Solanum carolinense has a gametophytic self-incompatibility (GSI) system that is typical of the Solanaceae in which pistils produce specific S-RNase proteins that disrupt the growth of pollen tubes sharing the same S-allele. However, unlike most self-incompatible plants Solanum carolinense is a weed. Self-incompatibility is uncommon in weeds because disturbed habitats require frequent recolonization (hence populations are repeatedly founded by few individuals bearing a limited number of S-alleles), effective population sizes are small (supporting few S-alleles) and habitats are ephemeral (so there is limited time for the migration of additional S-alleles into populations). We carried out a series of greenhouse experiments using clonal replicates (rhizome cuttings) of plants from two natural populations of S. carolinense to determine if there is variation in the strength of GSI within these populations. We found that the growth rate of self-pollen tubes and self-fertility increases with floral age. That is, flowers become more self-compatible as they age. Moreover, we found that self-fertility increases on plants in which the first 20 flowers receive no cross pollen. That is, when few or no fruits are produced on the first 20 flowers, self-pollination is more likely to result in fruit/seed set. Finally, we found that genotypes differ in their degree of self-fertility indicating that there is broadsense heritability for plasticity in the strength of self-incompatibility. These findings indicate that some genotypes of S. carolinense are capable of producing self-seed when cross pollen is scarce, even though the plants have a functional GSI system.