By Grady Miller1, Drew Pinnix1, and Casey Reynolds2
1North Carolina State University, Raleigh, NC and 2Texas A&M University, College Station, Texas
Athletic fields are defined by lines and enhanced with logos. This makes painting a ritual repeated every week during the playing season for thousands of fields worldwide. Athletic paints are formulated with the intent that they will not cause harm to the turfgrass when properly applied, yet most sports turf managers have experienced some level of paint-induced damage during their career. While there is still a lot to learn about paints and painting, North Carolina State University has been testing products and application methods for several years to better understand why paints damage turfgrasses.
Before discussing paint and how it can influence turfgrass health, it may be helpful to understand a few components of paint. Athletic paints are very similar to common household paints in basic ingredients. They are generally made of four components: binder, solvent, pigment, and additives. The binder (or resin) is a film-forming component of paint that binds pigments together and allows them to adhere to a surface. The solvent in turfgrass paint is water. The pigment is an organic or inorganic particle that provides colour. Additives may be surfactants, thickeners, emulsifiers, etc. that give paint particular properties that make it easier to handle, mix and extend shelf life. Athletic paints have less volatile components than household paints and consequently there is no need for anti-microbial or algaecide components.
The opacity of the pigment in the paint that covers the turfgrass’ surface alters the normal micro-climate around the plant. Although research has shown that paint may positively influence turfgrass, it routinely has the opposite effect. After repeated applications of athletic paint, we typically see a decrease in turfgrass quality, or in extreme cases, complete death. A positive influence has been noted during cooler, sunny weather when a paint-darkened surface can result in heating of the turfgrass plant which may produce limited growth enhancement. However, in most situations paint provides an additional stress. When considering the “quality” of a turfgrass, we look at the combination of turfgrass colour, cover, and density. The decrease in quality that is seen after multiple applications of athletic field paint is mostly due to a decrease in turfgrass stand density as the frequency of athletic paint applications increase.
Like most plants, turfgrasses are unable to survive without light. Light is emitted from the sun through wavelengths of particles known as photons. There are a broad range of wavelengths that determine the energy state of light that is emitted from the sun but we are only interested in photosynthetically active radiation (PAR) which is the group of wavelengths (400-700 nm) that is visible to the human eye. Most importantly, PAR is also the range of light that is used by plants. PAR is separated into three different colours of light. The 400-500 nm range is considered to be blue light, the 500-600 nm range is green light, and the 600-700 nm range is red light. So what happens when light strikes the leaf of a turfgrass plant? Light that comes in contact with the leaf surface is transmitted, absorbed, or reflected. From a plant perspective, the best scenario is to maximize the amount of light that is transmitted through the paint since that will be directly utilized by chlorophyll. PAR is separated into these different colours because when light strikes a chlorophyll molecule, it reflects these colours at their respective range of wavelengths. This is why when we look at a turfgrass plant we see the colour green. Reflected light may be partially transmitted as well, as it can be reflected to other areas of the turfgrass canopy and transmitted there. Absorbed by the paint, light can provide heat, but is largely lost for photosynthesis.
Turfgrasses use PAR to produce carbohydrates that provide energy and food storage compounds. The reaction that produces these carbohydrates is known as photosynthesis. For this light driven process to occur, plants exchange water vapour for carbon dioxide through transpiration. Photons of light excite chlorophyll molecules that are housed in the chloroplast of plants and as a result, initiate a chain of reactions that enable the turfgrass plant to capture carbon dioxide and convert it into usable forms of energy such as carbohydrates.
Before our research it was not well known how paint colours would influence light reflection, transmission, and absorbance. Our research indicated that lighter colours such as orange, yellow and white could transmit between 12 and 18 percent of available PAR while reflecting 47 to 93%. Darker colours such as blue, green, maroon, purple, and black transmitted and reflected much less (0 to 8%) while absorbing up to 95% of PAR. Our research found that much of the difference was due to the innate properties of the pigment colour and part was due to the percent of solids by volume. So the formulation and colour can impact light transmission.
A good test of these transmission results was to measure total canopy photosynthesis. This would account for the high degree of reflected light for a light-coloured paint (e.g., white paint) that may still be used in photosynthesis despite a poor transmission percentage. As a percentage, leaves that were painted white maintained approximately 80% of the photosynthesis of the non-painted. Yellow and orange was about 70%; whereas red was about mid-50s. Purple and maroon were about 40%. Dark blue and black maintained less than 20% of the non-painted. So, the shading effect of paint by colour turned out to be a very real and significant limitation to the plant.
