False brome is not your ordinary grass species. This introduced bunch grass spreads into shaded woodlands and can carpet the forest floor, excluding native species and increasing fire risk. Sometimes used as an ornamental grass, false brome (Brachypodium sylvaticum) first spread extensively in the Willamette Valley in Oregon and has since moved into areas of Washington and California. It has a broad native range in Europe, Asia and North Africa where it most often grows in forested areas. Unfortunately, because of this shade-tolerance, false brome poses a higher risk of invasion here in the Pacific Northwest because of our extensive forests and woodland habitats.
In King County, we first encountered false brome in a landscaped area on Vashon Island where it may have been planted or brought in accidentally. This small patch has been easily controlled because of its size and it appears not to have spread beyond the planting bed.
Sadly, the same cannot be said for the numerous patches of false brome recently discovered in east King County by our noxious weed specialists and the King County Roads vegetation management crew. There appear to be two main areas of infestation. The largest so far is in the Lake Kathleen/Lake McDonald area, where there are patches along roadsides as well as some large infestations in forested areas. The second main area is in the Mirrormount neighborhood, where initial surveys show it mostly along roadsides but also spreading into forests as well.
According to Minwook Park, our noxious weed specialist for east King County, false brome is likely being spread through mowing as well as helped along by deer and elk, based on the patches found along wildlife trails heading into the woods. Where it has spread into forests, it is behaving like in Oregon, creating a solid carpet and excluding other plants.
Because false brome is a Class A noxious weed and still very limited in distribution, we are focused on early detection and rapid response. The initial goal is to contain it along the roadsides and to manage the few large patches found last year, as well as new sites found this spring. Hopefully, we have found the extent of the infested areas, but more surveys may turn up new patches. Grasses are notoriously hard to identify, and it is very possible that this one has escaped our notice in other parts of the county.
It will take a team effort to contain this difficult invasive grass. So far all of the false brome sites are in unincorporated King County, and King County Roads has been working hard to control it on the public roadsides. When false brome is spotted on private property, Minwook and our other staff contact the property owners and offer to control it for them, or they show people how to do it themselves if that’s their preference.
It would be great to get help finding new locations, but because false brome is difficult to identify and easily confused with other grasses, it is very important to get confirmation of the species before it’s controlled. Also, we would like to know where it is growing. As with most invasive species, there is usually more around than you first suspect, so it’s important to search nearby areas for more infestations.
False brome is a perennial bunch grass that stays green most of the year. The leaf blades are lime-green, flat and arching or drooping. The flowers (called spikelets) are also drooping on tall, slender stems and either stalk-less or on short stalks. Key identification traits include hairs along the edges of the wide, flat leaves, lower stems covered with fine, even hairs, and an open leaf sheath (meaning the base of the leaf freely releases the stem when the leaf blade is pulled back). False brome looks like a true brome but differs in that the spikelets aren’t on long stalks.
There are helpful, closeup pictures of false brome on the websites listed below. However, the best way to be sure is to contact one of our noxious weed specialists or send the location and a photo through our online Report a Weed form or the King County Connect mobile app so we can check it out in person.
For more information and photos of false brome: