A laboratory examination of the effectiveness of a winter seasonal lake drawdown to control invasive Eurasian watermilfoil (Myriophyllum spicatum)

A set of laboratory studies were conducted to identify abiotic conditions associated with winter lake drawdowns that might be capable of sufficiently damaging root systems of Eurasian watermilfoil (Myriophyllum spicatum) to prevent shoot regrowth. Two separate conditions prevented regrowth: exposure of roots in hydrated sediment to a temperature of -5 C for 24 h or longer with a slow thaw over 12 h, or exposure to a sediment water content equivalent to 0.02 m(3)/m(3) for 48 h or longer. Eurasian watermilfoil can therefore be killed by freezing at -5 C or desiccation at 4 C in as short a time as 24-48 h, and a percent relative electrolyte leakage (% REL) of 80% or higher is sufficient to prevent shoot regrowth. The ability to regrow shoots after freezing at -5 C did not correlate to the sediment hydration level. Treatments that did not damage roots sufficiently to prevent shoot regrowth included freezing at -5 C with a 10 cm snow cover above the sediment; freezing at -5 C for 1 d with roots submerged in standing water, as would occur if exposed plants were standing in pooled water prior to freezing; and freezing at 0 C for 4 d. Treatments that resulted in partial root damage but still allowed subsequent shoot regrowth included freezing at -5 C in standing water for 4 d, and freezing at -5 C for 4 d with a rapid thawing of roots. Monitoring of temperature and moisture content of the exposed littoral zone in sequential years indicated that both damaging conditions were observed in the 2009-2010 drawdown season, but neither condition was observed in the 2010-2011 season, suggesting that the interannual variability in response to drawdowns is probably the result of variable annual climate conditions. Monitoring the exposed sediment could indicate if and when conditions suitable for killing root crowns have been satisfied and the appropriate timing to reverse the drawdown.