It's stone. Basalt melted at around 1600 °C then spun like cotton candy, which gives a very light tangle of mineral fibres. Our rockwool starter cubes come pre-drilled with planting holes: simply insert the cutting into the pre-drilled hole, which makes seed starting and plant propagation easy.

Why serious propagators use it: a saturated rockwool cube holds about 80 % water and still keeps around 20 % air. That's exactly the combination a rootless cutting is looking for: it needs constant moisture not to dry out, but a base starved of oxygen rots before it roots. Ordinary potting mix swings between the two extremes depending on watering; rockwool holds both at once. It's also completely sterile out of the bag — no damping-off spores, no weed seeds — and inert, so nutrition is entirely your decision.

The mistake that spoils almost every first attempt: rockwool is naturally alkaline. Used as-is, it pushes the pH around the roots well past what plants tolerate, and the first weeks are disappointing without your knowing why. It has to be soaked before use in water adjusted to a pH of about 5.5 to 6.0, for a good hour, then drained without squeezing. That's the move that makes all the difference, and it's the one nobody mentions on the packaging.

Two other things to know: don't compress the cubes — the air they contain is precisely what you're buying; and wear a mask when handling dry cubes, fine mineral fibres are irritating. Unlike our biodegradable pots, rockwool doesn't break down: it's a propagation medium, not a container to plant in the ground.

• Rockwool starter cubes, pre-drilled with planting holes.
• Holds about 80 % water and 20 % air: the ideal compromise for rooting.
• Sterile and inert: no spores, no weed seeds, nutrition entirely under control.
• Important: soak before use in water at pH 5.5-6.0 for about an hour. Don't squeeze.
• Wear a mask when handling dry cubes.
• Available in several sizes. For: cuttings, propagation, seed starting, hydroponic growing.