Lava rocks are the jack-of-all-trades in the plant world. These reddish-brown rocks can be used at the bottom of pots for drainage, as a substitute for mulch, as a soil amendment, and as a growing substrate. Lava rocks are similar to other houseplant materials, vermiculite and perlite, both of which are byproducts of volcanic activity. Lava rocks generally are available in bigger pieces than the other two and because of this difference, allow for many more uses.
As drainage -
Houseplants need air around their roots in order to survive. This means pots that have no drainage hole can become water-logged and drown/rot your plant to death. If you have a cute planter you’d like to use, but want to make sure your plant survives, try putting a layer of lava rock at the bottom before adding in your soil. The porous nature of the rocks with absorb the water and keep the soil from staying too damp.
As a mulch substitute -
In drier climates - think Arizona - lava rock can be used as a substitute for outdoor mulch. The rocks reflect heat and plants can grow in lava rock just as easily as mulch. They also minimize slug damage!
As a soil amendment -
Some plants prefer well-aerated soil. This means you should add things like perlite, orchid bark, vermiculite, or lava rocks to your general soil mixture before planting your plants. Ariods, succulents, philodendron, and rubber plants all prefer this type of soil.
As a planting substrate -
Some plants, like herbs, can be grown directly in or on lava rocks. Tillandsia (aka Air Plants) and some succulents can also be planted on lava rocks. Because lava rocks themselves do not contain nutrients, the water used to water the plants should contain fertilizer or other nutrient-additives.
Photo from Home Depot