Most of us gardeners have to read carefully which plants in our gardens are problematic—what we often call weeds. These plants usually appear seasonally, such as common mallow (Malwa Neglect) that grows in my home meadow with an impressive, if disturbing, degree of hardiness. Every year I weed as much as I could, digging their fibrous, white roots out of the backyard soil. And every year as the soil temperature warms to tomato planting, a fresh crop emerges from the seed bank anew before I start weeding the spot. Occasionally, however, a new unwanted plant makes its debut on our gardening stage.
A new attacker to watch
In short, I wanted to use this month’s post to draw attention to this emerging invader. It has yet to become widespread in our area but it is very alarming, and it has managed to gain a foothold on the Colorado Front Range. The first official Colorado record I could find for this plant was, Epilobium hirsutum (hairy willow herb), came from the early 2000s. Map produced by the Colorado Department of Agriculture Flags plants in at least eight counties.
Considering how many neighborhoods today have natural areas that often double as retention areas for neighborhood runoff, this plant can’t get away from you. Keep your eyes open for it, and take appropriate action to eradicate it if you find it (say, if the plant is on your property), or notify land managers who can do so (if it is on public property).
If you see what you think this plant might be but you’re not sure, your Local Colorado State Extension Office or County Weed Management Office Should be able to help. Worryingly, I’ve also seen species in nursery pots that were blown to seed (yay!).
I’ve been able to find reports of hairy willow herb primarily from the Colorado Front Range, where wind-dispersed seeds have helped it spread relatively quickly. Only a few specimens have been reported from the Wasatch Front and Heber City area in Utah, as well as from the Idaho-Washington border. It is a “List A” noxious weed in Colorado.
Scientific name: Epilobium hirsutum
Common name: Hairy willow herb, giant willow herb, codalines and cream
Habitat: Wetlands, riparian zones, and wetlands.
- Plants are 4 to 6 feet tall.
- Spread aggressively by rhizomes and airborne seeds
- They leave each other opposite.
- Dense, soft hairs are present on most of the above-ground parts of the plant, especially on the upper stems and leaves
- Magenta flowers with four petals appear in early summer and are followed by slender seed pods that split to reveal small seeds surrounded by cottony tufts.
There are several related but well-behaved native species in our region that occur in the same elevation band as this grass, so don’t eliminate them unless you are sure of their identity! The small number of native species that can reach the height of this plant have very small, soft pink flowers and do not run to form a monoculture. Our native species also lack the dense, soft hairs of hairy willow herb.
fire bush (Chamerion angustifolium, zones 2–7) is a native that may resemble the best regional form of this plant and grows primarily at higher elevations where it can form monocultures in disturbed sites. So far, hairy willow herb has not shown that it can colonize the higher reaches of our region, further helping to separate the two species. Fireweed also lacks the dense, soft hairs of hairy willow herb and produces flowers in a dense, single spike rather than a loose cluster. While the flowers of fireweed and hairy willow herb are similar in size and color, fireweed has spaces between the flower petals. Hairy willowherb petals touch and overlap to form a cup.
Prevention is the best management for this plant. Maintaining healthy plant communities, whether in your garden or in the wild, reduces the chances of an invader gaining a foothold. Once established, small populations of hairy willow herb can be dug out by hand, but broken rhizome pieces regrow, requiring several rounds. Stopping flowering by cutting back the stand or mowing can slow the spread of this plant. Large, established stands are best managed by professionals, as chemical control on this scale is the only proven method. Regular scouting is required after eradication to ensure that plants do not re-establish from seed or rhizome fragments in future years.
Learn more about invasive plant management:
Identification and control of common invasive plants
Remove these regional invasive plants.
How to get rid of invasive plants
Brian Fisher lives and gardens at the intersection of the Great Plains and the Rockies. He is a horticulturist and curator of plant collections for the local botanical garden.