In addition, there are some introduced plants which have become very abundant, in some cases blocking streams and rivers. Among these is watercress, Nasturtium officinale, one of the plants introduced by early colonists as a food, which became a nuisance in the Avon River and streams about Christchurch. Pond weeds, which have become very abundant and in certain instances a serious problem of control, are Ottelia ovalifolia, from Australia, and Aponogeton distachyon, a water hyacinth from South Africa. A greater threat comes from another water hyacinth, Eichhornia crassipes, the growing of which is strictly prohibited in New Zealand. It has attractive pink-mauve flowers with yellow and purple markings on one petal. It grows very rapidly, has extensive rhizomes, and may root in mud or float on water.
by Olive Rita Croker, M.A., Botanist, Wellington.