Vegetative overwintering and viable seed production explain the establishment of invasive Pistia stratiotes in the thermally abnormal Erft River (North Rhine-Westphalia, Germany)

• We studied the invasive alien aquatic plant Pistia stratiotes within the Erft River.
• We documented both vegetative over wintering and viable seed production.
• Heat images showed the effect of high water temperatures on plants during winter.
• We documented the massive drift of >6000 Pistia plants per day into the Rhine River.

Pistia stratiotes, a native of South America, has been established in the thermally abnormal Erft River (a tributary of the Rhine River) in Western Germany since 2008. The species formed mass developments since then. Plants mainly overwinter as small flat plants, which are protected against air frost temperatures due to high water temperatures of >11 °C. Heat images document that during winter the temperature of flat leaves was mostly less than 2 °C below water temperature, while large leaves above the water surface have been killed by air frost. During all years, numerous flowering plants and viable seed production were observed. We have no direct evidence of seed germination in the field, but under laboratory conditions seeds showed a high germination potential even after exposure to −4 °C. The high number of >6000 drifting plants per day into the Rhine River demonstrated the high potential for this species for spread into the Rhine River and connected water bodies, and this threat potential stresses the paramount need for effective management strategies.