Soil & compost
Vermiculite
A light, spongy mineral added to compost to hold moisture and air, and often used to cover seeds for even germination.
Vermiculite is a natural mineral that has been heated until it puffs up into light, flaky, golden-brown granules, a bit like miniature concertinas. Those layers trap both water and air, which is exactly what makes it so useful in the potting shed. It is sterile, pH-neutral and lasts for years, so a single bag goes a long way.
Vermiculite versus perlite
Gardeners often reach for vermiculite or perlite for similar jobs, and it helps to know how they differ. Both are lightweight minerals that open up a compost mix, but they pull in opposite directions. Perlite (the white, polystyrene-like granules) drains freely and adds air, so it suits cuttings and plants that hate sitting wet. Vermiculite, by contrast, soaks up and holds far more water, releasing it slowly back to the roots. As a rough rule: reach for perlite when you want sharper drainage, and vermiculite when you want a mix that stays reliably damp. Plenty of growers keep both and blend to taste.
Using it for seeds
Vermiculite's best-loved trick is covering seeds. Sift a thin layer over freshly sown seed in place of compost: it lets light filter through to seeds that need it, keeps the surface from drying out or forming a hard crust, and lets delicate shoots push through with ease. Because it stays evenly moist, it gives steadier, more even germination, which matters a great deal in a cool UK spring when seed trays can dry out fast on a sunny windowsill. It also lets in air at the surface, which helps reduce the damp, stagnant conditions that lead to seedlings keeling over.
Using it for potting
Mixed into potting compost, usually at something like 10 to 30 per cent by volume, vermiculite acts as a moisture reservoir. It is handy for hanging baskets, containers and any pot that tends to dry out between waterings, and for moisture-loving plants generally. It is also a tidy medium for rooting cuttings and for storing dahlia tubers or bulbs over winter, where you want them just barely moist rather than wet.
A word of balance: because it holds so much water, an over-generous helping can leave a mix soggy and airless, especially in heavy or slow-draining situations. For plants that prefer free-draining conditions, lean on perlite or grit instead. Buy the horticultural grade rather than the coarse loft-insulation type, store the bag somewhere dry, and a little will see you through many seasons of sowing.
In a UK garden
UK garden centres and online sellers stock vermiculite in handy bags; it is a staple for windowsill and propagator sowing, where its moisture-holding really earns its keep through our cool, slow springs.
Example
Sow tomato seed on damp compost, then sift a thin layer of fine vermiculite over the top instead of compost so light still reaches the seed while the surface stays evenly moist.