Gardening glossary
Vermiculite
Vermiculite is a naturally occurring silicate mineral — usually mined as a form of mica — that has been heated until the layers expand into accordion-like flakes. The resulting product is lightweight, spongy, and capable of absorbing 3-4 times its weight in water along with dissolved nutrients.
In a potting mix, vermiculite acts as the moisture reservoir. The flakes hold water and slowly release it back to roots, which is exactly what you want for seeds, cuttings, and moisture-loving plants. It also has a slight cation exchange capacity, meaning it grips onto positively charged nutrients (potassium, magnesium, calcium, ammonium) and feeds them back to roots gradually.
Best uses: - Seed-starting trays: a 50/50 vermiculite and peat (or coco coir) mix keeps tiny seeds evenly moist without crusting over. - Tropical houseplants and ferns that hate drying out: 10-20% vermiculite in the mix. - Lettuce, spinach, and cool-season greens that prefer steady moisture. - A thin top layer over freshly sown seeds — it holds moisture against the seed coat and is light enough for sprouts to push through.
Where to skip it: succulents, cacti, snake plants, and any rosette plants prone to crown rot. Vermiculite holds too much water for these. Use perlite or coarse sand instead.
Two practical notes. Vermiculite is pH-neutral and chemically inert (modern horticultural-grade vermiculite is asbestos-free — older deposits from the Libby mine, closed in 1990, were the concern). And because the flakes are soft, vermiculite eventually crushes down in long-lived containers; refresh the mix every couple of repots.