This paper reports the use of chlorophyll fluorescence analysis (CFA) to determine the effects of herbicides on photosynthetic activity in Arundo donax L. (giant reed). A field experiment was conducted with five systemic herbicides. Glyphosate 36% was applied in each of the following ways; 1) on adult mass of giant reed plants (3–4 metres in height), 2) on sprouts after initial cutting (80 cm in height) and glyphosate 54% was applied by injection into the stem with full-strength product. The other herbicides were applied on giant reed after the initial sprouting following cutting, on-label application rate. The CFA showed that glyphosate 36% applied on adult mass of plants at 10 L a.i. ha-1 had a significant effect (70% reduction) on photosynthetic activity 60 days after treatment (DAT). Glyphosate 36% at 4 L a.i ha-1 applied on sprouts resulted in 50% reduction and glyphosate 54% at 4 L a.i ha-1 applied by injection into the giant reed stems reduced their photosynthetic activity by 60%. Profoxydim 20% at 0.75 L ha-1 caused a 50% reduction at 60 DAT, cyhalofop-butyl 20% and penoxsulam 2.04% reduced 12 and 15% respectively, and azimsulfuron had no significant effect on photosynthetic activity of giant reed. Visual evaluations of giant reed presence showed similar results at 168 DAT. These results show that CFA could be used to measure the response of invasive plants to herbicides, and that glyphosate 36%, and possibly profoxydim 20%, might be used as an integrated control of giant reed. © 2013 The Author(s) and 2013 REABIC.