Beautiful Invasions, Part II: The Generalized Invasion Curve

Read part I here

Reading about scotch broom as I drove away from my wedding was the first time I explicitly remember being able to identify an invasive plant, though I don’t remember wondering what the term meant, so I must have heard it somewhere before. Himalayan blackberry was the next I became intimately familiar with, after buying a few acres and more than a few blackberry patches in the summer of 2020. In King County, the most populous county in Washington state, and the one where Seattle is located, Himalayan blackberries are classified as a class C noxious weed, along with dozens of other species. Noxious weeds are

non-native plants that, once established, are highly destructive, competitive and difficult to control. They have economic and ecological impacts and are very difficult to manage once they get established. Some are toxic or a public health threat to humans and animals, others destroy native and beneficial plant communities.

To be Class C just means that they are so widespread that control is effectively impossible and therefore not required. It’s a surrender, an acknowledgement that humans are not capable of modifying a landscape in any way they see fit after all, at least not without effort or expense beyond what would be gained. Earlier in a plant’s uninvited spread throughout the region, before it becomes Class C and when control or eradication is still possible, attempts must be made. The county says so, but either can’t or doesn’t do much to enforce this. Private landowners are responsible on their own land for removing these invaders that haven’t yet completed their invasion, and counties, cities, and nonprofit organizations will often do so on public lands, bringing their own work crews or rounding up volunteers to participate. Where I live, while we aren’t required to pull blackberries, we must still remove knapweed, milk thistle, gorse. There are others — pages and pages of others.

Not so once a plant is Class C — recommendations about when to control these typically revolve around the impact the offending plant is having on the function of the land, or any economic impact. Said another way: remove it if there’s a tangible cost to not doing so, otherwise don’t bother trying. That economic impacts are so regularly cited…