Ludwigia hyssopifolia (water primrose).
Abstract L. hyssopifolia is an extremely widespread weed of rice and wetlands across three continents. Holm et al. (1979) record it as a 'serious' or 'principal' weed in Indonesia, Borneo, Thailand, Malaysia, Nigeria and Trinidad, and common in Colombia and the Philippines. More recent publications confirm its importance in Malaysia (Begum et al., 2008) and in Sri Lanka (Chandrasena, 1987). While usually no more than 1 m high, it can grow to 3 m in height according to some authorities. Although there is some uncertainty over its native distribution, it is recorded as introduced and invasive on several Pacific islands, including Fiji, Kosrae, Pohnpei, Upolu (Western Samoa) and Christmas Island (HEAR, 2012). It is thus an aggressive weed of rice and of wetlands that poses a real threat to these ecologies. It has recently been identified as a 'new invader' in Spain (Ferrer and Laguna, 2009; Ferrer et al., 2009). Although this may be a slight exaggeration of its status there it does apparently have the ability to naturalise well north of its current distribution.