Peatmoss comes from sphagnum fibres that have partially decomposed in a bog. It is light and spongy and crumbles into small clumps. Peatmoss has a remarkable ability to hold water and to retain nutrients from leaching out of the soil. The finer the fiber, the more water-holding capacity it has; it can absorb up to 16-26 times its weight in water, but can be difficult to re-wet if left to dry out. Peatmoss is slightly acidic, so if you choose to use peatmoss, a pH ajuster such as lime should be added to the mix to balance out the acidity.

Peatmoss is a biodegradable material and will naturally decompose over time sending it's carbon content to the atmosphere. It doesn’t have a very long shelf life and after 1-2 years (from the time you purchase it), the fiber starts breaking down, making it ineffective at its primary job: to hold moisture. Expiration in this case doesn’t mean the peatmoss turned moldy or smelly (and it doesn’t as long as it’s been kept dry) ‒just that it loses its structure.

Usage of peatmoss should be kept to a minimum considering it's environmental impact, and some countries altogether ban it's usage in horticulture and horticultural products. Biochar is a great alternative to peatmoss.