"The regional council has really snatched away an indigenous food source."
On Wednesday, Greater Wellington Regional Council officers sprayed glyphosate - a herbicide more commonly known as Roundup - around the waterway, and put up signs to warn people not to take any plants for consumption for a seven day period.
Mr Howden said there were only a couple of signs up at first. Alarmed at the lack of signage, he called the council to tell them it was a health risk.
"My staff stopped two people from picking some - they hadn't seen the warning signs and they were about to pick the watercress.
"It's potentially going to hurt someone ... they should put up a physical barrier."
After his complaint, the council workers came back and erected more signs.
Mr Howden also questioned the need to spray as the watercress "wasn't choking" the stream and the foragers helped keep it clear.
Some of the signs appeared to offer conflicting warnings, advising people not to take and eat plants from the waterway for a 14-day period. A council spokeswoman said that following a request by Mr Howden to display more signs, council officers returned to the site with more notices - to ensure the warnings were up quicker, they used template signs which had already been printed with a 14-day avoidance period.
This period exceeded the manufacturer's recommendations, she said.
There are around 20 signs now stapled to fence posts along the waterway.
The spokeswoman said the council's flood protection department carried out regular maintenance of waterways across the region.
"It is necessary to keep some waterways clear of vegetation to enable water to flow through easily and not cause flooding on adjacent land."
She said the chemical spray used, Agpro Green Glyphosate 510, was approved for use over water and carries "zero day" withholding period for stock, meaning livestock can graze on sprayed areas.
If swallowed, it is recommended to seek medical help immediately.
