Alismataceae (water plantain)
What does it look like?
Marginal aquatic perennial (occasionally annual) to 2+ m tall with thick vertical basal rhizome which produces buds. Soft, spongy 3-sided stems. Strap-like submerged leaves mostly on young plants. Emergent leaves (< 25mm long) strongly arrowhead-shaped. Flowerhead a leafless stem, whorls of 2-12 white flowers (<25mm diam) with purple patch at base, with male flowers above females. Produces many seeds.
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 6 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 NZ.
What can I do to get rid of it?
Notifiable Organism. Suspected new infestations should be reported to your regional council.