Weeds are considered weeds because they’re good at what they do (growing & spreading). So good that perhaps you have wondered how certain plants could ever not be an issue, even in their native range. It’s hard to definitively say why some weeds are not weedy where they come from. But, with enough digging we were able to figure out just how three species we consider “pests” in the PNW (Pacific Northwest) maintain balance in their native habitats.

Before diving into the borderline philosophical question of why some plants are invasive while others are not, we must clarify what exactly an “invasive plant” is in the first place and where the concept of “invasion” in ecology gets is roots. 

The roots of invasive ecology

The concept of [plant] invasion has wartime roots. It describes the act of entering a space by force in large numbers. It was first applied to ecology by animal ecologist Charles Elton in his 1958 book, The Ecology of Invasions by Animals and Plants. During World War II, Elton was responsible for protecting food rations from rodents like mice and voles. This is important to note because his writings are the basis of the language that we still use when it comes to invasion ecology. Thus, much of our understanding of invasion ecology is based on a survivalist, wartime mentality. The result is militaristic language such as “invasion”, “aggressive”, and “takeover”, etc. being projected onto plants and the belief they are capable of ill intent. This narrative of plants “invading” is built into much of our current understanding of weeds. Recognizing this is not to dismiss the impact that many of these introduced plants have, but to acknowledge that plants do not have ill will. Plants do not grow with the intention of causing problems, but rather to survive in a space they would have never been if not introduced by humans. 

What makes a plant “invasive”?

Plants labeled as invasive typically grow quickly and spread readily once introduced. So how are they not invasive in their native range? Removing a plant from its native habitat also removes it from its native controls (factors that keep the species in-check). This may be a particular pest, climate, or cultural practice. It’s not to say that some plants do not spread “like weeds” in their native range, just that there they have co-evolved with their ecosystem since time immemorial and are an inseparable part of the landscape. Thinking about why these plants are out of balance here and not in their native range prompts a deeper understanding of these species. There is limited research on the topic of weeds in their native range, so some natural controls mentioned for the species below are strong inferences (versus experimental/peer reviewed science) based on scientific reasoning and botanical knowledge. If you have any questions, concerns, or insight regarding any of these species in their native range you can email us at noxious.weeds@kingcounty.gov. 

Three (PNW) weeds & their native range controls

Itadori knotweed: volcanoes, soil, & grasses

Itadori knotweed (Fallopia japonica) is known around the globe as being an incredibly invasive species. With growth rates of 2-4 inches per day in the spring and roots that can extend 7+ ft down and 25+ out from each parent plant, how could this plant NOT be invasive wherever it grows? Well, like all “invasive plants”, if knotweed had been left where it was found, it never would have become the weed that we know today.

Knotweed grows naturally in the lava fields alongside active volcanoes in Japan where it has several controls that keep it in check. Knotweed plants in this native range are usually smaller due to poor soils and growth is limited due to repeated coverings of volcanic ash and landslides. Once you imagine a young knotweed plant punching up through layers of volcanic strata, perhaps you can understand how it is able to grow through a concrete driveway. Further, it is a pioneer species that helps break up soils and add nutrients for other native plants like bamboo and grasses that quickly take over following its introduction, keeping knotweed from becoming dominant in the landscape. Here there are also more natural pests and diseases that target knotweed and keep it in check. 

Himalayan blackberry: topography & climate

Himalayan blackberry (Rubus armeniacus) is one of the most well-known introduced species around the world. So how is it not invasive in its native range?

A plant’s invasiveness is subjective. For instance, salmonberry shrubs are native to the PNW region, but a person unfamiliar with the local plant life might assume that it is a weedy species if they see a large patch of it. Many bramble (thorny thicket) forming plants are perceived as weedy. So, it can be assumed that in its native range Himalayan blackberry grows much like it does in the spaces that it has been introduced. And while humans may interpret any thorny thicket as a problem, they are an invaluable part of their native ecosystem, nonetheless.

To our knowledge, the main factor that keeps this species in-check in its native range is the geography, namely topography and climate, of its region. From genetic testing, it is believed that Himalayan blackberry originates in the valley areas within the Lesser Caucasus Mountains (Armenia & northern Iran). While the plants spread in part via their seedy berries, what makes them “weedy” is their ability to form wildly large patches. They do this by spreading outward from established plants via a network of hardy roots that can survive through most seasons in temperate climates.

If left in this valley, it is likely that the species would have been relatively contained by the surrounding mountain ranges and desert areas with the exception of seed spread by migrating birds. Blackberry’s roots have a hard time surviving ground temperatures of less than 15°F, temperatures guaranteed at the surrounding mountain peaks where snowpack averages 5 ft in depth. Additionally, large patches of the surrounding area are desert or desert-like, also not an ideal host for blackberry. In order to become “weedy”, plants must be allowed to establish and spread. This is why Himalayan blackberry does so well in temperate regions. In these surrounding climatic extremes of its native range, to thrive would be a triumphant feat.

Scotch broom: specialized pests

 Scotch broom (Cytisus scoparius) is incredibly adaptable and spreads by seeds that can live in the soil for decades. It was introduced to the Western United States as an ornamental plant and further established when intentionally planted along roadsides as an erosion control species when the highway system was first established.

Known as common broom in its native range of Western Europe, this species is kept in check primarily by particular bugs. Namely the seed weevil Exapion fuscirostre and bruchid Bruchidius villosus are two of many insects that occur in the broom’s native range that have been introduced to the United States to target scotch broom. These and other pest species co-exist in abundance with scotch broom in its native habitat, many of which target the seeds before they can spread. This pre-dispersal predation by insects may reduce scotch broom seed yield by as much as 75% in its native habitat. Both insects have been released in King County by the King County Noxious Weed control program in partnership with the Washington State University Extension. To learn more about the release process check out our blog post.

So why not just introduce more of these known insects? In order to introduce a pest to Washington state as a biological control agent, it takes years of intensive studies that can prove that the species target only the undesirable species. Thus it is impossible to safely introduce all the species that keep scotch broom in-check in its native range. To learn more about biological controls visit Washington State University website.

Conclusion

It’s easier to “hate” a species when it is having a negative impact on your environment. By acknowledging the reality that it is just a plant trying its best, we hope to foster a deeper connection and richer understanding of the novel environment created by these species. Doing so does not dismiss the reality of the impact of these species, but rather makes it easier to pull your weeds with empathy versus apathy. For more information on noxious weeds in King County, visit our website at kingcounty.gov/weeds. Happy weeding!