What's the big idea?: Taming trees to improve turf acclimation

Learn how effective tree care and pruning can help preserve your warm season grasses.


As we’re in late summer, thoughts of efficiently acclimating grasses for the upcoming winter come to light. And light is a major factor. In the transition zone region of the U.S., it’s quite common for winter injury on warm-season grasses to occur in areas where shade is prevalent, for they receive less “photosynthetically active radiation,” or PAR. Shade prevents warm-season grasses from producing carbohydrates through photosynthesis that are important in the cold-acclimation process, reducing their ability to survive winter. Warm-season grasses require more PAR than cool-season grasses, and some species also differ in how well they tolerate shade.

A common routine on warm-season grasses in areas that experience cold winters is to increase the mowing height in late summer and autumn. Why? More leaf area also enhances the ability of the grasses to absorb PAR. That said, it’s sometimes easy to not recognize the lack of PAR that is occurring in areas that experience morning or afternoon shade.

On some golf courses, shade issues become more significant years after trees have been planted. As trees grow larger, they can impact the visibility of greens, playability and performance of turf. However, trees can also form strong relationships with golfers, which can make their removal more difficult.

But it can be done. At Oakmont (Pa.) Country Club, the original architect, Henry Fownes, valued the benefits of an open golf course lacking trees, and that’s largely how the course was presented from the early 1900s to the mid-1940s. Thereafter, thousands of trees were planted and began to impact play years later. Beginning in the mid-1990s until the U.S. Open in 2016, over 12,000 trees were removed at Oakmont. More information on this from the USGA can be found here.

I recently visited a private club in the Kansas City area that also began as a relatively open, tree-free golf course over 50 years ago. That changed many years after thousands of small trees were planted shortly after opening. If trees cannot be pruned efficiently or removed to maximize light, grasses will suffer from reduced growth due to shade, and are inclined to experience more winter injury. Trees may also extend roots into soil on fairways and tees, which can also impact turfgrass growth. Bermudagrass and zoysiagrass in the Midwest often exhibit winter damage on shaded tees, perimeters of fairways that are lined with trees, and other areas where shade extends across the surface.

On golf courses where a superintendent’s request for tree removal can be problematic, turf species are sometimes changed to assist in shade tolerance. For example, some zoysiagrasses, such as Emerald or Diamond, are more tolerant of shade than bermudagrass and are used to provide better turf quality in shady areas in the South. In the Midwest, it’s quite common for significant cold injury to occur to zoysiagrasses or bermudagrasses in shady spots. This may result in frequent purchase of sod to replace turf, or conversion to cool-season grasses in those areas.

There are ways to measure levels of shade on areas of golf courses throughout the growing season, and some companies can provide such services. Superintendents can also use technical instruments that measure PAR. Mike Richardson, Ph.D., and his co-authors outlined strategies for light measurement in this GCM article from October 2019.

Such information can serve as valuable evidence for superintendents attempting to communicate shade issues with those who are emotionally attached to trees. In addition, highlighting courses that recognize the benefits of open space, such as Oakmont, can also be of benefit. Undoubtedly, trees are beautiful. Nevertheless, quality of turf on the golf course is commonly reduced when trees reach a size where they begin to impact photosynthesis of warm-season grasses that are sensitive to cold.

Jack Fry, Ph.D., is a professor of turfgrass science at Kansas State University, currently working at the school’s Research and Extension Center in Olathe, Kan. He is a 25-year member of GCSAA and was recently awarded GCSAA’s Outstanding Contribution Award for 2022.