This has been a terrible year for thistles in West Marin. Or perhaps I should say it has been a good year for the thistles and a terrible year for landowners doing battle with them.

By now I’ve had to spend seven full days slashing thistles and then bagging them lest their seeds get picked up by the wind. Even so, new thistles are constantly appearing.

All this made me curious about the identity of the thistles on my property. When I then happened to take several walks through federal parkland just downstream from the Green Bridge in Point Reyes Station, I could immediately see the Park Service has its own thistle problem.

This area is part of the former Giacomini dairy ranch, which the Park Service bought, and is immediately east of the wetland-restoration project. Most of the Park Service’s thistles were the same as mine, so I asked Stacy Carlsen, the county agricultural commissioner, about West Marin’s thistles.

Three of the photos I shot on parkland and emailed Commissioner Carlsen turned out to be Bull thistles, Cirsium vulgare.

“Bull thistle, he wrote back, “is associated with disturbed soils and shaded areas, with moist conditions being preferred. Bull thistles,” he added, are “native to Europe.”

Different stages of Common teasel, Dipascus fullonum, on federal property.

The teasel, Carlsen noted, is also “native to Europe. It is not classified as invasive in California, but some counties take action against the weed.

“Teasel is often associated with moist conditions [and] shallow soil. The seed heads were [at one time] used to process wool as a combing structure.”

Part of a sizeable thicket of Italian thistle, Carduus pycnocephalus, on Park Service land.

Italian thistle is “native to the Mediterranean region,” the agricultural commissioner noted.

I had written him, “My guess is that with the late dairy rancher Waldo Giacomini, as well as his family and cows, no longer keeping the area clear, thistles have begun moving in.” In response, Carlsen wrote, “Livestock will eat this plant in the early stages of growth, assuming they have access to it.

“Italian thistle is the most common of the [above] three in Marin County. The three thistles are not native and, by nature of their wide distribution in the state, are not clearly defined as invasive.

“However, they can be a nuisance and interfere with best use of both agriculture and open-space areas, including your walking trails.

Wooly distaff thistle.

“Our biggest problem species in Marin County,” Carlsen added, “are Wooly distaff, Purple and Yellowstar thistle.”

Yellowstar thistle (at right).

Yellowstar thistle is especially harmful to horses. If a horse does not have enough feed in its pasture, it may turn to yellowstar thistles.

And if horses eat a large amount of yellow star thistle over one to three months, they can “develop dysfunction of facial, mouth and throat nerves and muscles,” the Doctor of Veterinary Medicine online newsmagazine reports.

Horses reach the point where they can chew but not swallow, which is why the poisoning is often called the “chewing disease.” They have trouble drinking and breathing and often become dehydrated, malnourished, lethargic, and depressed.

Horses next develop lesions, some of which damage the brain and can lead to starvation. There is no treatment for chewing disease, and even if horses partially recover on their own, they never again have their full faculties.

Purple thistle.

Plumeless thistles (at left).

“We have eradicated Plumeless thistles from the Point Reyes National Seashore, but it pops up from time to time from residual seeds,” the county agricultural commissioner wrote.

Some thistle seeds can, in fact, lie dormant for years if buried.

“There are some native thistles in California,” Carlsen reported, “but the vast majority of prickly and spiny types were introduced with feed and livestock from Europe and the Mediterranean areas.”

When thistles began regrouping near Mitchell cabin last month, I warned them in the words of General MacArthur, “I shall return.” True to my word, I have once again engaged the enemy.