There is increasing evidence that some of the most successful and troublesome invasive plants can suppress the growth of their native competitors through allelopathic effects. One of the most dominant and at the same time least understood group of plant invaders in North America and Europe are the clonal knotweeds Fallopia japonica, F. sachalinensis and their hybrid F. x bohemica. Lab assays have shown that exotic knotweeds contain several bioactive compounds, and that plant extracts can suppress the growth of other plant species. However, we still do not know whether allelopathy really plays a role in the invasion success of exotic knotweeds. To thoroughly investigate the ecological significance of Fallopia allelopathy, and its underlying ecological and evolutionary mechanisms, we are undertaking a series of complementary ecological experiments. In one experiment, we tested for general allelopathic effects of Fallopia x bohemica on experimental communities of six native European plant species.

Results/Conclusions

We found that addition of activated carbon to the soil greatly reduced Fallopia rhizome biomass, and that it reduced the suppressive effect of Fallopia on native forbs by 35%. This positive effect of activated carbon was similar to that of regular cutting of Fallopia shoots, which suggests that Fallopia indeed exerts strong allelopathic effects on native forbs. These effects must act on the growth rather than the germination of natives, since we did not find any evidence for decreased germination rates or germination times in a follow-up experiment in which we tested whether Fallopia leachates and soil residues affected the germination of several native species. Other, ongoing follow-up experiments investigate the effects of litter leachates and soil residues on later life-history stages of natives, the role of soil biota in mediating allelopathic effects, and the consequences of knotweed hybridisation for the species’ allelopathic potential and invasiveness.