FORT KENT — There are any number of regulations border officials can cite to bar entry into their country.

Now you can add mud to that list when it comes to entering Canada from the United States.

Simply put, U.S. dirt is unwelcome across the border, no matter how it arrives.

Tasked with enforcing the Canadian Food Inspection Agency’s regulations prohibiting the importation of soil into the country, officers with the Canadian Border Services Agency can — and have — barred the entry of personal and commercial vehicles deemed too dirty or mud coated to enter the country.

It’s a law that has been on the books for a while, but it becomes more of an issue as northern New England moves from winter into spring by way of mud season.

“We do not block vehicles that have a little dust or dirt on them,” a border crossing supervisor in Clair, New Brunswick, who declined to be named said late last week. “But if we see big clumps of mud or dirt, we turn them around.”

Only when the vehicle has been given a good wash job is it allowed to enter Canada.

“The importation of soil into Canada is generally prohibited because regulated quarantine plant pests can travel in soil,” Elena Koutsavakis, media relations with the Canadian Food Inspection Agency said in an email Friday. “These invasive pests are often not easily visible to the naked eye and can hitch a ride in the soil under people’s vehicles.”

Keeping that from happening is something the Canadians take very seriously.

“Vehicles found to be contaminated with soil may be refused entry under the authority of the [Canadian] Plant Protection Act,” Koutsavakis said.

Several drivers and truck operators based in Maine this past week said they have been turned back at either the Clair or Edmundston, New Brunswick, ports of entry, but declined to comment on the record given the frequency of their cross-border trips.

One who did speak openly was Denis Bossie, a Clair lumberman who works in Maine near Millinocket.

“I’ve been turned back [from entering Canada] five times because my [pickup] truck is too dirty,” the French-speaking Bossie said Saturday morning, speaking through his daughter Emily Bossie, who translated for her father. “It’s not a big thing, I wash it and can then come home.”

Bossie admitted his truck is often pretty dirty after a week working along the Golden Road in central Maine.

“So dirty you can’t even tell what color it is,” he said.

The closest car wash to the Clair port of entry is a self-wash at Daigle Oil Co., about a quarter mile from the international bridge in Fort Kent where workers this week said they have heard from both U.S. and Canadian residents coming to to wash vehicles before crossing into Canada.

“All vehicles should be clean and free of soil and soil-related matter [such as] muck, earthworm castings, leaf litter prior to arriving in Canada,” Koutsavakis said. “Soil is a high-risk pathway for regulated quarantine pests [such as] potato cyst nematodes that can cause serious harm to Canada’s natural resources.”

Cleanliness is not the sole province of the Canadians, as U.S. Customs and Border Protection officials are also on constant lookout for small, dirty hitchhikers trying to come into this country.

“Customs and Border Protection is working cooperatively with other agencies to protect America’s agriculture and certain items brought into the U.S. can harbor foreign and invasive animal and plant pests and diseases that could seriously damage American crops, livestock, pets and the environment,” Michelle Benson-Fuller, spokesperson for Customs and Border Protection, said. “Our CBP officers and agriculture specialists are trained to detect and prevent entry of these plant pests and exotic foreign animal diseases with inspection and prevention efforts designed to keep prohibited agricultural items from entering the United States.”

As far as the Canadians are concerned, no U.S. soil is welcome on their turf.

“It makes no difference what part of the US the vehicle is coming from,” Koutsavakis said. “All vehicles should be clean and free of soil and soil-related matter prior to arriving in Canada [and] this requirement applies to all vehicles, regardless of origin.”

For cross-border travelers such as Bossie, it’s a minor inconvenience.

“It’s not really a big deal,” he said. “The Canadians just don’t like the grass or dirt of Maine.”