Bohemian Knotweed Reynoutria × bohemica Chrtek et Chrtková Seems Not to Rely Heavily on Allelopathy for Its Persistence in Invaded Sites in the Southwest Part of the Zagreb, Croatia

Notorious invasive Bohemian knotweed Reynoutria × bohemica Chrtek et Chrtková is a hybrid of two species, Reynoutria japonica Houtt. and Reynoutria sachalinensis (F. S. Petrop.) Nakai in T. Mori which spontaneously developed in Europe, outside the natural distribution of its parental species. Its success could potentially lie in its allelopathic activity, which was confirmed in a number of experiments conducted with the leaf and root exudates, testing their effect on the germination and growth of various test plants. Here, we tested its allelopathic potential using different concentrations of leaf exudates on two test plants, Triticum aestivum L. and Sinapis alba L., in Petri dishes and pots with soil and by growing test plants in the soil sampled in knotweed stands on the edges of stands and outside of stands. Tests in Petri dishes and pots with soil to which leaf exudates were added have shown a decrease in germination and growth in comparison to the control, hence confirming the allelopathic effect. However, this was not confirmed in a test with in situ soil samples, where no statistically significant differences were observed, neither in the growth of test plants nor in the chemical characteristics (pH, soil organic matter, humus content) of the soil. Therefore, the persistence of Bohemian knotweed at already invaded sites could be attributed to its efficient use of resources (light and nutrients) through which it outcompetes native plants.