As mentioned earlier, in addition to colour, dilution can also play a role in total canopy photosynthesis. When comparing both white and red athletic paint (diluted and non-diluted) we found that the non-diluted formulation can have a profound effect. White and red non-diluted paint decreased total canopy photosynthesis by 25% more than when it was diluted using a 1 part paint:1 part water dilution. Red, non-diluted paint decreased total canopy photosynthesis by 75%. The reduction in pigment per unit area reduces the opacity so that more light can be transmitted to the chlorophyll. While this may be beneficial to photosynthesis and plant health, diluting paint with water often reduces the brightness, coverage, longevity, and quality of paint applications. As a result, the need for brightness and uniformity must be balanced against the effects on plant health and may vary based on a particular setting.
Without adequate photosynthesis the plant cannot maintain necessary metabolic functions. Athletic paint coating the pores (stomata) on turfgrass leaf surfaces compounds the issue. If the plant cannot freely lose water and take in carbon dioxide from these pores, then canopy temperatures can rise to become an additional stressor. The colour of athletic paint can have a great effect on turfgrass transpiration. In unpainted bermudagrass, water loss increases with canopy temperatures, but it was just the opposite with painted leaves. We typically found that the darker colours affected transpiration to a larger extent. Turfgrass coated with lighter colours such as white, yellow, and orange had similar but slightly lower transpiration rates than the unpainted turfgrass. The darker colours including red, blue, and black showed much higher canopy temperatures and much lower transpiration rates. These results mirrored what we saw with total canopy photosynthesis in that the darker coloured athletic paints have a greater negative effect on turfgrass physiological processes.
It is important to note that shading by paint pigments may not always be detrimental. Cool-season grasses grown in northern climates often do not utilize all of the light that is possible for photosynthesis. For example, ryegrasses have a relatively low light requirement and the daily light integral on athletic fields with little to no shade may be sufficient to drive adequate growth,
even when accounting for shading effects by paint. Furthermore, regular use of the field and mowing may remove some of the paint from the leaf’s surface. The plant will also generate new growth that is more efficient at utilizing the light. Of course, the next coat of paint may soon follow the emergence of this new growth. The result is that chronic paint use, especially darker colours,is even more damaging.
We looked at application rates and timing in relation to mowing. The results indicate that earlier removal of the paint is beneficial to the plant. We found that one less mowing during the week provided faster recovery. Although a higher frequency of mowing would suggest that more of the paint is being removed from the turfgrass leaf blade, the area of the plant that is not coated with paint (new growth) is also being mown off. Allowing new tissue that is not coated with paint an extra day to expand will increase the leaf area that is able to actively photosynthesize and as a result, promote faster recovery. The timing of athletic paint application had a greater effect on the quality of the painted surface compared to recovery of the turfgrass over time. As expected, earlier applications of paint were of reduced quality compared to applications made later in the week (closer to “gameday”). Furthermore, we found that paint applications made earlier in the week may minimally increase turfgrass recovery, but not to the degree that would merit sacrificing the overall appearance of the paint application. As mentioned before, an athletic field manager may need to balance plant health and quality of paint appearance.
Also some limited work on binder concentrations looked very promising for alternative formulations that would be less damaging. Remember that binder is the ingredient in paint formulations that is responsible for the paint’s ability to adhere to the leaf blade, so manipulating the concentration of binder can affect how long the paint will “stick” around. One issue that must be addressed when looking at alternative formulations in terms of binder concentration, is the susceptibility of the paint to transfer onto an absorbent material, i.e. an athlete’s uniform. This can be very problematic as this may result in increased staining of uniforms. The severity of this issue may be anywhere from a minor laundry matter or as severe as a need for total uniform replacement.
The painting of athletic fields is a ritual that has evolved tremendously as sports have become more and more popular over the years. Not only are athletic paints needed for boundary lines and other field markings for playability of the game, paints are also needed for advertising logos and brand marketing. Logos may not be required for successful completion of a sporting event, but at the time of high-definition television and major companies seizing opportunities to market their brand, athletic field painting will continue to be a major part of athletic field management.
We have identified some of the underlying negative effects that athletic field paint has on natural turfgrass surfaces. While there is not much likelihood of teams changing their colours to help minimize turfgrass decline, there are ways to help combat the harmful effects. •