The following presentation is brought to you
by the University of Arkansas System Division of Agriculture.
Primary funding for this research was provided by the USGA.
Turfgrass needs light to survive.
In shady conditions, direct sunlight is diffused or blocked out,
reducing the amount of photosynthetically active radiation,
or PAR, that reaches the turf.
“So PAR is photosynthetically active radiation, and it’s a narrow bandwidth within the visible
spectrum, from 400-700 nm, that plants use to complete the photosynthesis process.”
Insufficient levels of PAR compromise the ability of turf to photosynthesize energy
in the form of carbohydrates.
“What tends to happen is the turfgrass will then try and grow vertically instead of horizontally.
And when it’s elongating, it’s not as healthy of a plant, and it can lead to lack of density.
This vertical growth and decreased density lead to reduced traffic tolerance and increased
risk of scalping—generally compromising turf health and quality.
Despite the challenges, shade is an integral part of many golf courses.
Trees add to overall course aesthetics and can influence how certain holes are played.
Buildings and other unalterable topographical features can also cast shadows that affect turf health.
The minimum amount of light needed to maintain acceptable turfgrass quality can be quantified
in terms of the Daily Light Integral, or DLI, which is a measure of cumulative PAR
for a given day and area.
Previous studies of minimum DLI threshold values for putting greens primarily focused
on warm-season turfgrass species like ultradwarf bermudagrass, so no minimum DLI threshold
values have yet been established for putting greens utilizing the cool-season creeping bentgrass.
Researchers at the University of Arkansas Department of Horticulture designed a study
to evaluate shade timing and intensity on creeping bentgrass putting greens.
Researchers also evaluated the effects of trinexapac-ethyl growth regulator
and Turf Screen plant protectant to determine whether these products can lower the
recommended DLI threshold or improve shaded turf quality.
This study was carried out over the 2016 and 2017 growing seasons
at the University of Arkansas Agricultural Research and Extension Center in Fayetteville, Arkansas.
The experimental area consisted of “Tyee” creeping bentgrass, established from seed
and constructed according to USGA putting green guidelines during the fall of 2013.
This putting green was mowed 6 times per week at a height of one-eighth inch,
and a nutrient and pest management regimen typical to the region was followed.
Researchers utilized a randomized split-strip plot design with 32 treatment combinations,
replicated three times.
Moveable shade structures were used to evaluate three levels of shade intensity:
70%, 80%, and 90% reduction of full sun, as well as 0% shade, or full sun.
Shade timing was also evaluated, with half of the plots receiving shade in the morning,
from sunrise to solar noon, and the other half receiving afternoon shade from
solar noon to sunset.
Each plot was also treated with some variation of spray treatments, including:
Primo Maxx trinexapac-ethyl growth regulator applied every other week at 0.0625 ounces
per one thousand square feet;
Turf Screen turf protectant applied weekly at 1.25 ounces per one thousand square feet;
Both the Primo Maxx and Turf Screen at the aforementioned application rates; and
A control with no treatment.
Spectrum LightScout Quantum Light Sensors were mounted under each shade screen and recorded
PAR measurements to a data logger every 15 minutes.
Researchers performed visual quality evaluations of the turfgrass plots every two weeks, assigning
each plot a visual rating on a scale from one to nine,
where nine corresponds to ideal turf quality and one to dead turf.
Minimum acceptable turf quality was set at a rating of seven for the purposes of this study.
Other variables measured include turfgrass percent coverage and dry clipping yield.
In 2016, the plots receiving full sun began with visual quality ratings of less than 7—below
acceptable quality, but beginning in June, improved to 7 or greater
for the remainder of the growing season.
Plots receiving 70% shade, regardless of morning or afternoon timing, fell below acceptable
quality by August 9, but recovered in September.
80% shade applied in the morning also resulted in less-than-acceptable quality by August 9
with recovery in September, but 80% afternoon shade resulted in unacceptable turf from August 9
through the remainder of the growing season.
90% shade, regardless of timing, caused turf quality to fall below acceptable for
a majority of the 2016 trial period;
however, plots receiving 90% afternoon shade regularly experienced greater deterioration
compared to plots receiving 90% morning shade.
In 2017, neither the observation dates nor the effects of shade timing were found to
be statistically significant predictors of turfgrass quality.
Application of 70% and 80% shade resulted in slightly lower quality turf than the full
sun plots, but not to a statistically significant extent.
90% shade, however, resulted in significantly lower quality on average, than any of the
other shade treatments.
On average, application of Primo Maxx and the Turf Screen turf protectant maintained
or improved the overall turfgrass quality throughout both growing seasons.
In 2016, there were three dates on which Primo Maxx or Turf Screen significantly improved
turf quality compared to the untreated control plots.
In 2017, both Primo Maxx and Turf Screen significantly improved turf quality, by 0.4 and 0.3, respectively.
Throughout both the 2016 and 2017 growing seasons, turfgrass coverage was significantly
affected by the various shade levels and timings.
In 2016, 90% shade overall resulted in significantly lower coverage than the other treatment options,
with 90% afternoon shade resulting in significantly lower coverage than morning shade.
In 2017, 90% shade applied in the afternoon was the only treatment to consistently result
in significantly lower coverage than the full sun treatment.
During both years of the study, application of Primo Maxx and Turf Screen generally did
not have significant effect on turfgrass coverage under morning shade conditions.
Under afternoon shade, all spray treatments improved coverage compared with
the untreated control—by at least 11.9% in 2017.
In order to calculate a recommended DLI for minimum acceptable turf quality, turfgrass
visual quality ratings and percent turfgrass coverage were plotted against measured DLI
for each month.
Regression analysis was used to determine what DLI on average would achieve a visual
quality rating of seven, or 90% turfgrass coverage.
Evaluation of both years of data results in a recommended threshold DLI for creeping bentgrass
of 30 moles per square meter per day.
The minimum DLI value of 30 moles per square meter per day determined by
University of Arkansas researchers provides a valuable piece of information to golf course superintendents
managing creeping bentgrass turf.
“This number can help turf managers when they’re assessing the light conditions at their course.
It’s going to be beneficial in the long term to provide them with objective data that
they can provide not only for themselves in making agronomic decisions,
but also to their bosses as well.”
This study found that shade intensities of 70% or greater, even when applied for only
half of the day, caused turf quality to decrease,
with 90% shade causing the most significant deterioration.
During the first growing season, shade timing was also found to play a significant role in turf quality.
“In the first year in 2016, we observed that afternoon shade was more detrimental
to turfgrass health than AM shade.
We didn’t necessarily see that in 2017, but it was an interesting find that we did
see in 2016.”
This study also found that use of both Primo Maxx trinexapac-ethyl growth regulator and
Turf Screen turf protectant may help maintain or improve turf quality, especially during
peak summer stress conditions.
“So while trinexapac-ethyl and Turf Screen did improve turfgrass quality during the late
summer months, based off our statistical analysis, neither one of those applications significantly
lowered the minimum DLI requirement.”
Golf course superintendents can expect to see their creeping bentgrass putting greens
decline in quality if the total light received is less than 30 moles per square meter per day.
Superintendents may want to evaluate shade reduction options if observed DLI values are
consistently less than this threshold.
For more information about The University of Arkansas Department of Horticulture and
their research program and projects, please contact Dr. Wayne Mackay.
Primary funding for this research was provided by the USGA.
Additional funding for this research was provided by a gift from the Arkansas Turfgrass Association
and from the Arkansas Golf Course Superintendents Association.