Arundo donax L. (Poaceae) is an invasive grass that severely degrades riparian habitats. It grows in many-stemmed clumps and, in California, spreads vegetatively only. Currently, A donax is thought to invade new areas through fragments broken from established clumps during flood events. But the role of flooding in generating fragments is based on anecdotal evidence only and has not been adequately studied. I examined A. donax clump break-up and reproduction via fragmentation in the Tijuana River Valley, California. I found that: (1) the majority of the new recruits from fragments grew from rhizome fragments (85% of 54) rather than stem fragments; (2) during the record rainfall of 2004–2005, flood waters damaged the rootstock of only a small proportion of clumps in the flood zone (7%; n  =  46 clumps), and relatively few recruits from fragments subsequently became established in the valley at large (0.048 recruits 100 m⁻²); and (3) during emergency channel maintenance along one tributary, bulldozers severely damaged the rootstock of all clumps growing on the channel bank (n  =  3 clumps), and many recruits from fragments subsequently became established downstream of the bulldozer activity (2.92 recruits 100 m⁻²; 61 times the number in the valley at large). These results indicate that, in the Tijuana River Valley, flood events only rarely break up A. donax rootstock and wash rhizomes downstream, and bulldozers play an important, and overlooked, role in the break-up and dispersal of A. donax. To reduce the spread of A. donax via rhizome fragments, regulatory agencies should require appropriate management practices when bulldozers are used in the presence of A. donax, and land managers should not use bulldozers when attempting to eradicate A. donax.