What does “horsegrass” look like? According to Ciscoe, it can’t be gotten rid of and I want to see if this is what I have.
I wonder if you are referring to horsetail, or Equisetum, which is a very persistent weed.
Wikipedia has a picture, and here is another from CalPhotos
Here is an article on Field Horsetail and Related Species from Oregon State University Extension.
Here is what Ciscoe Morris said about this plant in the Seattle P-I (April 29, 2006):
“Hands down, horsetail (Equisetum arvense) is the worst weed you can get in your garden. If you’ve got it, just be glad you weren’t gardening in prehistoric times. Back then, horsetail grew to 90 feet tall and you were in danger of being stepped on by a brontosaurus while weeding.
The worst thing about horsetail is the speed with which it returns to make your life miserable after you weed it. No matter how great a weeding job you do, it will be back, practically to full size, within a week!
Do what we did at Seattle University. Plant a mix of shrubs, ground covers and fast-growing perennials that are thick and tall enough to hide the horsetail. Shrubs that hide horsetail include Cistus (rockrose) Lonicera pileata (privet honeysuckle) Lonicera nitida (Box honeysuckle) and rosemary. My favorite perennial to hide horsetail is the prolific hardy Geranium oxonianum ‘Claridge Druce.’ It will seed all over your garden, but new seedlings are easy to remove in spring. These drought-tolerant plants look great in their own right and because they are so thick and tall, no one will see the hoards of horsetail growing within.”
Washington Toxics Coalition recommends controlling it by persistently hand-pulling or hoeing the above-ground growth as soon as it appears. This will weaken the plant over time. It does die back over winter, when you could cover the affected area with black plastic (for a duration of 2 years), but even this may not be entirely successful.
An article by Irene Mills in the Fall 2008 issue of the Northwest Perennial Alliance’s Perennial Post says that pulling, digging, and covering with black plastic are a waste of time. The author recommends keeping an eye out in April for emerging spore-bearing stalks, and cutting these off and disposing of them in the garbage. She suggests improving the soil texture (improve drainage, add organic matter, increase soil fertility, and in some cases increase soil pH). She recommends this guide called “Controlling Horsetail” from Swanson’s Nursery, originally published in Gardens West by Carol Hall.(note: guide may not open correctly in Firefox browser)