"It's just a dark green bush," Davey said.
"When you do a survey, you discover that it's more and further than you thought. It's a tricky plant to take on."
Removing it from one Whanganui street cost $2500 in dump fees, and there is lots of it near Fordell, South Beach and Castlecliff.
Five of the other most difficult species are water plants, such as hornwort, the "big dog" of aquatic weeds.
It has somehow got into Lake Namunamu near Hunterville and will be near-impossible to remove. A good herbicide is available, but it will need several expensive applications, and divers to check how well it has worked.
The council has listed 55 pest plant species for eradication from parts or the whole of the region. It only lists four pest animals - rabbits, rooks, possums and wallabies.
It wants to prevent some plants, such as the Chilean needle grass found in dry pasture east of Taihape, from getting into the region at all. Others it would like to totally eliminate - such as woolly nightshade, the subject of a campaign four years ago, or the Chilean rhubarb, which grows on coastal Taranaki cliffs.
Old man's beard is entrenched and cannot be eradicated, Davey said, but it is being controlled in some areas.
"Where we have chosen to work against it, it's suppressed to hell. We are right on top of it."
He was asked whether drones could be used to spray it, but said they would have to carry too much weight and obey Civil Aviation Authority restrictions.
"We just use big drones called helicopters."
Wild ginger, the subject of a 2013 campaign, has been dropped from the pest plant list, he said. Controlling it seemed too ambitious and more research was needed.
Horizons has recently introduced a fourth biological control agent for wandering willie, a yellow rust fungus. The aim is to reduce biomass of the smothering weed from over 1kg per square metre to about 200g - enough space to let seedlings through.
Field horsetail has been a problem weed in Rangitīkei and biological control seems the best hope for that. New outbreaks are targeted with herbicide, and landowners are expected to prevent it spreading to neighbouring properties.
The pest plant job will probably never get easier, Horizons chairwoman Rachel Keedwell said, and councillors will discuss whether to allocate more resource in their Long Term Plan.
