Rockwool (also known as “mineral wool”) is a fibrous compound made from molten rock spun into cotton candy-like fibres. It is very widely used in hydroponics.

Rockwool is sterile, inert, has low pH, it is light when dry, has excellent water retention and air circulation properties and it is not biodegradable. It is available as compressed slabs, cubes and in granulated form.

Very few orchid growers are using it. I have seen once or twice orchid seedlings growing in little rockwool cubes.

BE AWARE: Home insulation grade rockwool can be toxic. As I understand it, rock wool is classified as a possible carcinogen. If you must use it, use horticultural grade, it suppose to be safer.

Also be aware, it is prone to water logging.