TO STUDY THE RESPONSE OF DIFFERENT CLONES OF COMMON REED TO GLYPHOSATE AND DETERMINATION THE BEST TIME FOR CHEMICAL CONTROL, A COMPLETELY RANDOMAIZED DESIGN WITH THREE REPLICATIONS WAS CONDUCTED AT GREENHOUSE IN THE AGRICULTURAL SCIENCE DEPARTMENT, UNIVERSITY OF TEHRAN. THREE FACTORS INCLUDING DIFFERENT CLONES OF COMMON REED (KO-MA, KZ-D1, T-VGO, KE-S1, G-GR3, A-MO1, MA-S1, MA-S2, Q-BE2, AND T-MO1), THE GLYPHOSATE DOSES (1, 1.8 AND 3 KG AI HA -1) AND THE TIME OF TREATMENT (2-5 LEAVES, 5-7 LEAVES AND EARLY STAGE OF PANICLE FORMATION) WERE USED. GLYPHOSATE TOXICITY WAS QUANTIFIED BY VISUAL EVALUATION OF INJURY AT 40 DAYS AFTER HERBICIDE TREATMENT. AFTER SHOOTS WERE HARVESTED, SHOOT RE GROWTH WAS EVALUATED IN 56 DAYS AFTER SHOOT HARVEST. DIFFERENT CLONES RESPONDED DIFFERENTIALLY TO GLYPHOSAT RATES. HERBICIDE TREATMENT AT EARLY STAGE OF PANICLE FORMATION PROVIDED THE BEST CONTROL BUT SHOOT REGROWTH INDICATED THAT CONTROL TREATMENT MUST BE REPEATED IF COMMON REED IS TO BE ERADICATED.