Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant and Environmental Science

Committee Chair/Advisor

L. B. McCarty

Committee Member

W. C. Bridges

Committee Member

T. Whitwell

Committee Member

M. Cutulle


Annual bluegrass (Poa annua L.) is one of the most problematic winter annual weeds in managed turfgrass systems, having rapidly adapted to different climates and management practices. Characteristics such as prolific seedhead production, clumped bunch-type growth habit, and lack of stress tolerance negatively impact turfgrass quality. Achieving adequate control can be difficult because annual bluegrass exhibits high levels of genetic diversity, the ability to germinate year-round, and has a high tendency for evolving herbicide resistance, currently resistant to at least nine different herbicide sites of action. Current management programs rely heavily on herbicides for control; however, frequent use without implementing other management methods has led to widespread herbicide resistance. Therefore, this research aimed to survey and document the extent of annual bluegrass resistance to key turf herbicides, develop methods for rapid resistance detection, fill knowledge gaps in annual bluegrass biology and ecology, and evaluate cultural management tactics.

Field surveys were conducted across South Carolina to determine the distribution of annual bluegrass resistance among escaped plants in four turfgrass systems (golf, athletics, sod production, and residential lawns) and collect samples for subsequent herbicide resistance evaluations. The number of samples collected from each turfgrass sector is as follows residential lawns (2), golf courses (53), sod production (7), and athletic fields (3) for a total of 65 samples collected in 2019 and 2020.

One study assessed prodiamine and indaziflam preemergence herbicides’ effect on resistant (R) and susceptible (S) annual bluegrass biotype germination. Germination, coleoptile length, and radicle length increased in prodiamine concentrations from 0 to 0.01 mM but decreased as concentration increased from 0.01 to 10 mM. At the 0.01 mM prodiamine concentration, germination was >68%, and coleoptiles were 54% longer for the S biotype than R biotype. However, the S biotype was completely inhibited at 10 mM prodiamine concentration. Similarly, germination, coleoptile length, and radicle length decreased as indaziflam concentration increased. Complete inhibition occurred for the S biotype in indaziflam concentrations from 0.1 to 10 mM, and the R biotype was inhibited at concentrations from 1 to 10 mM. These results do not indicate or confirm annual bluegrass resistance to prodiamine or indaziflam.

To verify and quantify the level of resistance to ALS-, EPSPS-, and glutamine synthetase-inhibiting herbicides, whole-plant dose-response experiments were conducted on mature annual bluegrass plants using trifloxysulfuron, glyphosate, and glufosinate. Biotypes were chosen from populations categorized as resistant in initial herbicide resistance screens conducted on populations from SC and 13 other states, including TX, MS, AL, GA, FL, TN, NC, VA, OR, IN, NJ, and PA, with a known susceptible population and non-sprayed control maintained for each batch as a check. For the ALS dose-response assay, GR50 values were 2.13 g trifloxysulfuron ai ha-1 for the susceptible biotype, and values ranged from 7.62 to 55.86 g trifloxysulfuron ai ha-1 for various resistant biotypes, especially FL-4-GC-2 and TX-4-GC-3. For the EPSPS dose-response assay, GR50 values for susceptible biotypes were 0.08 and 0.17 kg glyphosate ha-1 and 0.27 and 0.07 kg glyphosate ha-1 for resistant biotypes, respectively. However, according to the extra sum-of-squares F-test, GR50 estimates were not significantly different (p < 0.05) between susceptible and resistant biotypes. For the glutamine synthetase dose-response assay, GR50 values were ~0.22 kg glufosinate ha-1 for suspected susceptible and resistant biotypes. Values were not significantly different (p < 0.05) according to the extra sum-of-squares F-test.

A replacement series experiment was conducted to ascertain if growth or competitive differences occur between herbicide (glyphosate) resistant (R) and susceptible (S) biotypes. Plant height and shoot dry weight were greater in a monoculture of the R biotype than in a mixed or monoculture of the S biotype. However, biotypes only differed in height and weight by 3.84 cm and 0.0224 g, respectively. Differences in seed production trends (number of inflorescences, inflorescence length, and seed weight per 100 seeds) were insignificant. Therefore, glyphosate resistance only results in minimal alterations to annual bluegrass growth with little significant impact on seed production trends.

Growth chamber experiments were conducted to investigate whether chlorophyll fluorescence (FV/FM ratios) could be used to rapidly detect glyphosate-resistant annual bluegrass biotypes as an alternative to time-consuming and labor-intensive greenhouse methods typically used for herbicide resistance screening. Visual control (%) was used as a comparison to determine if FV/FM values could be used to distinguish resistant from susceptible biotypes, and separation in visual control between the three biotypes was similar to separation observed for mean FV/FM values for the 1x herbicide rate 21 DAT. Thus, measuring and comparing FV/FM values (chlorophyll fluorescence) appears to have potential as a method for distinguishing between resistant and susceptible biotypes at 1x and 2x herbicide rates once sufficient time has passed after treatment and visual control has occurred between biotypes.

