Peat moss is a dark, fibrous material harvested from peat bogs, consisting primarily of decomposed Sphagnum moss and other organic matter that has accumulated over thousands of years. Peat moss exhibits several physical characteristics that make it valuable in horticulture. It is highly absorbent, capable of holding up to 20 times its weight in water, allowing it to function as a reservoir of moisture for plants. Its spongy structure resists decomposition and compaction, providing long-lasting benefits to the texture of a growing medium. It is also naturally sterile, meaning it is free from weed seeds and most pathogens, making it a clean material for various gardening applications.
Primary Component in Soilless Potting Mixes
The primary use of peat moss is as the foundational organic material in soilless potting mixes, designed specifically for container gardening, hanging baskets, and seed starting. Unlike dense garden soil, peat moss provides a lightweight structure ideal for roots growing in confined spaces. Its fibrous nature creates air pockets within the mix, promoting excellent aeration and preventing root suffocation.
Potting mixes typically combine peat moss with other aggregates like perlite or vermiculite to achieve the necessary balance of moisture retention and drainage. Peat moss acts as a sponge, ensuring water is available to the roots while the aggregates keep the mix loose so excess water can escape. This combination minimizes the risk of waterlogging and root rot, issues problematic for sensitive young seedlings. The sterility of peat moss also provides a clean start for seeds, protecting vulnerable sprouts from soil-borne diseases.
Amending Outdoor Garden Beds and Soil
Peat moss is widely used as a structural amendment to improve the physical properties of existing native soil in outdoor garden beds. Incorporating it into heavy clay soil is effective because its spongy texture forces apart the fine clay particles, which significantly improves drainage and aeration. This action prevents the soil from compacting into a dense mass, allowing roots to penetrate more easily and preventing water from sitting on the surface.
Conversely, in sandy soils that drain too quickly, peat moss increases the soil’s water-holding capacity. The material’s high absorbency allows it to retain moisture and nutrients that would otherwise rapidly leach away through the coarse sand particles. To achieve these structural improvements, the peat moss must be thoroughly incorporated into the top 6 to 12 inches of the existing garden soil. Simply applying it as a surface layer is ineffective, as it can dry out and become hydrophobic, meaning it resists water absorption.
Specialized Application: Creating Acidic Conditions
A specialized function of peat moss in the garden is its application to create or maintain acidic soil conditions for specific plant groups. Peat moss, particularly Canadian sphagnum peat moss, has a naturally low pH, typically ranging between 3.0 and 4.5. This low pH makes it a valuable amendment for plants known as acid-lovers, which require a soil pH below 6.0 to properly absorb essential micronutrients like iron and manganese.
When mixed into the soil, the acidity of the peat moss helps to lower the overall soil pH, which is a requirement for plants such as blueberries, rhododendrons, azaleas, and camellias. For these plants, the chemical property of the peat moss is the primary reason for its use, rather than its physical benefits of drainage or moisture retention. However, the acidifying effect is not always permanent, especially in native soils with a high buffering capacity. Repeated applications or the use of elemental sulfur may be necessary to maintain the desired low pH over time. For container-grown acid-loving plants, using a mix that is predominantly peat moss ensures a consistently acidic environment from the start.