Thatch and organic matter in bentgrass/poa annua systems have been managed differently over the years
By Mike Jiggens
May 17, 2019 – Managing thatch on golf courses is a practice superintendents have engaged in for years and one they must continue to perform if they are to deliver faster, firmer surfaces for their clientele. What has changed over the years is the means by which thatch and organic matter in bentgrass/poa annua systems is managed and controlled.
Dr. Beth Guertal of Auburn University in Alabama shared her findings on thatch management with superintendents attending March’s Canadian Golf Course Management Conference in Banff, Alta.
“Controlling and managing thatch and organic matter is really the first step in IPM,” she said.
There are several causes of thatch buildup in turf, including vigorously growing turf species, abundant nitrogen fertilization, overwatering and the natural growth of the plant. A number of cool season grasses are prone to producing thatch or the organic matter that accompanies it.
As bentgrasses are bred and selected, one of the selection criteria is shoot density. Thatch increases organic matter and in turn carbon. Guertal questioned whether an increase in carbon is necessarily a bad thing, especially when the aim is to increase sustainability. When fairways 25 to 40 years old have been managed that long, a significant carbon resource and base has been created.
“We represent an industry that sequesters and stores carbon which is a great way to offset pollution.”
Guertal said it’s important to note that as things age, such as putting greens that are 40 years old on native soils, the soil pH decreases as a function of age.
“That’s the natural mineralization and release of stored organic matter and nitrogen. As nitrogen mineralizes, you will also reduce the soil pH. As things get older and you produce mature surfaces, you’re going to store organic carbon and store organic matter.”
When shoots are high, thatch depth is high. With low shoot density, there is less thatch.
Methods to manage thatch
The means to manage thatch include core aerification, vertical mowing, topdressing and the use of microbial thatch degraders. Superintendents are expected to create playing surfaces that meet stringent demands by golfers. If aerification or severe vertical mowing is done, there is bound to be some commentary about it, Guertal said.
“The days are largely gone where you can close your course for a week and a half and vertical mow the stew out of the whole thing and then let it grow back.”
There was a time in the 1950s and 1960s in the southern United States when such a strategy could be undertaken, but those days are gone, she added.
Aeration is the go-to strategy when layered soils are present or a depth problem needs to be remediated. It may be a less effective practice if the goal is to alleviate a particularly severe thatch problem. Aerification units are generally used to relieve compaction than to manage thatch. Guertal said they are less likely to help with agronomic matters and are more suited to help with such physical properties as water movement and compaction.
She said when sand is added to control the amount of organic matter, depth is being artificially inflated because the sand is diluting it.
Although hollow tines are more effective than solid tines in relieving compaction, solid tines are useful for ridding algae or for controlling mat, moss or a dried surface yet may not be the right tool for alleviating organic matter, she added.
For controlling organic matter in thatch, Guertal said aerating is a “maybe.” Topdressing tends to be underserved, but it helps, she said. Studies conducted at Rutgers University have found topdressing seems to help reduce anthracnose with the assumption that the applied sand protects the crowns of the poa plant, ensuring they aren’t as open to being damaged.
Vertical mowing has come through a transition and is “a pretty significant way to reduce organic matter.” Guertal recommended vertical mowing as deep as possible with about three passes, a cleanup and topdressing to get rid of the accumulated organic matter.
Research data compared vertical mowing done twice a year at 19.1 millimetres (a severe vertical mowing) with light, frequent vertical mowing four times a year at 6.4 millimetres. Guertal said both variations produced the same results, giving superintendents some flexibility as to when the practice can fit into a maintenance schedule and what club memberships are willing to tolerate.
Golfers might prefer the less frequent route, she speculated, but added, “You’re going to go to town on it.”
Grooming and thatch
Studies have also been done to learn more about the relationship between grooming and thatch, including groomer spacing. If cutting, nicking or slightly grooming turf with closer spacing and a node is nicked or a stolon is slightly touched (because the creeping bentgrass plant has stolons), the node will be encouraged to produce shoots.
“If we have a groomer with closer spacing, are we going to encourage improved shoot density?”
The groomer study involved various scenarios. Grooming was done once, three times and six times a week. Mowing was done every day of the week to simulate real golf course conditions. The two-year study also looked at groomer spacing of a quarter-inch, a half-inch and the use of no groomer at all.
“As we increased grooming frequency, we tended to increase thatch depth.”
The frequency of three times a week seemed to foster an increase in thatch depth, but it came about due to the increase in shoot density, Guertal said. Increased grooming stimulated shoot growth, suggesting the groomers were doing their job.
Research showed topdressing reduced organic matter five per cent while vertical mowing – no matter how deep or frequently – reduced it 10 per cent. Grooming, meanwhile, increased organic matter 11 per cent.
Guertal said grooming is desirable if the intent is to eliminate organic matter because it’s going to stimulate shoot growth.
She said studies also show that nitrogen has the biggest impact on green surface hardness. Fertilizing with a high rate of nitrogen contributed to a softer green while lower rates of nitrogen produced harder surfaces. When spiking was included, it made for a softer surface. The hardest surface came only with grooming. Vertical mowing sometimes had little effect. Heavy topdressing contributed to a harder surface.
Higher topdressing, only grooming or vertical mowing and lower rates of nitrogen were the tools for the hardest surface, Guertal said. Higher nitrogen, however, reduced moss and pink snow mould, but softened the surface and led to ball roll decrease. Spiking had benefits for better water infiltration.
Biological dethatchers have been around for about a decade, she said, noting commercial grade products are a bacillus type that have produced a significant response in trials.