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 dehydration during the establishment process. It also can be used on open, windswept grass to help avoid winter drying.
When applied following core aeration , the topdressing material filters into the holes opened by the aeration process, speeding grass recovery.
For these uses, the topdressing 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. For example, on soils with a high clay content, topdressing containing sand or a combination of organic materials can improve water, air and fertilizer absorption.
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 ensure 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 grass roots since water moves throughout one layer of soil texture before infiltrating the differing texture below.
Applications of too much topdressing material, or applications made too frequently, over an existing thatch layer can bury the thatch. Without proper aeration to rectify this, grass roots may grow into and within the thatch layer rather than extending into the soil profile. This makes the grass more susceptible to drought, heat and other stresses.