Technical Abstract: Giant reed may invade riparian habitats throughout California and the United States. Imazapyr has only been recently approved for use in controlling this species in California. There is little published data on the efficacy of imazapyr which can be used to select an appropriate application regime for California habitats. We monitored a site where imazapyr had been applied with the purpose of testing the hypotheses that a 1.5% solution of imazapyr applied as a foliar spray was effective at killing giant reed. Plants treated with 1.5% imazapyr had reduced leaf chlorophyll content after treatment but recovered the following spring. Treatment with 1.5% imazapyr did not significantly reduce the proportion of living stems or prevent the production of new stems during the spring following treatment.