Thatch management is a challenge on athletic fields, but the nature and extent of the challenge varies. On a sand-based field, just about any amount of thatch is bad. On a native soil field, some thatch can actually be beneficial. Regardless, thatch management is a combination of art and science and there are a couple of different tools available.
Cultivation – in the form of core aerification – is performed on turf to improve root zone conditions, primarily to alleviate soil compaction and reduce excess thatch. But other methods of cultivation are available, including vertical mowing (or verticutting) and the more aggressive fraze mowing.
Verticutting is more of a preventative tool and fraze mowing is a renovation process if the thatch layer has gotten out of hand.
The root of the problem
Thatch is the intermingled layer of undecomposed organic matter located between the soil surface and the zone of green vegetation. This includes crowns, roots, rhizomes and stolons. Because of this, grasses that spread laterally (such as Kentucky bluegrass, bermudagrass and zoysiagrass – the kinds we generally prefer for sports turf) tend to produce more thatch.
Generally, any amount of thatch on a sand-based field is a negative because of the effects of the organic matter layer on water infiltration rates. But when present in moderation, thatch can be an advantage on native soil fields. A moderate thatch layer will provide some resiliency on sports surfaces and can result in modest improvements in field wear tolerance. Also, grass plants that are properly rooted in soil may be insulated from temperature extremes by the thatch. Finally, thatch can provide a barrier against germination of crabgrass and other annual weeds.
An excessive amount of thatch, however, will cause many problems on an athletic surface that outweigh its benefits. First and foremost is poor rooting and localized dry spots, and this is especially a problem when the grass is rooted in the thatch instead of the soil. Thatch doesn’t retain water and nutrients as well as soil and it’s not wetted by upward movement of water from the soil. Plants rooted in thatch are more subject to drought and nutrient deficiencies as well as extremes of heat and cold. Because thatch deposition tends to vary over a surface, a large amount of it increases the likelihood of scalping injury from mowers. Thatch also acts to restrict water and air movement into the soil and can decrease penetration of pesticides into the soil. A large amount of thatch can also increase disease and insect potential.
Accumulation of excess thatch occurs when organic matter production exceeds the rate of decomposition. This occurs if high amounts of nitrogen are applied, the soil pH is unfavorably low for microbial activity and/or if aggressively growing species or cultivars are utilized, which is what is most often recommended on athletic fields. Return of clippings doesn’t contribute to thatch accumulation.
Verticutting may also be referred to as power raking or vertical mowing. It’s used either preventatively or curatively where an excessive layer of thatch exists or if other cultivation practices are inadequate to remove excess thatch. But verticutting may also be done less intensively and more often in order to manage the grain of the turfgrass, which is a condition where the turf lies down in one direction.
Machines used to remove thatch include the verticutter or vertical mower, which is a machine with vertically mounted blades that slice into the thatch layer. (Solid blades are best, but can damage irrigation lines.) The spacing of the blades is anywhere from 0.25 to 1.5 inches apart on most units. When verticutting, the blades rotate through the turf and thatch layer to bring up thatch and organic matter to the surface for removal. This process is less intensive than core aerification.
Verticutting can serve an important function in sports turf management. On fields that use zoysiagrass, bermudagrass or Kentucky bluegrass, thatch buildup can be excessive, particularly if an aggressive watering and fertility program is utilized. If the goal of verticutting is mechanical thatch management, then it shouldn’t be considered a substitute for core aerification because it doesn’t contribute to thatch decomposition as effectively as does coring and topdressing.
Current research suggests that mechanical thatch removal via vertical mowing doesn’t substantially decrease the total amount of thatch that’s present, unless many passes are made over the surface. The author of the aforementioned research used one of these machines for a research project on a thick thatch layer and found that seven passes in three directions was necessary to remove one-third of the thatch. However, this caused a significant disruption to the turf surface. In practice, one to three passes is more typical, with three passes expected to remove perhaps 10 to 20 percent of the thatch present. Thus, verticutting should be considered a periodic or annual management tool as part of an overall thatch management program, as would be needed on a sand-based field.
