The plants that take over a pond edge are a different problem from green water. They root in the wet ground or the shallows, then creep out across the surface. On farm dams, golf ponds and drains in New Zealand, the ones that cause the real trouble are alligator weed, parrot’s feather, primrose willow, reed sweet grass and Manchurian wild rice.
Healthy Ponds products have been clearing ponds since 1999. They do not kill these plants. Rush, Natural Blast and AquaSphere PRO deal with organic matter and the nutrients in the water. A mat of alligator weed is a weed job. A couple of these species are serious enough that the regional council wants to hear about them before anyone starts cutting.
The photos are of the plants, so you can match what is on your bank. They were not all taken in New Zealand. If you are not sure which one you have, leave it alone for the moment. A piece in the tray of a mower, or in the bucket of a digger, is enough to start the next patch.
Alligator weed
Alligator weed is the one with a white, pom-pom flower and leaves in pairs up the stem. The stems are hollow, which is why a broken piece floats and keeps growing. It sprawls across wet ground and shallow margins, and a bad patch will stand about a metre tall. In deeper water it does not usually root past about 2 to 3 metres. It holds on at the edge and floats out over the rest.
It is well established in Northland and Auckland, with scattered patches in Waikato, Bay of Plenty and Manawatū-Whanganui. It will grow in fresh water and in slightly brackish water. It moves as stem fragments, on machinery, in soil and down a drain. If the patch is new for your district, that is a phone call to the regional council, not a weekend with a brushcutter.
Parrot’s feather
Parrot’s feather looks like a bright green feather standing out of the water. The leaves are finely divided and sit in whorls around the stem. The part above the water is stiff, often 10 to 30 centimetres tall. Under the surface the same plant is softer and more spaced out. Only female plants are known in New Zealand, so it does not set seed here. It spreads by pieces of stem.
It likes still or slow water with plenty of nutrients and light, which is most farm dams and a lot of golf ponds. It is widespread in the North Island and scattered in the north of the South Island. Native milfoils are relatives. Parrot’s feather is the one that stands up out of the water in that stiff feather. If you are looking at a soft plant that stays under the surface, you may have something else, and ripping it out “just in case” is how the wrong plant gets spread.
Primrose willow
Primrose willow is the yellow flower. Ludwigia peploides sits on the margin of a calm pond or a slow drain, then runs out as a floating mat. The leaves are glossy and alternate, up to about 6 centimetres long, and the flower in summer is a clear yellow, up to about 2 centimetres across.
There is a smaller relative, water purslane (Ludwigia palustris), with reddish leaves in pairs and no real petals. Both root from the stems wherever a node touches mud or water. A yellow flower on a glossy mat at the edge of the dam is primrose willow.
Reed sweet grass
Reed sweet grass is a stout grass from Europe. It stands upright, with broad leaves, and it runs by thick rhizomes along the edge of a lake, a river or a drain. A stand of it will slow the water and fill a channel. It is common in the Waikato and on the Hauraki Plains, and it turns up in parts of Otago.
Stock will eat it. It has also been linked to cyanide poisoning, so a bank solid with reed sweet grass is not spare feed. If cattle are living on that edge through a dry spell, that is a reason to thin it, and to keep the cuttings out of the next paddock and the next waterway.
Manchurian wild rice
Manchurian wild rice is the tall one. A mature stand is 2 to 5 metres high, with a heavy rhizome system under it. It came in with ship ballast in the early 1900s. It rarely flowers or sets seed in New Zealand, so it moves as rhizomes and pieces carried by water, stock and machinery. It is in Northland and Auckland, and along the Wairoa River.
It will grow on a river berm, then walk into a swamp or a paddock, in fresh water or slightly brackish water. This is not a plant to mow and cart. If you think this is what you have, tell the regional council before you cut it. The rhizomes in a trailer are a new infestation.
What to do with them
Do not tip a chemical into the pond and hope the edge cleans itself up. Do not rake a mat out and leave the pieces on the bank, where the next rain takes them back in. Fragments are the whole problem. They travel on boats, mowers, digger buckets and loads of soil.
A small patch of parrot’s feather on a garden pond can be lifted, dried well away from the water, and bagged. Do not compost it where the runoff runs back to the pond. Alligator weed and Manchurian wild rice are a different scale. Ring the council, then decide what comes out and where it goes.
Once the weed is actually out, the water still has the nutrients that made the edge so comfortable. That is the part Natural Blast and AquaSphere PRO are for. They will not replace the weeding.
If the water is green as well
A weedy edge and green water often show up together, and they are still two jobs. Green water in spring is the nutrient bloom. Duckweed is the tiny plant that carpets the surface. Submerged weeds such as hornwort and oxygen weed sit under the water and are a different identification again. The pieces on those are green water in spring, duckweed, aquatic weed control, and herbicides compared with a natural follow-up.
Not sure what is on the edge? Send a photo and the pond details through the free pond treatment plan, or call 09 415 0300. We would rather see a picture than have the wrong plant spread.
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