We’ve all heard horror stories of invasive submersed aquatic plants expeditiously expanding across a water body in what seems like a matter of days. Many of us have likely even seen this with our own eyes. Two of the most well known invaders for their seemingly prolific expansion are Hydrilla and Eurasian watermilfoil (EWM). In fact, a claim made by Langeland (1996) suggests that Hydrilla specifically, can grow an inch per day. Two scientists from the United States Army Corps of Engineers – Aquatic Research and Development have taken on the challenge of investigating such claims. The Journal of Aquatic Plant Management recently published “Does hydrilla grow an inch per day? Measuring short-term changes in stem length to describe invasive potential” by Leeann Glomski and Michael Netherland. Glomski and Netherland highlight the need to investigate changes in plant stem length as a useful technique to explain the spread of such plants. “Infestations are often reported as percent cover or percent frequency…….yet measuring the change in total stem length may be a useful technique for explaining rapid rates of lateral plant spread in a water body.”
A series of mesocosm studies were completed by the researchers to address such growth. The focus of the study were Hydrilla (H. verticillata), Eurasian watermilfoil (M. Spicatum), American pondweed (P. nodosus), and water stargrass (H. dubia). Plants were observed during the summer growing season and several variables, including total length of new growth, number of lateral branches, new stems, and stolons, were assessed. Growth of each species was assessed weekly over approximately 5 weeks, with the exception of EWM, which was assessed over two, five week periods.
The results from the study certainly shed light on the growth of hydrilla over a relatively short period of time. Hydrilla exhibited 7.3 cm (2.9 inch)/day growth in week 1, 29.3 cm/day in week 2, 149.3 cm/ day in week 3 and a whopping 463.1 cm (185 inch)/day and 486.8 cm/day in weeks 4 and 5! In comparison, American pondweed and water stargrass ranged from 1.8 to 32.9 cm/day and 2.4 to 12.6 cm/day over the five week period. Surprisingly, EWM growth was considerably less than that of Hydrilla as EWM never saw more than 31.7 cm (12.6 inch)/ day. The results of this study also makes suggestion on interspecific competition between the two invaders assessed. The large difference between the two would lead us to believe that a bout of hydrilla vs. EWM would be easily won by Hydrilla.