NEWBURYPORT — The invasion of the Great Marsh continues. This time it's perennial pepperweed.

Perennial pepperweed is an invasive plant that has been found in the past several years invading the salt marshes in the Great Marsh, said Sarah Janson, invasives coordinator for the Parker River National Wildlife Refuge. Pepperweed can grow as tall as 6 feet and has stems protruding from it that sprout white flowers, making it easily identifiable.

Pepperweed poses a serious risk to other local plants and wildlife in the marsh because it can spread easily through root fragments and seeds floating on water.

"It is one of the very few invasive plants that is able to survive and actually do well in the salt marsh," said Nancy Pau, a wildlife biologist at the Parker River National Wildlife Refuge. "The salt marsh community here is pretty rare. It supports a lot of plants and animals that have adapted to the marsh and can't live anywhere else."

The Parker River National Wildlife Refuge and Mass Audubon are looking for volunteers to help eradicate pepperweed from the Great Marsh.

"We don't really know what pepperweed does in terms of other functions of a salt marsh," Pau said. "We're not sure how pepperweed fits into it. We want to keep the salt marsh as natural as possible."

Janson said they would like to have 100 volunteers this year to continue the fight against pepperweed that began in 2003.

Last year, 80 volunteers were able to treat pepperweed at more than 70 sites and removed it from 8.5 acres of the salt marsh.

"The more (people) we get, the more eradication we can do," Pau said. "Hopefully we can slow it down and prevent spreading. We definitely have more pepperweed that we could be pulling."

This year the goal is to map 95 percent of the Great Marsh and remove pepperweed from about 50 percent of the areas that are currently affected.

"This year I think we probably want to do more control in the Great Marsh itself, from Essex to Hampton, N.H.," Pau said. "We need to figure out where pepperweed are so we can figure out a strategy to control it and stop growing."

Janson predicted that controlling pepperweed in the Great Marsh will be arduous.

"It is going to be a long-term effort to control pepperweed," Janson said. "It's really important to prevent it from spreading farther. It's important to do as much work in the beginning as possible before the problem gets out of hand."

There are two treatments to remove pepperweed that the Parker River National Wildlife Refuge has been using, Janson said.

The first is applying herbicides to selected dense areas once a year, and the second treatment is pulling pepperweed, which occurs twice each summer.

"One or two years of herbicide will kill the plant," Janson said. "We think that probably three years of pulling twice a year works."

Pau is hoping that pepperweed is still new enough to the wetlands so that with enough people volunteering to identify and remove it, it won't spread as quickly and it can be reduced.

"We don't want it to become like other established invasives," Pau said. "It is very hard to control something once it's already established."

Janson said the effort to remove pepperweed will resume in May, when volunteers are allowed to begin pulling pepperweed and will continue until it goes to seed in August.

The Parker River Clean Water Association, Ipswich River Watershed Association, Boy Scouts, Girl Scouts and Plum Island Kayaks have all teamed up with the Parker River National Wildlife Refuge and Mass Audubon to help eradicate and control pepperweed in the upcoming season.

Plum Island Kayaks will be training its staff in Newburyport and Salem on how to identify pepperweed and will educate people it brings to the marsh each year about this invasive species.

On Thursday from 7 to 8:30 p.m., the Parker River National Wildlife Refuge will hold a training session on how to identify and map pepperweed. Volunteers are needed to help map, pull and monitor pepperweed in Salisbury, Amesbury, Haverhill, Newburyport, Newbury, Rowley, Ipswich and Essex. The training session will be held at the Refuge Headquarters at 6 Plum Island Turnpike in Newburyport. Anyone who is unable to attend the meeting but would like to volunteer for the project can e-mail Sarah Janson at sarah_janson@fws.gov.