Arundo donax L. (Poaceae) is an invasive, perennial grass that grows in many-stemmed, cane-like clumps. It does not produce viable seeds in California and is currently thought to invade habitats rapidly by rhizomes and fragments only. But, during a two-year field study in the Tijuana River Valley, California, expansion of A. donax clumps via rhizomes was slow, only 0.29 m 2 yr−1, and new recruits from fragments were rare, only 4.7 ha−1 yr−1. Whereas layering, a mode of spread heretofore ignored by researchers, was common in the flood zone. Layering is the adventitious sprouting of stem tips in contact with the ground. Layering can be considered to be both expansion of a clump (while the layering stem is still alive and attached to the clump) and asexual reproduction (after the layering stem dies). When viewed as clump expansion, layering was 7.4 times faster than the annual expansion via rhizomes. When viewed as reproduction, layering produced 25 times more new recruits than fragments. Layering was therefore an important means by which A. donax was spreading within the flood zone.
A new general view of A. donax invasion is presented illustrating that fragmentation is the means by which A. donax invades a new site in the flood zone, expansion via rhizomes maintains an A. donax clump, and layering is the means by which A. donax spreads quickly and episodically within the flood zone. Outside the flood zone, A. donax expands slowly via rhizomes only and no new recruits arrive from either fragmentation or layering.
The Tijuana River Valley results challenge the current “top-down” management policy, which presumes that most new recruits come from upstream and that all clumps expand at the same rate. The results show that, on the contrary, most new recruits come from within the habitat, via layering, and that clumps in the flood zone expand faster than those outside the flood zone. I conclude that the top-down policy is counter-productive and suggest that managers shift to controlling A. donax “inside-out,” i.e., conduct treatments first inside and then outside the flood zone. In this way, the fastest expanding clumps – those in the flood zone – will be treated first.