Margaret Lauterbach

Know the proper ways to attack the Treasure Valley's plentiful weeds

Poison hemlock is plentiful in Idaho.
Poison hemlock is plentiful in Idaho. Idaho Press Tribune file

Weeds are a constant bane of gardeners and farmers, beginning in spring when we see sneaky tiny seedlings, lasting until the weeds are large enough to flower and set seeds, renewing their cycle of aggravation. If you recognize them early, you can often save yourself a lot of work and stress by just cutting off their tiny leaves with a scuffle hoe or another scraping tool. On farms, some of these weeds are so invasive that they foul harvest equipment and/or displace planted economic crops. Many are also toxic to livestock. Most of them can be spread by seeds, and that means that farm or range seeds can be planted in urban areas by birds. One reader of this column said she’d seen many of the noxious weeds in the Winstead Park area this year.

Many weeds are irksome or obnoxious, but some are even noxious by law. Homeowners are advised by Ada County officials to control certain noxious and invasive weeds now, even though we’ll all stand a better chance of killing them in fall, when plant vigor changes course. Right now, plant vigor is moving from roots to leaves, but near winter that vigor will reverse, coursing from leaves to root. Adam Schroeder, director of Ada County Weed, Pest and Mosquito Abatement, says “spraying now will help stunt plants, kill small patches, and keep the plants from spreading” and reduce seed production. At this time of year his department specifies prevention of flowering and seed-setting of poison hemlock, Canada thistle, whitetop (hoary cress), Scotch thistle and rush skeletonweed.

These named weeds are invasive to the point of overwhelming and displacing native plants. The fewer of these, the happier we’ll all be. Reading about poison hemlock (juices of which killed Socrates), it’s a wonder I’m alive, because as a child, I used the hollow stems of poison hemlock for pea shooters. Our parents didn’t know what the plant was either, and I’ll bet you and many other youngsters used the stalks the same way, mouths blowing missiles.

Poison hemlock is a biennial that reproduces only by seed. Its first-year growth is a large rosette of leaves at soil level; the next year it can shoot up to about 10 feet in height. Its hairless stalks are light green with red to purplish blotches on the stalk or trunk, with graceful, carrot-like leaves. It has a distinctive odor that some describe as smelling like mouse urine, others say is parsnip-like. If you cut down a large poison hemlock plant, put the remains in a garbage bag in the sun to let it heat and decay, and then send to trash removal. Dried poison hemlock retains its toxic qualities. If you get its juices on your skin, wash quickly with soap and water. Control may be by shovel-pruning, especially when it’s still a ground-hugging rosette of leaves, or by glyphosate. If you use glyphosate, be sure to follow label directions. The label on all pesticides is the law.

If you don’t know what any of these plants look like, use a search engine such as Google. Thistles are easily identified, and as far as I’m concerned they all should be destroyed (unless they’re the blue-ish Eryngium giganteum, known also as Miss Wilmott’s Ghost. (Miss Wilmott loved that plant and scattered its seeds in others’ gardens; when the plant grew, gardeners knew she’d been there). Thistles are easily identified, even in their first year when they appear as large, thorny, ground-hugging rosettes.

Scotch thistle is a biennial, rising from its first year rosette to bloom and set seed its second year. This thistle can grow huge its second year, up to 12 feet tall and 5 feet in diameter. The easiest control of this weed is the application of a spade thrust under its rosette of leaves at about a 45 degree angle, severing leaves from the root before it begins its second-year growth.

Canada thistle is harder to control because it’s a perennial, growing from a network of horizontal and vertical roots as well as from seeds. USDA researchers found that household vinegar (ca. 5 percent) killed weeds during their first two weeks of life, but stronger vinegar (20 percent) was required to kill older weeds. Oddly, Canada thistle proved most susceptible of weeds tested to being killed by strong vinegar, even though it’s usually one of the toughest weeds to control. Even though you attempt to kill a mature Canada thistle by spraying anything, you should carefully remove any seedheads and put those in trash before spraying.

Whitetop, or Hoary cress, is a very tough perennial weed that spreads by rhizomes and seeds. It’s no surprise that whitetop has a white top. It’s a perennial mustard that spreads by rhizomes, and can dominate an area 12 square feet in one season. Leaves are covered with soft white hairs, and blossoms are white, hence its common name, whitetop.

Its root system grows very quickly, up to 12 feet in its first season, and a single plant can produce up to 4,800 seeds in one season. It begins growth in fall, then spurts in spring. A pesticide rated for control of this weed, paired with a surfactant, may be used for control. If you have a substantial patch of this, contact Ada County Weed, Pest and Mosquito abatement for free control advice ( or 208-577-4646).

If you see Rush Skeletonweed (Chondrilla juncea), a perennial weed on your property, do not till or dig in that area, for pieces of root will form new plants. It reproduces by seed as well. Small infestations can be controlled by hand-pulling each year, for six to 10 years. Its first-year rosette resembles dandelion, with leaf lobes pointing back toward the leaf base, fine hair on both sides of the leaf and spines on the margins. It has already spread underground by the time it blooms (small yellow blossoms), and it is difficult to eradicate. Biocontrol doesn’t kill it but can reduce its seed production.

Send garden questions to or Gardening, The Statesman, P.O. Box 40, Boise, ID 83707.