Japanese knotweed (Reynoutria japonica) is a highly invasive plant in the United States. It has previously been thought to only reproduce asexually in its introduced range (i.e. spreading through rhizome fragments). However, it has recently been discovered that knotweed and its hybrids can produce viable seeds. In order to understand why knotweed seedlings are not common in existing stands, we first tested the germination of seeds collected from Pittsburgh and Stroudsburg, Pennsylvania under conditions under varying light conditions, with four light levels ranging from complete darkness to full light. 6 replicates of 20 seeds were tested in each light condition and each population.
We then designed a growth chamber study to test how light and soil feedback affect seedling development. 162 Knotweed seedlings were grown in one of three light conditions (full light, partial shade and full shade) with either sterile soil, soil inoculated with soil collected from a knotweed stand, or soil collected from an uninvaded riparian area. Height, and leaf dimension measurements were taken approximately every 10 days to document growth.
Results/Conclusions
The germination experiment showed that light does not affect the germination of knotweed seedlings. In all cases germination rates were high, with an average germination rate of 96.8%. There was no effect for light, meaning that germination is not the limiting factor restricting sexual reproduction. The growth chamber study showed that knotweed seedling growth is affected by both light and soil feedback. Seedlings in the full shade condition were smaller than seedlings in the full light and partial shade conditions. Seedlings grown in full shade had a 79% reduction in leaf length compared to the full light condition, and those grown in partial shade showed a 15% reduction in leaf length. Effects from the soil are significant only in the full light condition. Knotweed seedlings grown in the knotweed soil showed a 12% reduction in height compared to those grown in sterile soil and a 7% reduction compared to those grown in the riparian soil. Future work will analyze how light and soil affected plant mass and rhizome development These results indicate that knotweed seedlings would struggle to develop under the canopy of their own stand, leading to the lack of seedlings and genetic diversity in knotweed stands.