EVERETT — It’s everywhere along the roadsides this time of year: long swaths of exploding yellow.

The smell, however, is as bitter as the color is sweet. It’s Scotch broom, a non-native, invasive weed that thrives along roadsides and gives a bad time to people who suffer from allergies.

“It’s everywhere,” said Ron Morton, a maintenance superintendent for the state Department of Transportation.

If it seems like there’s more of it this year, it may be part reality and part perception, based on comments by officials.

From 2005 to 2008, the state worked on and alongside I-5 in south Everett on a $263 million project to widen the freeway. During the project, crews likely kicked up some dormant seeds and more Scotch broom sprouted, Morton said.

“As that soil was all disturbed, that’s the type it loves,” he said. “Now it’s getting to height where it becomes more noticeable.”

The rainy early spring and sudden, extended burst of sunny weather in mid-May might also have caused the plants to bloom all at once, drawing attention for that reason as well, said Sonny Gohrman, coordinator of the Snohomish County Noxious Weed Control Board.

Scotch broom, also known as Scot’s broom, was imported from the British Isles to California in the 1850s for erosion control, said Sharon Collman, an educator for the Washington State University Cooperative Extension in Snohomish County.

It also was used in its early days here as an ornamental plant, according to other sources.

Once Scotch broom made its way to Western Washington, it began thriving in the cool, wet weather similar to that of its native area, Gohrman said.

The plant is classified by the state as a noxious weed, but because there’s so much of it, crews don’t spent much time or money trying to eradicate it. They focus their attention instead on controlling other types of weeds such as tansy ragwort and giant hogweed.

There’s just too much Scotch broom to make a dent in it.

“We try to pick off those scattered ones, those small patches here and there, to stop it from spreading,” Gohrman said.

When it comes to large areas covered by the plant, though, “it’s a little beyond us,” he said.

The seeds stay fertile for up to 60 years and each plant can produce up to 10,000 of them. When the seed pods mature, they pop and throw the seeds out as far as 20 feet.

“That’s pretty awesome,” Collman said.

Scotch broom does cause problems. It displaces native plants; destroys rangeland and grasslands; dense stands impede the movement of wildlife; the seeds are poisonous to humans, horses and livestock, and it’s considered a fire hazard, according to a King County noxious weed alert.

The state and county receive a few complaints about Scotch broom this time of year, primarily from allergy sufferers. On the county’s allergy scale for plants, it’s fifth out of the top 10, Gohrman said.

Scotch broom is pollinated by insects and not by wind, Collman said, so it’s likely not the pollen that causes the reactions.

“There may be oils or irritants in the odor,” she said.

Scotch broom works in concert with grasses to trigger allergy symptoms, Gohrman said.

“It has a real smell to it, and that smell kind of triggers the allergy reaction,” he said.

In some areas, such as the I-5 median south of 128th Street SW, state crews have planted native shrubs in hopes of crowding out Scotch broom and other invasive plants, Gohrman said.

In some cases, Collman said, state staff have worked with local weed boards in using insects to try to control Scotch broom. Two types of weevils lay eggs on the seed pods and the larvae eat the seeds. This works better in some areas than in others, she said.

The plant can be mowed to keep it from going to seed, Collman said, though this must be done frequently. The stumps can be treated chemically or pulled out by the roots with a device called a weed wrench, but that’s a lot of work.

“I’ve seen fields where you would be there the rest of your life,” Collman said.

Bill Sheets: 425-339-3439; sheets@heraldnet.com.