Slug Tales From Zone 6: Common noxious weeds

By Soncirey Mitchell
Reader Staff

When you start to identify the plants growing wild around the yard, garden or alongside roads, you may begin to notice a worrying trend: many of those pretty flowers are actually invasive species. Humans have intentionally and accidentally transported innumerable species from one continent to another, introducing plants to habitats where, without any of the competitors or pests that evolved alongside them, they run rampant, choking out the native flora. Here’s how to identify a handful of local noxious weeds that you probably see every day. For a complete list with resources and management tactics, visit bonnercountyid.gov/noxious-weeds.

Yellow and orange hawkweeds (Hieracium caespitosum or aurantiacum)

These one- to three-foot-tall flowering plants have dark orange blossoms with yellow centers or plain yellow blooms that resemble one-inch dandelions. Flower clusters sit atop one long, hairy stem protruding from a rosette of long, sword-like leaves at its base. Native to Europe, this perennial spreads via roots, fluffy seeds or stolons — vine-like runners that travel along the top of the soil — often taking over fields. Anything that looks like a hawkweed is likely invasive, with one exception being the native white-flowered hawkweed (Hieracium albiflorum).

Giant, Japanese and bohemian knotweeds (Polygonum cuspidatum, sachalinense or X bohemicum)

Indigenous to northeastern Asia, these perennials can be found along roadsides, in ditches or in wetter pastures. The bamboo-like stems are thick, but hollow in the center, and covered in reddish-purple speckles. Knotweed can be anywhere from four to ten feet tall and produces plenty of heart-shaped leaves reaching lengths of up to 12 inches. In the autumn, long fingers of white flower clusters bloom from the upper branches.

Rush skeletonweed (Chondrilla juncea)

So named for its branching, bone-like frame, rush skeletonweed is a perennial that spreads via its roots and fluffy, dandelion-like seeds. It can grow anywhere from one to four feet tall, producing long, skinny stems covered in coarse hair at the base and small, yellow flowers at the top. All the plants’ leaves grow in a rosette at the base and are easily mistaken for a dandelion.  If you’re not sure of your identification, break off a piece of the plant: skeletonweed will ooze a white, latex-like sap.

Hoary alyssum (Berteroa incana)

This Eurasian herb enjoys dry, sandy or rocky soils in pastures or along roadsides, spreading by flat, oval seed pods. It is poisonous to livestock — especially horses — making it particularly dangerous to Idaho’s farmlands. It can grow as an annual, biennial or short-lived perennial, reaching one to three and a half feet tall. Its long stalks, branches and skinny, pointed leaves are covered in fine hair, giving them a gray coloring. Clusters of white flowers with eight petals each sit at the top of the plant, blooming from May through September.

Absinth wormwood (Artemisia absinthium)

Yes, like the bright green spirit. Native to Eurasia, absinth wormwood is toxic, which is why it makes such strong booze and has historically been used to kill intestinal parasites. The herbaceous perennial is a light green color, due to fine, silver hairs coating the leaves, and has a sage-like smell when crushed. It generally caps out at three feet tall but can reach up to five feet, with branching stems covered in many-lobed leaves (think parsley). Wormwood’s flowers look like dozens of small, yellow pompoms that eventually give way to myriad tiny, brown seeds, which can remain ungerminated and viable in the soil for years.

Tansy ragwort (Jacobaea vulgaris)

One of Bonner County’s most famous noxious weeds is tansy ragwort, a native of northern Africa, Asia and Europe that’s toxic to both humans and livestock due to high levels of alkaloids.

It can be biennial, annual or perennial, spending part of its life as a rosette of ruffled parsley- or daisy-like leaves. It later produced four-foot-tall stems with regular layers of leaves, branching only at the top into bouquets of one-inch yellow flowers with 13 petals each, resembling daisies. It reproduces by seed or vegetatively in disturbed soil or fields and is often made worse by mowing.

Spotted knapweed (Centaurea stoebe)

Knapweed could easily be mistaken for a thistle due to its flowers, whose small purple petals emanate from a spiky, vase-shaped base. Unlike a thistle, though, this perennial has no thorns or spikes anywhere besides the base of the flower, and is sparsely covered in small, sword-like leaves. Its three- to five-foot-tall stems branch toward the top, each producing a single flower, and the entire plant grows from a rosette of multi-lobed leaves. Native to Europe, the tumbleweed easily takes over disturbed soil by producing plenty of small black and yellow seeds that it distributes on its wind-propelled journey.