Red or partially-red stems are common among weedy, colonizing species, and stem color often varies widely within and among individuals. For example, in Apocynum cannabinum, plants growing in harsh environments produce bright red stems, while stems of plants grown in the greenhouse remain entirely green. In leaves of many species, red pigments such as anthocyanins serve to protect the photosynthetic apparatus by absorbing excess photons, and/or by acting as strong antioxidants. It is possible that red pigments found in photosynthetically active stems may also act as protective compounds for plants under stress. We sought to confirm that exposure to strong light increases the production of anthocyanins in A. cannabinum stems, to investigate whether there is family-level variation in anthocyanin production within and among A. cannabinum populations in south-central Indiana, and to compare the response of red and green stems to photoinhibition, using chlorophyll fluorescence to estimate photosynthetic efficiency. We quantified anthocyanin production and light interception in the field, and found a positive relationship between available PAR and stem redness. Using a reflectance spectrometer, we also monitored change in stem color among individuals from several maternal families in two light environments over a growing season in a common garden. Anthocyanin production was significantly higher among plants in full sun than among their siblings in shade. There was significant variation among maternal families for overall pigment production; families also varied in the magnitude and rate of their response to increased light. We suggest that anthocyanins in the stems of Apocynum cannabinum may function similarly to those in leaves of other species, by helping protect the photosynthetic apparatus from photoinhibition in stems of plants experiencing strong light.