Photosynthesis stomatal conductance and water use efficiency were compared between Lonicera japonica (Japanese honeysuckle) an invasive species in the southeastern United States and its native congener Lonicera sempervirens (coral honeysuckle) to determine the role of seasonal patterns of water loss and carbon gain in the invasive ability of the exotic. Diurnal measurements were taken monthly for 1 year under a closed forest canopy and in an open field. There were few significant differences in photosynthetic rates between the two species in either environment. However at both sites Lonicera japonica retained its old leaves over winter while old leaves of L. sempervirens senesced. Also new leaves of L. japonica had significantly higher photosynthetic rates than the emerging leaves of L. sempervirens (6.2 vs. 4.4 µmol m-2 sec-1 under the canopy; 4.4 vs. 3.0 µmol m-2 sec-1 in the open). Although differences in conductance and water use efficiency between species were seldom significant L. japonica tended to have higher maximum values than L. sempervirens. Retention of old leaves by L. japonica during new leaf formation (January-March) as well as higher photosynthetic rates in new leaves contribute to greater annual carbon gain and help explain the invasive ability of Japanese honeysuckle.