Modeling accumulated degree-days for the invasive aquatic plant Oxycaryum cubense in the southeastern United States

Cuban bulrush (Oxycaryum cubense [Poepp. & Kunth] Lye) is an invasive floating aquatic plant that causes negative ecological and economic impacts in the southeastern United States. Temperatures in the United States have increased over recent decades which can result in geographic expansion of invasive plants in North America. Accumulated degree-days (ADD) were utilized to develop predictive models (state and regional models) for Cuban bulrush growth from harvested biomass collected over one year in Mississippi, Louisiana, and Florida. Peak emergent biomass occurred from early to mid-fall (September-October) with growth continuing into winter. Accumulated degree days needed for Cuban bulrush to reach peak emergent biomass ranged from 6,469 (Mississippi), 7111 (regional), 7,643 (Florida), and 7,903 (Louisiana). Calendar days needed for Cuban bulrush to reach peak emergent biomass ranged from 292 (Mississippi) to 334 (Florida). Base temperature thresholds for Cuban bulrush were −6 C, −3 C, −3 C, and −2 C for Mississippi, Louisiana, regional, and Florida models respectively. The models suggest Cuban bulrush has a tolerance to lower air temperatures that could allow for survival in moderate winter conditions. Overall, model predictability was less accurate for populations further south (Florida) due to warmer winter temperatures, year-round growth, and difficulty defining when peak emergent biomass occurred. Results from this study indicate that Cuban bulrush growth is greater in warmer temperatures, though low base temperature thresholds suggest this species may be capable of expanding its invaded range to cooler climates beyond the southeastern United States.