Creeping bentgrass (Agrostis stolonifera) is a cool-season turfgrass species that produces a bimodal growth pattern during the year. Basically, most of the turf growth and development occurs in the spring and fall months, with declines in the summer and winter months. This growth pattern is visibly noticeable with turf shoot growth and clippings. But what about the roots?
To get a closer look at roots, turf managers typically use a cup cutter, soil probe or soil profiler to extract a section or slice and view the roots. These methods provide a good indication of rooting depth but not total root biomass. However, measuring root biomass under field conditions is difficult.
Research was conducted at North Carolina State University (Raleigh), under the direction of Fred Yelverton, Ph.D., with the goal of measuring and quantifying creeping bentgrass roots over time. Specifically, the research examined how roots were affected in creeping bentgrass maintained at three mowing heights and treated with monthly applications of two plant growth regulators.
The study site was a 2-year-old stand of Penncross creeping bentgrass grown in a sand-based root zone, and did not contain any annual bluegrass contamination. The site was fertilized to maintain adequate growth, treated with fungicides to prevent disease and was vertically mowed and sand topdressed each spring to minimize thatch accumulation.
This two-year, replicated field trial consisted of plots measuring 5 feet × 15 feet (1.5 meters × 4.5 meters). The creeping bentgrass was mowed five times per week at three heights-of-cut of 0.125, 0.155 or 0.190 inch (3.2, 4.0 or 4.8 millimeters). The two plant growth regulators were trinexapac-ethyl and paclobutrazol, applied at label use rates. Mowing began in March, and plant growth regulators were applied in April and every four weeks after that. Trinexapac-ethyl was applied first to allow for foliar absorption, then paclobutrazol was applied and watered in to facilitate root uptake.
To measure and quantify roots, two sod and soil cores were extracted from each plot every month. Each piece measured 110 cubic inches (1,800 cubic centimeters). With each sample, roots were separated by carefully removing the turf at the thatch/soil interface. Next, the root zone sample was tediously and skillfully washed over a mesh screen to remove soil particles while retaining the roots. Clean root samples were oven-dried at 150 F (70 C) for 96 hours, then placed in a muffle furnace at 932 F (500 C) for 12 hours to burn off any organic matter. The result was a measure of the actual root biomass.
To use the young person’s vernacular, “Where my roots at?” Overall, creeping bentgrass mowed at 0.125 inch had consistently lower root biomass compared to the turf mowed at 0.155 or 0.190 inch. Specifically, turf mowed at 0.155 inch had 32% more roots, and turf mowed at 0.190 had 36% more roots, versus turf mowed at 0.125 inch. Thus, the research indicated a higher height-of-cut as a favorable management strategy, particularly during severe summer stress periods.
The largest amount of roots was measured in May, with 14% of the total annual root biomass, and the lowest amount in September, with 3% of the total annual root biomass. The root biomass measured from May through September represented a 76% decline in rooting from May through September, thus confirming the need for plenty of healthy and growing roots during the spring before going into the summer.
With all three mowing heights, creeping bentgrass treated with either plant growth regulator at the labeled application rates did not adversely affect root growth. Also, keep in mind that the variation in air and soil temperatures over time influences turfgrass root growth and development.
Yelverton began his academic career in tobacco research, but since 1995 he has focused on applied turfgrass weed research at North Carolina State University. Thank you, Dr. Yelverton, for your contributions to turfgrass science, and best wishes on your retirement!
Source: Fagerness, M.J., and F.H. Yelverton. 2001. Plant growth regulator and mowing height effects on seasonal root growth of Penncross creeping bentgrass. Crop Science 41(6):1901-1905 (https://doi.org/10.2135/cropsci2001.1901).
Mike Fidanza, Ph.D., is a professor of plant and soil science in the Division of Science, Berks Campus, at Pennsylvania State University in Reading, Pa. He is a 23-year member of GCSAA.