Proceedings of the Sixty-first Annual Meeting of the Northeastern ...

101
CRITICAL WEED-FREE PERIOD FOR OVERSEEDED BERMUDAGRASS IN
NORTHERN CLIMATES. B.W. Compton and S.D. Askew, Virginia Tech, Blacksburg.
ABSTRACT
In the transition zone, it is common to overseed warm season bermudagrass (BG)
(Cynodon dactylon) with cool season perennial ryegrass (PRG) (Lolium perenne) to
improve winter aesthetics on golf courses. Most golf revenue is generated during spring
and early summer due to ideal weather conditions. PRG is needed to provide desirable
quality and playing conditions for fairway turf but competitively injures BG during this
period. BG has the ability to recover from PRG competition given enough time during the
summer. It has been suggested that healthy BG needs 100 days of weed-free growth in
summer, yet research has not been conducted to validate or test this claim. “Healthy” BG
is a subjective term that is usually based on biomass accumulation, total nonstructural
carbohydrate (TNC), and ability of plants to survive stresses such as cold, heat, or UV
light. Our objective is to measure how duration of PRG competition influences BG health.
Studies were conducted in Blacksburg, VA on Patriot BG at VA Tech’s Glade Road
Research Facility and on Midiron BG at the Turfgrass Research Center. Foramsulfuron
(Revolver) at 17 oz/A, was applied at weekly intervals for 24 weeks between April 4 and
August 29, 2006. To assess BG “health”, BG and PRG cover was visually evaluated on
September 29 and 80cm 2 plugs of turf were collected from each plot on October 15 to
assess dry biomass, TNC, and electrolyte leakage (following cold stress). Electrolyte
leakage and TNC data are still being collected and will be discussed later. Data were
subjected to analysis of variance using a repeated measures technique and regressions
were used to describe effects of BG weed-free period on measured responses.
Green shoots were evident on Patriot and Midiron BG on March 25 and April 6,
respectively, and both cultivars stopped growing at first frost on October 1. Therefore, the
greatest possible growing season was 178 to 190 days, depending on cultivar. However,
growing degree days at base 65 (GDD 65 ), the typical growth model for BG, were only
accumulated between May 26 and September 24, a period of 121 days. PRG left to
compete with BG beyond July 25 (less than 68 weed-free days) reduced BG visual cover
10 to 20% and 13 to 35% in Patriot and Midiron BG, respectively. As the duration of
weed-free growth increased, BG above-ground dry biomass increased from 777 to
1322g/m 2 for Patriot and 350 to 525g/m 2 for Midiron. Patriot dry biomass increased
linearly at a rate of 3g/m 2 for each additional day of weed-free growth while Midiron
biomass exhibited a hyperbolic response with an asymptote at approximately 89 days.
This differential biomass accumulation indicates that Patriot continues to compete with
ryegrass and grow during the entire season while Midiron does not. Thus, Midiron has a
higher requirement for weed free period than Patriot. The 89-day asymptote for Midiron
biomass accumulation represents 309 GDD 65 out of a total of 621 GDD 65 for the entire
season. Although the TNC and electrolyte leakage data yet to be collected represent the
most important indicators of BG health, we can tentatively conclude that 100 days of
growth is a safe assumption for less competitive cultivars like Midiron but may be overly
conservative for cultivars like Patriot.
83