By Florence Sessoms
As a homeowner, I (my husband) mow our lawn regularly with the simple focus of keeping it at lower height. However, several of my neighbors love to create patterns every time they mow and these patterns can be as elegant as those observed in many sports fields. I love seeing these different stripes of grass color when I watch a soccer on TV, when I visit training facilities (Figure 1) and when I go watch our Minnesota United play (Figure 2).
Like every plant, turfgrass leaves possess two different sides: the upper side is called adaxial and the lower side is called abaxial. These sides each have different tissue morphology in term of trichomes (leaf hairs), the presence of waxes, and the number of stomata. In turfgrass species, the number of stomata (where gas exchange occurs) can be very different between the two sides; fine fescue species do not have any stomata on their abaxial side while for other cool season grasses, the number of stomata is lower on the abaxial side than for the adaxial side (Green et al., 1990). Similar differences have been found for warm season grasses (Atkins et al., 1991; Green et al., 1991).
In certain species such as perennial ryegrass, the two sides have very different colors and textures (Figure 3). For perennial ryegrass, it seems that the upper side is lighter and that the lower side is glossier.
A common misconception is that the stripe pattern observed after mowing is a result of re-arranging the side of the leaves during the mow. This would mean that all leaf blades would become arranged with the same side facing up or down. The probability of this happening is low. In fact, the mowing pattern observed is the result of light reflection, the size of surface area on which the light can be reflected and where the viewer stands.
If you stand behind the mower, as the grass is being mowed, the grass will be pushed backward (your position) increasing its surface area where sunlight can be reflected creating a lighter green. Now imagine you are standing in front of (facing) the mower during a mow, here the grass will be pushed toward you, decreasing its surface area where the light is reflected leading to a darker green.
I decided to run a quick experiment to confirm this explanation--I tested different leaf angles and observed the color differences. In a dark room, I placed several leaves of perennial ryegrass and Kentucky bluegrass leaning backward or forward; the source of light came from an artificial light behind the camera (Figure 4). Both upper and lower leaf sides were tested.
The results can be seen in Figure 5.
For all species, regardless of which side of the leaf was observed, leaning backward resulted in a lighter color because the leaf surface area where the light could be reflected was larger. However, when all leaf blades were leaning forward toward the camera, all of them appeared darker, since less surface area of the leaves was in contact with light.
In conclusion, the angle of the leaf blade facing the light is responsible for the lighter or darker color. The stripe pattern can be re-enforced with higher heights of cut, (more surface), species with leaves that can bend easier (cool season > warm season grasses) and the use of a roller after the mow.
References
Atkins C.E, Green R.L., Sifer S.I., and Beard J.B. (1991). Evapotranspiration rates and growth-characteristic of 10 St. Agustinegrass genotypes. HortScience, 26(12): 1488-1491.
Green R.L., Beard J.B., and Casnoff D. M. (1990). Leaf blade stomatal characterizations and evaopotranspiration rates of 12 cool-season perennial grasses. HortScience, 25(7): 760-761
Green R. L., Sifers, S.I., Atkins, C.E., and Beard, J.B. (1991). Evapotranspiration rates of 11 Zoysia genotypes. HortScience, 26(3): 264-266.