The phenology and biomass dynamics of Typha subulata were studied for 2 yr under two natural conditions, a marsh and an irrigation channel. Populations demonstrated a spring to summer growth pattern, with peak aboveground biomass in spring. The phenological phases identified for both populations were identical, but plants growing in the marsh averaged 38 and 28% less aboveground biomass and height, respectively, than plants growing in the channel. Noticeable differences were found in the timing of both phenological phases and biomass cycle between growth seasons, suggesting that the susceptibility of cattail to weed control during the seasons could vary between years.