runs are the 144, 384, 1386 and 2250-cube cartons.
5. Decide hole or no hole. Use a with-hole cube if you are dropping a cutting or a smaller plug into it; pick no-hole for sowing directly or for cutting the cube to size.

Preparing and handling rockwool

Make up a tray of pH-adjusted water — Growool specifies 5.5 — give the cubes about an hour to take it up, then drain before sowing. Grodan's SBS instructions follow the same shape: 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. Keep it damp rather than waterlogged afterwards. Stone wool holds a lot of water, and Growool notes it needs watering rarely.

Slabs need their own prep. Grodan's 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. That one rule covers every product here.

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