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The allelopathy of alien plants is critical to their successful invasion. In nature, two alien plants can co-invade the same habitat. Changes in the forms of nitrogen may have the potential to alter the invasion process of alien plants by causing alterations in their allelopathy. This study aimed to evaluate the individual and combined allelopathy of two alien plants from the Asteraceae family, Solidago canadensis L. and Conyza canadensis (L.) Cronq. This study examined the effects of aqueous leaf extracts of the two alien plants with different nitrogen forms (NO3−N, NH4−N, and CO(NH2)2−N, and the mixed nitrogen forms at a 1:1:1 ratio) on the seed germination and seedling growth of the horticultural Asteraceae species Lactuca sativa L. using a germination bioassay. The allelopathy of the two alien plants significantly reduced the seed germination and seedling growth of L. sativa. Extracts from S. canadensis produced stronger allelopathy on the seed germination and seedling growth of L. sativa compared with those from C. canadensis. The mixture of extracts from the two alien plants produced an antagonistic effect when compared with the effects of extracts from each plant species. The addition of nitrogen intensified the allelopathy of the two alien plants on the seed germination and seedling growth of L. sativa. The degree of influence of nitrogen on the individual and combined allelopathy of the two alien plants was similar. Thus, nitrogen deposition may facilitate the independent invasion and co-invasion of the two alien plants via intensified allelopathy.

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The phytotoxicity of exotic invasive plants (EIP) is vital for their successful invasion. At the family level, Asteraceae contains the largest number of EIP species in China. Drought stress may affect the phytotoxicity of EIP by altering the production and secretion of allelochemicals. Therefore, it is important to estimate the phytotoxicity of multiple EIP under drought stress. In this study, the phytotoxicity of five Asteraceae EIP, i.e., Solidago canadensis L., Aster subulatus Michx., Bidens pilosa L., Erigeron annuus (L.) Pers., and Conyza canadensis (L.) Cronq., were estimated using leaf extracts. Their effects on seed germination and seedling growth of the horticultural Asteraceae species Lactuca sativa L. under drought stress (simulated by polyethylene glycol-6000) were assessed. These five Asteraceae EIP, particularly S. canadensis, inhibited seed germination of L. sativa. Aster subulatus, B. pilosa, E. annuus, and C. canadensis increased seedling growth of L. sativa. The phytotoxicity of these five Asteraceae EIP decreased in the following order: S. canadensis > E. annuus > A. subulatus > B. pilosa ≈ C. canadensis. Drought stress intensified the phytotoxicity of all five Asteraceae EIP. The level of stress intensity mediated by these five Asteraceae EIP, particularly by S. canadensis, A. subulatus, and B. pilosa, increased with increasing degree of drought stress. Thus, an increasing growing degree of drought stress may accelerate the invasion process of these five Asteraceae EIP via the effects of intensified phytotoxicity on growth performance of adjacent species, particularly on seed germination and seedling growth.