Clematis addisonii Britton (Ranunculaceae) is a Virginia endemic restricted to calcareous soils in a four county region of the Ridge and Valley Province in Virginia. A two year study of the reproductive biology of this species reveals that it is self-compatible, showing no significant reduction in fecundity following self-pollinations.

Morphological observations indicate that this species is protogynous. In vivo pollen tube growth supports this conclusion. Field observations suggest that the morphological pistillate phase lasts significantly longer than the staminate phase and is sufficient enough in length that cross-pollination is likely to occur during the time period preceding the staminate phase. The secretion of nectar from the onset of anthesis enhances the probability that outcrossing will occur prior to the presence of self-pollen in flowers.

These findings suggest that, in spite of self-compatibility, populations of Clematis addisonii are capable of maintaining high levels of outcrossing by virtue of protogyny and nectar secretion from the onset of anthesis.