Horrible Hornwort

Hornwort is arguably New Zealand’s worst submerged waterweed and it is the curse of hydrostations in the Waikato as well as living in all kinds of North Island waterbodies : drains, irrigation systems, reservoirs and dams. NIWA is keeping a close eye on it and two South Island outbreaks have been successfully eradicated. But it ranks even higher than didymo so is definitely one to watch out for.

Dr Clayton has a doctorate in freshwater botany and a background in freshwater biodiversity and biosecurity. He specializes in applied research and technical consultancy on aquatic plants, their control and management.

With the aid of SCUBA he developed original survey methods, including using plants as a way to assess the ecological condition of lakes.

He is a principal scientist and programme leader with NIWA based in Hamilton. His previous roles include engineering scientist for the NZ Electricity Dept working on NZ hydro lakes and 14 years as a research scientist with MAF at Ruakura.

Recently, hornwort has been targeted more than usual due to the World Rowing Championships at Lake Karapiro

Hornwort is a huge threat to South Island waterways and power stations. It’s on a par with didymo and has MAF’s highest biosecurity status as an unwanted organism for the South Island, which means a commitment to total eradication.

It’s the worst submerged weed in New Zealand, not only because it can grow as tall as a three story building, but it can grow deeper than other weeds (down to 12m) and doesn’t have any roots.

Research so far has found excellent tools to control and eradicate it.

There have been two South Island outbreaks of hornwort, one in Timaru, the other in Nelson, where MAF has successfully controlled hornwort with NIWA’s advice.

Hornwort is a native of the Americas, but in New Zealand its growth is exceptional (a bit like possums and rabbits out of their natural environment).

It is an invasive aquatic weed found in fresh water. Originally propagated as an aquarium and ornamental pond plant, it is now banned from sale or distribution.

It has long slender brittle stems with stiff forked leaves and prefers clear deep water, but it’s not fussy and grows in a wide range of sites. It grows extremely quickly, outcompeting other plants and smothering them.

We have a great environment for it here and it was first found in 1961 in drains near the Tukituki River in Hawke’s Bay. It was an aquarium escapee. In only a couple of years it was found in Waikato, then in 1981 in the Bay of Plenty, the next year in Manawatu, and it also spread to Auckland.

Near Taupo it was found growing in a stream outflow in thermal water and in only a few years its growth had shut down a power station because of a massive influx of the plant. The hornwort had bent the screens on the penstock intakes, causing quite a serious problem.

Now power stations have better systems for cleaning off aquatic weeds, with screen cleaners activated when plants build up to a certain level. It’s expensive to remove and it also costs money in terms of lost generation potential.

Hornwort does very well in cold water, and it can grow anywhere, even when it is iced over. Although it is found throughout the North Island and in Lake Taupo, it’s not in every water body.

Grass carp would eat it but they need to be contained, and you’d need a risk assessment to make sure they wouldn’t escape too. It’s not very palatable and not highly preferred on the menu of vegetarian fish.

Power companies have looked at pulverizing it, but really there are only two key ways to control it: grass carp or chemicals. There are a couple of herbicides which can be used to good effect.

The unique thing about hornwort – or its Achilles heel – is that it doesn’t have a very strong stem, and it doesn’t have roots. The lower part of the stem embeds itself into sediment, but there is no anchor or root so it’s easy for it to break away, creating a problem because it forms islands or rafts of weed which float and wrap around anything. It can also survive for several months in wet spots such as under the floor or in boat anchor wells.

It grows a bit like a triffid and it’s heavy because it is 96% water. If you dry it out it is only 5-6% dry weight.

It has limited use and might have some potential for biogas generation. It has also been used in some worm farms but these aren’t solutions.

The main approach with the weed is to prevent it spreading. Little fragments attached to boat trailers or even in anchor wells can easily spread hornwort into uncontaminated sites.

Boating gear including nets and fishing gear must be cleaned before leaving a waterway. All recreational water users should follow the Check, Clean and Dry rules.

There are some quite novel approaches in the Rotorua Lakes with cordons on several of the lakes.

Surveillance information at the points of entry to the lakes is also important. For example Environment BOP, the regional council, has put up flashing signs that light up when you leave a boat ramp. The signs tell boaties about removing weeds from their boats.

They can now also impose fines on anyone caught transporting weeds out of any particular water body. 90% of the public are vigilant about this, but there are always a few who don’t care about this sort of thing. It only takes a small number of people to disregard advice and not to check, clean and dry their boats and boat trailers to spread hornwort.

The idea is to stop the problem at the top of the cliff, rather than dealing to it when it has spread into any new uninfested water body or into the South Island.

Both of the outbreaks in the South Island were dealt with very quickly, and since their treatment, nothing else has been found.

When comparing hornwort to didymo, the scourge of South Island rivers, NIWA has developed a Weed Risk Model (now being adopted in several countries overseas) that enables an objective comparison of a wide range of aquatic weedy characters.

When these two weeds are compared, hornwort scores higher. There are several reasons for this – the key differences are that hornwort has a much wider habitat tolerance and a greater degree of unfavourable impacts in comparison to didymo.