Daniel Gill, Koske, Thomas J., Beasley, Jeffrey S.
The term ‘topdressing’ is used for both the material used and the process of adding a thin layer of material to the surface of a sod or seedbed. This topdressing material can be sand, soil, compost, synthetic material (crumb rubber, zeolites) or a blend of each.
The extent of topdressing and type of materials depend on the purpose. Topdressing material is usually a sand or sandy soil. Some use a 50:50 mix of sand and fine compost. The mix selected is often similar to or lighter than the current root-zone mix so that it blends well with the root zone soil structure to prevent soil layering. If soil modification is the goal, then a medium-coarse sand is dressed and swept into open core-aerification holes. A sand-based green or sports field should be maintained with similar sand used in construction to avoid soil pore plugging and layering from fines and clay. Sandy soils may be dressed over sod on native soil. Crumb rubber dressing and zeolites can be used, but over application can build up to problem levels.
Turf managers usually prefer sands and organic materials. Fine compost will create a dynamic, bio-active root zone that creates competition to reduce disease pathogens. Compost will thus enhance microbial breakdown of thatch and organic mat layers.
All materials used should be reasonably weed- and disease-free. Keep topdressings dry and covered to protect their integrity. Dry materials spread better and will better migrate into the sod and core holes. Brushing or dragging, followed by irrigation, will help incorporate the materials deeper into the sod.
It is often best to topdress at lighter rates and more frequently than apply heavier amounts less frequently. Always be careful to avoid soil layering of dissimilar materials and textures; soil layering can affect water movement and root health. (see Layering in Soils) Moderate-heavy topdressing may be combined with coring and core removal (harvesting) to begin a root-zone modification on established turfgrass. This approach requires 2 to 3 years of aggressive effort requiring 4 to 6 procedures each season, and complete field renovation is a better choice.
Brushing the sand into the open core holes will create sand channels to promote a softer ground and expedite water percolation and air exchange. This may be accomplished with 1/8- to 1/4-inch dressings requiring 10 to 21 cu. ft. /1000 sq. ft. Seldom might we apply a 1/2-inch dressing requiring 42 cu. ft. and risk layering unless the material dressed was similar in texture to the current root zone.
You may core, harvest or leave core plugs, topdress, then drag or brush in the topdressing. Applying topdressing first, followed by coring and brushing may keep core holes more open if equipment is large and heavy, but this does not allow for harvesting of undesirable cores. Just don’t wait more than a day or two to topdress after coring, or roots may clog core holes. If cores are not harvested and reincorporated, less additional topdressing material is needed.
When and how much to a topdress depends on the purpose for topdressing and the season. This management is normally done in our warm growing seasons when turf growth is aggressive and thatch reduction is most feasible. Application frequency may be most aggressive at this time. Avoid dressing when turf is under stress. For root-zone soil modification, core and harvest plugs, then topdress every 3 to 4 weeks in the season. For thatch management, try 1/8-inch dressing every 2 to 4 weeks and monitor the thatch layer to gauge for program adjustment. Thatch degrades best when under warm and moist environmental conditions.
Topdressing can well be done without accompanying coring or vertical mowing. These disruptive procedures may bring on stress-related diseases like Curvularia and Bipolaris leaf blights. Dense ultra-dwarf Bermudas may also respond better to weekly light dustings (3-4 cu. ft./1,000 sq. ft.) than to vertical mowing. This sand should have less that 20% fines (1/4 mm or less) and little if any coarse (3/4 mm or more) material.
Topdressing is also used at planting and turf establishment. The dressing can cover sprigs or seed for faster crop establishment. Topdressing newly installed sod will help fill gaps and seams as well as help smooth the surface.
Topdressing is a very important turf-management practice and can be costly in equipment, materials and labor. If close-standards material is needed, it must be purchased from a materials dealer who provides that sieved and blended product. Inspect your dressing material before you have it delivered.
Years ago, most greenskeepers had a skilled laborer who could sling sand uniformly enough with a wide shovel. Now we use moderate-to-large spreaders and topdressers to produce a very uniform dressing.