Ask Mr. Smarty Plants

There are 11 species of the genus Equisetum in our Native Plant Database, of which 5 are native to California and 4 of those loooove your area of Marin County.  These are Equisetum arvense (field horsetail), Equisetum hyemale (scouringrush horsetail), Equisetum hyemale var. affine (scouringrush horsetail) and  Equisetum laevigatum (smooth horsetail). 

It doesn't really matter which you have in your garden, or perhaps you have one that has been imported from another area; they all love marshy ground, can get along in sun, part shade or shade and are very difficult to control. Improving the drainage in your garden will certainly help, but it won't solve the problem. We found an article from Iowa State University, Department of Agronomy, Equisetum-Biology and Management that discusses several options. 

Everyone wants a special spray that will just kill the one plant they are interested in eliminating. The Lady Bird Johnson Wildflower Center recommends neither for nor against the use of herbicides. We found two articles that dealt with elimination of this plant. The first is from University of Minnesota Extension Horsetail by Beth Jarvis.  Here is an excerpt from that article:

"Horsetails are difficult to eradicate without the use of herbicides. They favor damp, sandy or gravelly, shady places. Depending on where they're growing, improving drainage and fertility and increasing the organic material in the soil along with regular mowing or clean cultivation may make the site less hospitable. Digging the plants out, in all but the smallest sites, could be prohibitively difficult due to the depth and spread of the underground rhizomes."

Another article, from a UK newsgroup, has more herbicide information to add: Re: Horsetail Roots.

Our conclusion from all of this is that you may need to modify the environment in which your garden is growing. In other words, improved drainage, drier, richer, maybe with taller plants to shade out the Equisetum. Cut off the spore-bearing cones before they ripen and spread more spores, but with all those rhizomes under ground, it will be difficult to starve out the plant.

From our Native Plant Image Gallery:

Equisetum arvense

Equisetum hyemale

Equisetum hyemale var. affine

Equisetum laevigatum