Q & A WITH

DR . GRADY MILLER

We do not use our football field much after the fall season , so we decided to not overseed it this year so we can better tackle some problem winter weeds . My question is not about the weeds , but why our bermudagrass cuts so poorly late in the fall ? We have used the same cutting height all year , but once we got into September it seems to scalp . Is there something we should change so that it cuts more uniformly without scalping ?

This is actually a pretty common question

I get from turfgrass managers and homeowners who have bermudagrass . What has changed is the quantity and quality of your sunlight . Solar radiation , the energy from the sun , fuels the photosynthesis process , which converts carbon dioxide into sugars and carbohydrates that are used by the plant . Bermudagrass loves long days with lots of light and heat . Starting in late summer to early fall , the relationship of earth to the sun limits the light and heat it needs for heathy growth .

Solar radiation comes in three forms — ultraviolet , visible and infrared . Most of the light that reaches the earth is infrared and visible light . Infrared light is largely responsible for warming Earth ’ s surface , whereas visible light ( known as photosynthetic active radiation or PAR ) is the light wavelength range that is best for photosynthesis .

Solar radiation is the greatest when the sun is directly perpendicular to the Earth ’ s surface , such as midday in summer . When the sun ’ s rays hit the Earth at an angle , the sun ’ s rays are spread out over a larger area , so there is less energy per unit area . The sun ’ s angle increases the amount of ozone through which the light has to pass . This angle does not only change with the season , but also changes during the day . Solar radiation in the morning and evening has to pass through more of the atmosphere , which reduces its irradiance . Also , cloud cover will scatter the sun ’ s rays , decreasing irradiance . This is why we sometimes see reduced mowing quality after extended cloudy weather , even in the summer .

Solar irradiance is the intensity of solar radiation per unit area . As solar irradiance increases , plants experience higher rates of photosynthesis . It is like the difference

between a 20-watt light bulb and a 100-watt light bulb : both produce light , but the brightness and intensity are different . The greater irradiance over time also causes the Earth to warm due to conduction and convection . In the presence of visible light , increased temperature speeds up photosynthesis — thus heat plays an important role in growing dense , healthy turfgrass .

With reduced light intensity and shorter day length in the fall , the bermudagrass will begin to increase internode length and reduce tillering . The individual plants are using their limited resources to grow taller so that the plant may better intercept the limited sunlight that is available . Over time , the overall result is a thinner stand of turfgrass with longer , thinner leaves . That is why the turfgrass will scalp even though you have continued to use the same mowing interval and height of cut . You have several options to better manage this situation . 1 . Since we know there is a relationship between light and temperature , grow covers can increase heat in the canopy — therefore increasing photosynthesis efficiency .

2 . Reduce the plant from elongating . Plant growth regulators have shown efficacy in reducing vertical growth while concentrating chlorophyll necessary for more efficient photosynthesis .

3 . Increase light levels . This can be anything from reducing tree shade around the edge of open fields to using grow lights to supplement natural light . The more light that can be added , the better .

4 . Reduce your mowing interval while increasing the height-of-cut as light levels decrease in the fall . This can be challenging to manage , but can be effective for early fall . If it still scalps to an unacceptable level , using a turfgrass colorant can aid in masking the symptoms . SFM

Grady Miller , Ph . D . Professor and Extension Turf Specialist North Carolina State University

Questions ? Send them to Grady Miller at North Carolina State University , Box 7620 , Raleigh , NC 27695-7620 , or e-mail grady _ miller @ ncsu . edu

Or , send your question to Pamela Sherratt at 202 Kottman Hall , 2001 Coffey Road , Columbus , OH 43210 or sherratt . 1 @ osu . edu

50 SportsField Management | November 2023 sportsfieldmanagementonline . com