Peat moss is the partially decomposed remains of Sphagnum moss harvested from bogs, widely known in horticulture for its remarkable capacity to manage moisture. Peat moss holds water, making this organic material a highly valued component in various growing applications. This ability to absorb and retain moisture is a direct result of the unique structure of the moss as it decomposes.
The Mechanism of Water Retention
The exceptional water-handling ability of peat moss is rooted in the unique cellular structure of the Sphagnum plant. The moss contains two primary cell types: small, living chlorophyllous cells and large, dead, empty cells known as hyaline cells. These hyaline cells have thin, porous walls and function like tiny sponges.
These large, dead cells, along with spaces between the densely packed fibers, create a highly porous structure. This physical arrangement allows peat moss to absorb up to 20 times its dry weight in water. The water is held within these cells and the capillary spaces against the pull of gravity.
The moss’s structure allows it to retain a large reservoir of water while simultaneously maintaining necessary air pockets. This balance prevents water from draining away, yet avoids creating a waterlogged environment that would suffocate plant roots. Extracellular pore spaces hold the majority of the water content when saturated.
Addressing Dry Peat Moss
Despite its high capacity for water retention, peat moss presents a challenge when it is allowed to dry out completely. Once the material is fully desiccated, it becomes hydrophobic, meaning it actively repels water. Water poured onto the surface of dry peat moss will often bead up and run off the sides of the container instead of soaking in.
This water-repellent state occurs because the porous structure of the fibers changes when dry, and natural waxy compounds become exposed. To re-wet the material effectively, a wetting agent is necessary. One effective method is to use a few drops of mild dish soap in the water to act as a surfactant.
Wetting agents lower the surface tension of the water, allowing it to penetrate the waxy fibers and fill the collapsed pores. For smaller batches, soaking the dry peat moss in a tub of water until all air bubbles have escaped will ensure uniform saturation. Using warm water can also help speed up the rehydration process. Once re-moistened, the peat moss regains its high water-holding capacity.
Integrating Peat Moss into Growing Media
Peat moss is rarely used by itself as a growing medium; its value is realized when it is blended with other components to create potting soil or seed-starting mixes. Its high moisture retention means that when it is incorporated into a mix, the blend requires less frequent watering. This property is particularly useful in container gardening where the limited volume of soil tends to dry out quickly.
When mixed with heavy materials like compost or garden soil, the spongy structure of the peat moss improves the overall physical quality of the medium. It prevents the mix from compacting too tightly, ensuring that sufficient air space remains for root respiration. The balance between water-holding capacity and air porosity is often determined by the particle size of the peat moss used.
A secondary function of peat moss in growing media is its influence on soil chemistry. Peat moss is naturally acidic, generally having a pH range of 3.5 to 4.5. This characteristic is beneficial for plants that thrive in acidic conditions, such as blueberries and azaleas. However, for most common garden plants, this low pH must be adjusted by adding lime to the mix to prevent the soil from becoming too acidic, which can impede nutrient uptake.