Heat-treat a hydrated mineral — mica — and it opens into the soft, accordion-shaped pellets sold as horticultural vermiculite. Treat it as perlite's counterpart, not a substitute: perlite is there for air and drainage, and vermiculite is there to hold water and nutrients and give them back to the roots as the medium dries.
That is why growers use it at the start of a plant's life. It raises seed, strikes cuttings, and stops a potting mix drying out between waterings, and it is sterile, rot-resistant and roughly pH neutral.
Need drainage and air instead? Use perlite, or compare the full set of grow mediums.
