Noxious. Weedy. Non-native.
Whatever you call them, invasive plant species are the outlaws that disrupt an otherwise healthy frontier.
The U.S. Department of Agriculture legally defines them as alien species whose introduction would likely cause environmental or human harm, and the U.S. Forest Service reports they have led to the decline of 42 percent of native endangered and threatened species.
But doubtful naysayers may ask: How can invasive plants be evil if they grow so plentifully?
Invasive plants produce large amounts of seeds, which are distributed unknowingly by humans or wildlife who consume the vegetation, according to Clark County Noxious Weed Management, the department responsible for controlling harmful weeds.
The county’s weed board identified more than 100 noxious weeds residents should be aware of, each varying in the risk they present, whether it’s to human health, agriculture, habitat or property.
Depending on the plant, their roots can reach long distances while potentially smothering native plants’ root systems. Some release chemicals from their leaves or roots, which can slow the growth of their neighbors, or erode soil and degrade water quality.
Wanted dead (or regulated)
A “Class A” category encompasses the meanest, most vile plant criminals out there, which requires the destruction of an entire plant to minimize its risk. The less threatening bunch, “Class B,” only prompt a need for regulation, or prevention of seed production and distribution.
Washington prohibits the purchase, selling or transportation of these plants due to their threat on biodiversity, according to the state Department of Agriculture.
Here are six of the most significant noxious weeds in Clark County, according to county lead field inspector Justin Collell.
- Poison hemlock (Class B); Northern Africa, Asia and Europe
Poison hemlock grows abundantly in Clark County at the edges of forests and in pastures, as well as mixed in with blackberry patches, Collell said.
People and livestock who consume any part of poison hemlock, and in any quantity, will be fatally poisoned. Symptoms, such as trembling, respiratory troubles and dilated pupils, can appear anywhere between 20 minutes to three hours after it’s ingested.
Poison hemlock has tiny clusters of white flowers, bright green and fernlike leaves, and hollow stems with reddish spots and streaks. The plant germinates year-round but reaches its peak growth in late spring when it can reach up to 10 feet tall.
- Tansy ragwort (Class B); Northern Africa, Asia and Europe
Tansy ragwort seems misleadingly inconspicuous. Its bright, golden flower heads distract from the dangerous compounds in its branched dark-green leaves, which can knock over a cow days or months after being eaten.
Though the biennial already grows amply throughout coastal regions, as it thrives in rainy conditions, it is widespread locally in Clark County, Collell said.
All of tansy ragwort — leaves, roots and stems — contains levels of pyrrolizidine alkaloids, a natural compound found in plants. When ingested, these compounds mostly affect an animal’s liver by killing cells and preventing them from regenerating.
When mature, it has a potent “weedy” smell and produces many seeds, making it easy to invade pastures and uncultivated land where animals graze. It’s easily spread when mowed or pulled.
- Garlic mustard (Class A); Europe, Asia and Northern Africa
Garlic mustard is a threat to biodiversity.
It replaces native plants in a forest’s understory by developing before them, morphing into shadow-casting towers by the time other plants are ready to absorb sunlight. Even further, garlic mustard leaks chemicals from its roots, stalling the transfer of nutrients between plants through their fungal networks.
Though not widespread in Washington, it is becoming more common in Clark County, mostly near Salmon Creek and the city of Camas, Collell said. The plant has heart-shaped leaves with toothed edges and small, white flowers, which bloom in the spring.
- Slenderflower thistle (Class A); native to Northern Africa and Western and Southern Europe
Slenderflower thistle spreads rapidly and is skilled at forming dense collections of stems, making it a strong competitor against native plants.
“Pointy” accurately describes the weed, which has sharp-looking features from its stem to its leaves and pink flowers. It grows in dry open areas and has been identified in about 30 properties throughout the county, Collell said.
- Giant hogweed (Class A); native to Central and Southwest Asia
Giant hogweeds are distinguishable by their massive size — ranging up to 15 to 20 feet tall — and thick base.
The plant is widespread along creeks, where it forms dense canopies and erodes soil. The state Noxious Weed Control Board tagged giant hogweed as a public safety hazard, because its watery sap contains chemicals that cause light sensitivity, which can lead to severe burns and blistering.
Its stems and stalks, darkened with reddish splotches, have giant compound leaves that resemble parsley — though, these can grow to be 5 feet wide. Small clusters of white flowers sit at the plant’s top. Giant hogweed can be easily confused with cow parsnip, which is a native plant in Washington.
- Policeman’s helmet (Class B); native to India and Western Himalaya
Policeman’s helmet has a meager presence in Clark County, but its population can potentially surge. Collell said the prime reason is due to it being sold as a “Poor Man’s Orchid” for its similar features to the titular flower.
The weed thrives in moist riparian areas and can germinate underwater, making it easy to spread and develop downstream. Mature pods split and eject seeds when touched, earning it the nickname “touch me now,” according to the state Noxious Weed Board.
Those who identify an invasive plant on private or public property can report the sighting online to Clark County Public Works. For further help weeding out criminal plants, contact Clark County Weed Management via email at weed.management@clark.wa.gov or call 564-397-6140.
