‘Disease-like’ goatheads have riddled Boise for years. How the city plans to eradicate them

BOISE – The spiky, painful and nuisance-causing plants have been plaguing Boise bicycle riders for years.

But a new collaboration between Boise State University and City Hall aims to rid Idaho’s capital of puncturevine – commonly known as goathead – once and for all.

This summer, a team of researchers at Boise State University plans to unveil a hot-spot map of where the invasive species is most prevalent in the city to bolster efforts to eradicate it.

Trevor Caughlin, a biology professor at Boise State, said the study, which began in 2020, had an interesting finding: The areas of the city with the most goathead plants were correlated to areas with lower property values.

“The strongest predictor of where (a) goathead is in Boise is property value,” Caughlin said. “That finding for us really raised some interesting questions about how goatheads might intersect with transportation equity here.”

Caughlin said it’s possible marginalized communities are affected by goatheads more and “might not have the resources to repair punctured bike tires.”

Data from the U.S. Census Bureau has previously shown that poorer Americans are more likely to bike or walk to work than wealthier people.

“Because it’s a comprehensive, citywide map, we’re hoping that it can direct control efforts maybe a little bit more equitably to all of the parts of the city,” Caughlin said.

Caughlin said the map is expected to be finished by early June.

The Boise City Council on Tuesday allocated $56,000 of Open Space and Clean Water levy funds to complete the project.

“It’s a much more scientific approach than I ever would have envisioned to helping get rid of these things,” Council Member Jimmy Hallyburton said at a Tuesday council meeting.

Hallyburton directs a nonprofit, called the Boise Bicycle Project, that hosts an annual festival called the Boise Goathead Fest, which in addition to hosting a bike parade aims for volunteers to remove 10,000 pounds of goathead plants from the city.

Boise organizes volunteer groups to remove the plant, and the weeds can be reported to Ada County’s weed control service.

In an email, city ecologist Martha Brabec said invasive species treatments are usually reactive, and that the goathead map will allow the city to “proactively monitor and treat areas’ outbreaks before they get out of hand and spread.”

To collect the puncturevine data, Caughlin said, the research team selected 50 locations scattered across the city and at each location walked the approximate length of a city block.

Using a GPS device, the researchers recorded the location of each goathead plant they found. They also collected data on each site’s connection to the street network and the amount of bare ground in the area, since goatheads thrive on disturbed soil.

The project – which includes Garden City – found that one of the most prominent hot-spot areas was along Chinden Boulevard near Orchard Street and Curtis Road, as well as in Southwest Boise, south of Overland and south of Fairview Avenue. Referring to Chinden, Caughlin said the busy street contains hot spots because of lower property values and bare ground patches.

In the coming weeks, the study is scheduled to be published in a peer-reviewed journal called Ecological Solutions and Evidence.

While some other cities have produced maps of invasive species, Caughlin said the map will be one of the first instances that incorporates citywide data.

What is puncturevine, and how does it spread?

Puncturevine is a Mediterranean species that thrives in disturbed, sandy soils, Boise State Ph.D. student Richard Rachman said.

As the climate gets hotter and the U.S. continues to build past the limits of cities, the prevalence of goatheads could worsen, Rachman said.

Some of the first recorded instances of the species in Boise date from the 1970s, Rachman added.

Puncturevine plants produce bundles of fruit pods known as schizocarps, which then split up into mericarps, Rachman said. The small, spiky structures that get stuck in shoes, bike tires or animal paws are mericarps, which contain seeds from which new plants can sprout.

The burrs germinate in the spring and after rain in the summer, and the plants can quickly produce flowers and new fruits. The seeds within burrs can stay viable for six years or longer, according to the city of Walla Walla.

“I see them almost like a disease,” Caughlin said. “One way to think about this hot-spot map is that we are identifying the areas of the city that are super spreaders for this disease-like plant.”

To slow the plant’s spread, recreationists are advised to check their shoes and bike tires before going on trails in the Boise Foothills. When removing goatheads, residents should avoid putting the plants into compost because the seeds can survive the composting process, according to Boise Parks and Recreation.

While removing successful invasive plants is a challenge, Caughlin said he thinks it is possible to eradicate puncturevine from Boise.

In the areas sampled as part of the study, only 2% had goathead plants, and Caughlin thinks certain concentrated areas are producing lots of the seeds that continue the spread.

Kathryn Demps, an anthropologist at Boise State, plans to collect public feedback once the map is finished, Caughlin said.

“With the feedback from the public, and just observing how people use the map this year to decide where to go pull goatheads, we’ll update our map for next year and hopefully move closer toward eradicating this horrible species from Boise,” Caughlin said.