A block of compressed, dried coir (coconut husk fiber). Coir soaks up 5 times its weight in water, and holds it in the potting mix for plants. Unlike peat moss, coir won't repel water if it dries out. An abundant byproduct of the coconut industry, it's more sustainable than peat. I still add some peat to potting mixes for acid-loving plants like blueberries, but use coir for all other uses. Photo by Steve Masley