Verticutting on a native soil field can also be useful. But it’s not a single operation that will remedy an excessive thatch layer. Removal of thatch that’s brought to the surface is important because leaving it will just hasten the buildup of a subsequent thatch layer.
Another benefit of vertical mowing is that it slices through stems (rhizomes and stolons) which results in increased tillering, thus a potential increase in turf density. Because of this, some field managers with only moderate or low thatch levels will elect to verticut lightly three times per year – once in spring to help remove decaying organic matter left from winter, once during active growth in order to help increase density and once in the fall to help with overseeding.
There are some precautions to observe when verticutting. Because of the way these machines work, vertical mowing will cause some injury to turfgrass. While less intensive, the amount of time to allow the turf to recover following verticutting is similar to that for core aerification. You should also avoid vertical mowing if the turfgrass is stressed or not established. Excessive layers of thatch and or shallow rooted turf should be treated with multiple vertical mowing operations, allowing full recovery of the turf in between.
It’s also important to avoid times of the year that are favorable for weed seed germination. This can be difficult on cool-season turfgrass because spring is the optimal time both to verticut and for weed seed to germinate. Another time to consider verticutting cool-season turf is late summer to early fall, as the turf won’t recover as quickly but the weather is less stressful and weed competition is reduced. Warm-season grasses are best verticutted in late spring to summer. On bermudagrass, if verticutting is performed in the fall in preparation for overseeding, it should be with only one pass over the field because more than this may increase winterkill potential.
This process is much more aggressive than verticutting and will not only remove thatch, but also will contribute to field leveling and may help with management of certain weed species.
One of the original machines used for this purpose is commonly referred to as a “Koro” – this is in reference to the individual credited with developing the machine used for the process, Ko Rodenburg, who was a field manager in Holland. One of the original uses of the Koro Field Top Maker was to clean out fields using the Desso Grassmaster system, which allowed for removal of the real grass while leaving the artificial fibers intact.
These machines were introduced to the U.S. sometime after the year 2000, but for the different purpose of quickly removing and laser leveling an existing field that was in need of renovation. In the years following its introduction, field managers in Europe began to experiment with different blade depths leading to the development of the process for which Mr. Rodenburg coined the phrase “Fraise mowing.” (In the U.S. it’s most often seen spelled “Fraze.”)
While this has been practiced in Europe for many years, it has only been widely used in the U.S. for the past few years. Because of this, little research exists concerning management recommendations for how to fraze mow or best management practices for field reestablishment following fraze mowing.
The Koro Field Top Maker was the first machine developed for fraze mowing, but other units are on the market from various manufacturers, including the STEC Combinator. These are rotating machines, like a tiller, that remove the top layer of the turf. The machine is mounted to a tractor and adjustable, but depending on the depth setting can be used to selectively remove organic matter and shallow rooted weeds (such as Poa annua) while leaving crowns, roots and rhizomes intact.
The debris that it removes are sent out a side-mounted conveyor belt for collection and disposal. After the fraze mowing operation is completed, the desired grass may recover from the roots, rhizomes and stolons that were left behind. The initial result of fraze mowing can be quite shocking, as it completely removes all grass and organic matter down to the soil surface. Because of this, enough time needs to be allowed for complete recovery of the turf.
Fraze mowing is most often done on bermudagrass or Kentucky bluegrass fields. Some early experimentation with fraze mowing on Kentucky bluegrass suggests that it can be effective in reducing perennial ryegrass or annual bluegrass infestation. On bermudagrass, researchers have demonstrated that fraze mowing can decrease spring dead spot incidence.
There are an increasing number of sports field managers who are convinced of the benefits of fraze mowing. Research is underway to look at the effect of different intensities of fraze mowing on not only the benefits, such as removal of thatch and unwanted weedy species, but also on management practices that promote the recovery of the different species we manage as sports turf.
As this research continues, look for management recommendations that optimize the effectiveness of the practice and best strategies to aid in field regeneration and recovery post-operation.