The Penn State Extension states that “a weed is a plant out of place, not intentionally sown, whose undesirable qualities outweigh its good points.” There is no scientific definition of a “weed,” it is not a botanical classification. A plant is a “weed” if it strikes you as “undesirable” in whatever environment or situation it is growing in. Any plant can be a “weed” in just the right (or wrong) context.
What some of the undesirable characteristics that can make a plant a “weed?”
Weeds compete with “desirable” plants for space, for light, for nutrients, and for water. A plant can really express its “inner weediness” by growing very rapidly or growing on top of the other plants around it! Weeds also may be poisonous or irritating, or they may house or sustain plant pathogens or pests. They might also do damage to roads, sidewalks, drains, etc. “Undesirable” can be expressed in many dimensions, but most plants are called “weeds” simply because they are not the plants an observer (or gardener) wants to see!
Weeds can also hold vegetative ecosystems together. They can provide food for many herbivores and pollinators, and they can express their own qualities of aesthetic beauty and grace.
My neighbors define anything other than “grass plants” growing in their lawns as weeds. I look at my lawn and see dozens of tightly growing plants making a herculean effort to stay green all summer! There are several types of clover (great nitrogen fixers!), smartweeds, cinquefoils, chickweeds, speedwells, and plantain galore in my lawn. The bumble and honey bees swarm the clover flowers for nectar and pollen, and the rabbits avidly eat the clover and the plantain leaves (plantain is called “rabbit lettuce”).
The flower beds around our house also contain some very suspect plant species that we have planted or, at least, encouraged to grow (like milkweed, wild carrot (“Queen Anne’s lace”), and creeping bellflower). The edges of our woods are rich ecotones of Joe-Pye weed, ironweed, goldenrod, several types of tall clovers, thistle, jewelweed, milkweed, black raspberry and blackberry. The seasonal sequence of their flowers puts a cultivated flower garden to shame, and the ripening berries are a precious resource for both birds and jam lovers everywhere!
Deborah and I noticed on our Baker Trail hike a few years ago that field edges and drainage ditches were the best places to look for rare plant species. These ecosystems contained many more kinds of both native and introduced plants than the surrounding zones and formed complex, highly structured communities with both great ecological and aesthetic qualities. I remember many hours spent along the side of the roads that make up a great deal of the Baker Trail carefully poking through the ditch plants and finding orchids and other tiny flowers under the dense vegetative cover.
Remember, weeds are in the eyes of the beholders! Each plant species has its own matrix of worth and beauty and it’s up to us to appreciate as many of them as possible!