Ragweed (Ambrosia artemisiifolia L.), a member of the Asteraceae (Compositae), is an annual species native to North America. The species is a noxious invasive plant worldwide, with infestation by ragweed frequently posing challenges to protection and functioning of local ecosystems, crop production in agricultural areas, and public health. In North America, Europe, and Australia, ragweed is a major nuisance in arable areas and other habitats. At present, ragweed is relatively rare in agricultural areas of China compared with corresponding regions of other countries; nevertheless, the species is broadly distributed throughout many regions of China, including 21 of 34 provinces. Because ragweed is a monoecious annual that reproduces with high fecundity via seeds, its mating patterns may be closely related to its invasiveness and dissemination ability. In China, alien invasive plants in the family Asteraceae that are characterized by high fecundity tend to retain a high potential for self-compatibility, while many self-incompatible invasive alien plant species are perennials with high vegetative reproduction ability. Although ragweed does not reproduce vegetatively, its breeding system has not been fully elucidated. Some studies have indicated that ragweed can readily generate mature seeds autogamously and allogamously, whereas other experimental evidence implies that this species is self-incompatible. To shed further light on the breeding system of ragweed, we transplanted plants from an invasive ragweed population in Changshu, Jiangsu Province, China, and used them in a series of common garden experiments. We studied patterns of pollen viability and stigma receptivity. We also investigated the ragweed breeding system and apomixis by measuring seed sets of different individual plants subjected to four different treatments: self-pollination, cross-pollination, open-pollination, and bagging of female capitula. Viable ragweed pollen was first detected four days after flowering, with viability peaking eight to ten days post-anthesis. Viable stigmas were observed on the second day, with viable levels reaching a maximum five to eight days after flowering. The overlapping period of pollen viability and stigma receptivity was thus about five days. Seed set of ragweed individuals was highest under open-pollination (48.4%, on average) and cross-pollination (44.4%, on average) treatments, although the difference between these two treatments was not significant (P > 0.05). Seed set of self-pollinated individuals was much significantly lower (P < 0.01) than values obtained under open pollination and cross pollination, but the average value, 3.4%, was significantly higher than zero (P < 0.05). In addition, no seed set was observed from bagged female capitula. Taken together, our results suggest that ragweed is mainly self-incompatible, but possesses slight self-compatibility and no apomictic ability. Over the long run, self-incompatibility enhances ragweed diversity and adaptation to newly invaded areas; initially, however, this characteristic does not promote its spread, as it is obviously difficult for a few individual ragweed plants to form a large population via seeds relying solely on hybridization. Self-incompatibility may thus be one of the major reasons why ragweed seldom infests small croplands subjected to intensive weed management practices. Consequently, removal of sporadic individuals from new ragweed distributional areas should be made a priority to prevent serious invasion.