What makes a plant a weed?
By Sonja Johanson
When my family first moved into our home, there was a shade bed full of plants that I did not love. I got rid of several yews, some winter creeper, and an undergrowth of vinca. Not terrible plants, just not my taste, and I set to installing shrubs I liked better.
As I worked through the bed, I noticed one plant that had certainly volunteered - Ageratina altissima, white snakeroot. Snakeroot is a native herbaceous perennial, host to several moth species. It’s also toxic when consumed, to both ungulates and humans, and is responsible for the deadly “milk sickness” that likely killed Lincoln’s mother. But I do not have livestock, nor do any of my neighbors, so I decided to let that one plant stay.
Perhaps the best definition of a weed is a plant growing where a person does not want it to grow.
White snakeroot is very pretty when it flowers in the fall. The flowers are held in upright clumps, or panicles, and somewhat resemble a baby's breath. White snakeroot also spreads. A lot. So now I have the plant in just about every shady area of my yard. Gardening friends often advise me to root it out, and I do have to work to keep the population down, but I’m fond of the plant and don’t plan on a scorched earth campaign.
One of the challenges in rewilding an area can be overcoming the perception that the plants are “weedy.” What one homeowner considers excellent wildlife habitat may be an eyesore to a neighbor. What is a weed? Ralph Waldo Emerson famously defined a weed as “a plant whose virtues have not yet been discovered.” Generous of him, depending on how one defines virtue.
Another common response is that non-native plants are weeds, but by that definition we’d better get rid of clover, daisies, and dandelions—all beloved by pollinators. And then there is…the lawn. Lawn grasses such as perennial bluegrass or red fescue are considered desirable by many homeowners when they come up on one side of a line—the intended lawn. But move a quarter inch over into your foundation bed, and suddenly that same plant is a weed (often literally the same plant - tillering grasses are more than happy to move sideways in order to get a little more real estate). Perhaps the best definition of a weed is a plant growing where a person does not want it to grow.
One great way to shift your perspective from “weed” to “wild plant” is to start learning the names of the wild plants in your own yard.
There are all sorts of ways that “weeds'' can grow — as annuals, biennials, herbaceous perennials, and woody perennials. They can be familiar natives like Canada goldenrod, reviled vines such as poison ivy (native, and a source of food for birds, but nobody wants poison ivy growing on their property), or problematic invasives such as Japanese knotweed.
Trees can also be considered weeds, particularly those on the invasive list, like tree of heaven or amur maple. What is considered a weed can vary greatly depending on where you live. Most New Englanders enjoy a spring flush of violets, but the same plant is considered an incorrigible weed in the American South.
One thing all weeds (or wild/escaped plants) have in common is tenacity. There is a wild plant just waiting to grow in any environment. Weeds find a way. Salt loving weeds such as portulaca will do just fine in a hot sidewalk crack. Have compacted soil? Path rush is all about it. Super acid soil? Cue the heaths and heathers; they love it. And wild plants play the long game - their seeds have staggered germination. Bad weather year? No problem, only some of the seeds from that shepherd's purse broke dormancy this year. There are hundreds of others in the seed bank underground just waiting for the right moment. It’s very common for the horticultural weed velvetleaf (Albutilon theophrasti) to pop up in a newly cleared lot. Decades ago when that lot was farmland, velvetleaf was growing there. The farm was abandoned and field pines filled in. When those came down and the soil was disturbed, velvetleaf seeds seized the moment.
Not all weeds are good or even neutral. The Weed Society of America closely monitors weeds that are invasive, and individual states determine when a weed moves onto the invasive list. But the vast majority of wild plants are an important part of our ecosystem. They help retain and build soil, cool the environment, reduce evaporation, and provide food and shelter for a wide variety of insects and vertebrates.
One great way to shift your perspective from “weed” to “wild plant” is to start learning the names of the wild plants in your own yard. A truly fantastic resource for this is Newcomb’s Wildflower Guide. Filled with accurate line drawings and some color illustrations, Newcomb’s also has a beginner friendly key for figuring out which plant you might be looking at. How many petals does this flower have? What color are they? How many leaves and what do they look like? Using Newcomb’s is a great way to learn to look closely at your wild plants and get to know a few each year on a first name basis. Once you know that cool purple flower on what looks like a bean plant is the American groundnut, it will become a familiar friend in the landscape. You might even decide to let a few white snakeroot plants share a shady corner of the garden.