Topdressing is a sand or prepared soil mix applied to the surface of the lawn. The term topdressing also is used for the process of applying the material.
Topdressing materials are evenly applied in a thin layer, typically ¼ inch (6.35 mm) or less, for a variety of purposes. Topdressing can be used to smooth the surface of the lawn. It can reduce thatch buildup by encouraging decomposition. It can be used following seeding, overseeding or sprigging to protect the developing plants from desiccation during the establishment process. It also can be used on open, windswept turfgrass to help avoid winter desiccation.
When applied following core aeration, the topdressing material filters into the holes opened by the aeration process, speeding turfgrass recovery.
Typically, for all of these uses, the topdressing material should closely match the composition of the soil profile.
Topdressing also may be used to modify the soil profile, though this takes multiple applications over a number of years. For this use, the topdressing material differs from the composition of the soil to which it is applied.
Typically, this practice is combined with core aeration, with a coarse-textured material (sand or sand-organic matter combinations) applied to soils with high clay content (fine-textured). The goal is to alleviate compaction issues and improve water, air and nutrient movement into the soil (infiltration) and within the turfgrass rootzone (percolation).
Topdressing for soil modification can be detrimental if done improperly. Once the topdressing material is determined, that same material must be used throughout the process to insure the consistency that facilitates water movement and root growth.
The use of varying topdressing materials can result in layering, with different textured soils stacked in layers within the soil profile. The greatest problem occurs with a fine-textured soil forming a layer on top of a coarse-textured soil. This can hinder water’s ability to reach the turfgrass roots since water moves throughout one layer of soil texture before infiltrating the differing texture of the level below it.
Problems also can occur if the process of applying the topdressing materials is done improperly, resulting in uneven depths of material across the lawn. This can impede water and nutrient accessibility creating wet and dry areas that impact turfgrass growth and the overall appearance of the lawn.
Applications of too much topdressing material, or applications made too frequently, over an existing thatch layer can bury the thatch layer. Without proper aeration to rectify this, turfgrass roots may grow into and within the thatch layer rather than extending into the soil profile. This makes the turfgrass more susceptible to drought, heat and other stresses.