The Asian, annual, herbaceous plant Persicaria longiseta (Asiatic Waterpepper) is now a widespread alien, invasive species in Europe and North America. I found two previously unknown, very rare dwarf forms — a white-flowered one with narrow leaves and pink-flowered one with wide leaves — in a lawn and adjacent flower bed in Bethesda, Maryland in 2015. There were only about 15 plants of each form in their populations. In an attempt to understand their evolution, I tested hypothesis-1 that the plants’ dwarfism is genetically determined and hypothesis-2 that they produce seeds without animal pollination. To test hypothesis-1, I grew the dwarfs and the usual tall form from seeds in a common garden. To test hypothesis-2, I grew three plants of each dwarf form in a greenhouse without animal pollinators.
The two dwarf forms and taller form grew to their usual heights from seeds in their common garden (N = 10 of each kind). This indicates that their heights have a genetic basis. Both dwarf forms, grown without animal pollinators in a greenhouse, developed mature seeds, indicating that they produced seeds parthenogenetically, by intrafloral self-pollination, or both. Possible steps in the evolution of these dwarf forms include (1), lawn-mowing, browsing by Odocoileus virginianus (White-tailed Deer), or both selected for short plants that could produce seeds; and (2) the dwarf plants retained their dwarfism genetics in their tiny populations via selfing, parthenogenesis, or both. I did not see Odocoileus virginianus browsing on non-mowed, dwarf plants in the flower bed. A third known P. longiseta dwarf form occurs in Nara Park, Japan where it might have evolved under browsing pressure from Cervis nippon (Sitka Deer) (Suzuki, 2008).