Alismataceae (water plantain)
What does it look like?
Marginal perennial (occasionally annual) aquatic plant (<2+ m tall) with thick vertical basal rhizomes which produce buds. Soft, spongy 3-sided stems. Strap-like submerged leaves mostly on young plants, with strongly arrowhead-shaped emergent leaves (<25 mm long) on mature plants. Flowerhead is a leafless stem with whorls of 2-12 white flowers (<25 mm diam) with purple patch at base. Male flowers sit on stem above female flowers, and many seeds are produced.
Are there any similar species?
S. graminea ssp platyphylla (sagittaria), is also weedy and Alisma lanceolatum and A. plantago-aquatica (water plantain), are both minor weeds that are similar to arrowhead, but all lack the arrow-shaped leaves.
Why is it weedy?
Fast growing, matures quickly, produces widely dispersed, frost-hardy seed within six months. Overtops native species.
How does it spread?
Seeds spread by water flow, livestock, and possibly waterfowl and diggers can spread rhizomes. Can also escape from ponds during flooding.
What damage does it do?
Displaces most native marginal species, blocks waterways, contributes to flooding.
Which habitats is it likely to invade?
Flowing or still, shallow water, marshes, swamps, streams, potentially throughout New Zealand.
What can I do to get rid of it?
Notifiable Organism. Suspected new infestations should be reported to your regional council.