Like perlite, vermiculite is an effective soil conditioner that can loosen compacted soil, provide drainage, and hold three to four times its weight in water.
But—and this is an important but—vermiculite is more compressible and less porous than perlite, giving vermiculite higher water-holding capacity when it’s used as a planting medium.
You’re starting seeds. Because vermiculite holds moisture better than perlite, it helps keep seeds from drying out during germination. Vermiculite is an excellent choice for covering hard-to-start seeds or seeds that need light for germination.
You’re repotting outdoor container plants. Potted plants tend to dry out faster outside, especially if they’re in porous containers like terra cotta or fabric pots. Adding vermiculite to your potting mix helps them retain moisture better.
You garden in a dry climate. Adding more vermiculite to potting soil and soilless mixes conserves moisture in the growing media.