The DNA C-value is an important biological concept and it has been used in many areas of biological research. The C-value can be used as an index for evaluating angiosperm invasiveness. However, it is still uncertain how the DNA C-value influences plant invasiveness because there is a paucity of systematic experiments that test all the important aspects of this question. There is also a lack of data that identify the relationships between DNA C-value and invasiveness using invaders and non-invaders from the same genus or family. Therefore, expanding the database of plant DNA C-values is important for elucidating the mechanism by which the DNA C-value influences plant invasiveness. Using a cytometry method, we determined the DNA C-values for 138 herbaceous species collected from the Yangtze River Delta and its neighboring area. Among these species, 111 were newly reported. According to related literature and the plants' distributions in artificial habitats, we also evaluated the weediness of these 138 species, and classified them using five grades: 0, 1, 2, 3, and 4. Based on these data, we calculated the genome sizes of 127 species with known ploidy levels and compared the differences in the two nuclear values (DNA C-value and genome size) between families, monocots and dicots, polypoid and diploid species, and invasive and non-invasive groups. The results showed that: (1) The average DNA C-value of the 138 herbaceous species was 1.55 pg, with the maximum 37.17 times greater than the minimum. The average genome size of the 127 species with known ploidy was 1.08 pg, with the maximum 34.11 times greater than the minimum;(2) We compared the average DNA C-value/genome size (pg) among Poaceae (2.6803/1.2436), Asteraceae (1.7007/1.2436), Caryophyllaceae (1.1612/1.1842), Brassicaceae (0.9165/0.8029), Scrophulariaceae (0.9164/0.6314), Polygonaceae (1.0003/0.7064), Labiatae (0.7990/0.7164), and Umbelliferae (0.7259/0.7259), and found that these two values in Poaceae were significantly higher than in those in the other seven families (P < 0.01). Additionally, the two values in monocots were significantly higher than those in dicots;(3) The average DNA C-value of weeds was significantly lower than that of non-weeds (P < 0.01), while the difference in genome sizes between the two groups was more significant than that of the DNA C-values (P < 0.001). A similar situation was found for Asteraceae. With the decrease of genome size (x₁) and DNA C-value (x₂) in the focal plants, their weediness (or invasiveness, y) increased, following y=2.2334-1.2847ln(x₁) (r=0.4612, P < 0.01) and y=2.4421-0.7234ln(x₂) (r=0.2522, P < 0.01). We suggest that the DNA C-value and genome size could be used as indicators to evaluate plant invasiveness and;(4) The genome sizes of polyploid weeds were significantly lower than those of diploid weeds (P < 0.01), with the former 0.63 times greater than the latter. The genome size of non-weedy polyploid plants was not significantly lower than that of diploid plants (P > 0.5). In Asteraceae weeds, the genome sizes for polyploids were significantly lower than those for diploids (P < 0.1). Our analyses showed that genome downsizing with polyploidy enhances plant invasiveness. In the evaluation of polyploid plant invasiveness, knowing the genome size is more valuable than knowing the DNA C-value. The expansion of the plant DNA C-values database is also useful for comprehensively understanding the variation of plant DNA C-values among different taxa, and it has possible uses in taxonomic, systematic, ecological, and evolutionary studies.