Pond plant sometimes mistaken for weed

Above, below: Azolla, or Retoreto, is a floating fern that enhances living conditions in ponds.

It may look like a weed, but the bright green growth covering some of the city’s ponds and waterways at this time of year is actually a fern.

Known as Azolla or Retoreto, the floating plant is native to New Zealand. It grows in spring and summer when water temperatures heat up.

And as Christchurch City Council Botanist Kate McCombs says far from being a nuisance, Azolla provides a home and food for waterfowl, insects, worms and snails and protection for young fish from predators, such as kingfishers.

She admits passers-by could mistake the plant for a weed. "It can cover the entire surface of slow-moving water and look as if it’s blocking a pond. The plant actually floats freely, it doesn't need soil to take root."

Lime green in the shade, the pond plant turns a brilliant red in sunny locations. The plant may obscure people’s views of the water, but ironically, it’s clear ponds that often harbour the worst weeds.

"In ecological terms, the Azolla plant covers the surface area which keeps temperatures down and helps reduce the growth of aggressive sub-surface weeds which take oxygen out of the water," Kate says.

As to safety, Christchurch City Council landscape architect Alan Cutler says even if the plant covers the entire surface, the pond’s gently sloping sides and surrounding vegetation will alert people to the presence of water.

He says removing the plant is not cost effective, as a trial in Angela’s Stream in Travis Wetland proved.

"We took it away one week and the next it was back."

The perennial plant thrives in spring and summer only, dying back when the water temperature drops.