Annual bluegrass seedbank persistence was evaluated using a classical seed bag burial experiment conducted in a managed turf field in Clemson, SC. After 6 months, germination of all populations combined was 60% in dry storage compared to approximately 0.3% to 5% at the surface (0 cm) and 5 cm depths, respectively. After 1 year, germination increased overall to 75% in dry storage, 22% at the 5 cm depth, and 9% on the surface. Germination was higher for all populations retrieved from the 5 cm depth, except for TN. After 1.5 years, germination declined to 73% in dry storage, 3% on the surface, and 12% at the 5 cm depth. After 2 years, germination was only observed for populations from NJ (10.3%) and TN (22.5%) retrieved from the surface. These results suggest peak annual bluegrass seed germination occurs in South Carolina approximately 1 year after viable seeds are introduced to the soil seed bank. Seed burial depth appeared to have a significant effect on germination in this study, with lower germination on the surface than at the 5 cm depth.

A classical common-garden study was conducted in Clemson, SC, to shed light on annual bluegrass growth cycle, time needed for different developmental stages influenced by the environment, and specific traits potentially favoring adaptation. Establishment (77.5%) and tillering (1.50) were less for the FL population compared to other populations, with plants from TN having the greatest number of tillers (3.25) and plant vigor (5.88). In contrast, plants from TX had the lowest plant vigor (3.06). Populations from cooler, northern climates were observed to have less seed-shattering potential and seed rain (produced) prior to harvest, thus, suggesting these populations have a longer life cycle than those from warmer climates. Conditions appeared unfavorable for populations from AL, FL, IN, NJ, OR, and TX, characterized by lower biomass and fewer tillers per plant. The estimated total seed produced by a single plant was highest for the NC population and lowest for the PA population, ranging from 729.8 to 4018.2 seeds. Differences in growth attributes between the annual bluegrass populations tested support high genetic diversity within the species.

To understand the importance of turfgrass species and variety selection as a cultural tactic for annual bluegrass management, laboratory and greenhouse experiments were conducted to determine whether soil leachate collected from selected turfgrasses (centipedegrass and St. Augustinegrass) affects annual bluegrass seed germination. Annual bluegrass and crabgrass (year 1) germination was not affected by centipedegrass and St. Augustinegrass soil leachates. Goosegrass germination was higher in fertilized, non-fertilized, and distilled water controls than centipedegrass and St. Augustinegrass leachates, and crabgrass germination in year 2 was higher in fertilized (52.3%) and nonfertilized (50.3%) control leachates compared to 37.4%, 38.9%, and 36.6% in centipedegrass and St. Augustinegrass leachates and distilled water control, respectively. Crabgrass hypocotyl lengths grown in fertilized and non-fertilized leachates in year 1 were approximately 24-29% longer than in turfgrass soil leachates and distilled water control. Longer radicle lengths were found for annual bluegrass plants grown in centipedegrass (43.7 mm) and St. Augustinegrass (45.2 mm) leachates compared to the controls (39.4, 39.0, and 38.8 mm in fertilized, non-fertilized, and distilled water controls, respectively). Chemicals may have been responsible for higher germination and longer hypocotyl and radicle lengths however, this is not likely due to similar increases in some parameters observed in non-fertilized and distilled water controls. Furthermore, similar hypocotyl lengths from leachate treatments indicate low to no allelopathic effects from centipedegrass and St. Augustinegrass leachates on the indicator species tested. Results, however, support previous research where goosegrass germination was reduced in centipedegrass soil leachates.

To evaluate the effect of aqueous centipedegrass and St. Augustinegrass leaf extracts on annual bluegrass germination and growth, three seeded indicator species (annual bluegrass, crabgrass, and goosegrass) were germinated in both turfgrass leaf extracts and a distilled water control. Centipedegrass and St. Augustinegrass aqueous leaf extracts leaves did not affect the germination of annual bluegrass, goosegrass, or crabgrass in year 1. However, centipedegrass extracts did reduce crabgrass germination in year 2 by 57%. Hypocotyl length was longer when grown in centipedegrass and St. Augustinegrass extracts in year 1 but in year 2, centipedegrass extracts reduced annual bluegrass and goosegrass hypocotyl length. Radicle length was decreased by both turfgrass leaf extracts for annual bluegrass and crabgrass species compared to the control during both years, whereas St. Augustinegrass extracts reduced goosegrass radicle length.

To investigate relative differences in the establishment of annual bluegrass in established turfgrass stands maintained at three different mowing heights, two warm-season turfgrass (centipedegrass and St. Augustinegrass) with various mowing heights imposed were overseeded with annual bluegrass seed. Annual bluegrass shoot dry weight was reduced by approximately 72% and 79% in year 1 and 93% and 94% in year 2 in centipedegrass and St. Augustinegrass turf, respectively, compared to the bare soil control. Although annual bluegrass emergence and growth appeared to be suppressed by both turfgrasses, no significant differences were found between mowing heights, even though a slight decrease in shoot dry weight was observed. Mowing height was only observed to significantly affect the shoot dry weight of annual bluegrass grown in St. Augustinegrass in year 2, decreasing approximately 64% from high (0.14 g) to low (0.05 g) mowing height.


The research outlined in this dissertation was conducted as part of a multi-state cooperative project funded by the USDA-NIFA Specialty Crops Research Initiative (SCRI) program (award #: 2018-51181-28436) to address the herbicide resistance epidemic of annual bluegrass in managed turfgrass systems.

Included in

Weed Science Commons



To view the content in your browser, please download Adobe Reader or, alternately,
you may Download the file to your hard drive.

NOTE: The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS and if you are using a modern (Intel) Mac, there is no official plugin for viewing PDF files within the browser window.