And YES, weed is a valid "category" of plant, they are plants falling under one or both of the following categories:
Disturbance-adapted "disaster species" whose niche is to rapidly establish themselves in recently disrupted or destroyed environments. When a tornado or flood wipes out the existing plant community, the weeds are the first to return, and they get the restoration process started
Plants that have co-evolved with humans to live in human-created environments (like agricultural fields)
Yes! They are symbiotic with us! It's cool because everyone knows about artificial selection as a means of creating crops and domesticated animals, but humans have also selected the traits of weeds for thousands of years, and they have evolved a lot in response!
Many weeds of crops have changed the times and seasons when they bloom and produce seed so they can live in fields. Their growth shape and rate has changed. They have even evolved to mimic crops in a cool process called Vavilovian mimicry!
Is that weeds often get domesticated into crops themselves! Some of our crops probably started out as weeds.
And also, crops can turn back into weeds! In Europe, teosinte, the wild ancestor of maize, has become an agricultural weed...and the CRAZIEST part, is that it has obtained genes from genetically modified corn crops, and become resistant to herbicides as a result!
Weeds kind of follow us around and hang out at the edges of our settlements, perhaps in the same way domesticated dogs and cats must have done. Whenever you see Queen Anne's Lace at the edge of the road, consider that it is also known as Wild Carrot... (but don't try eating it, Poison Hemlock can look pretty similar!)
And they evolve FAST. We think about evolution as occurring slowly, over millions of years, but weeds constantly challenge what we think we know about evolution, demonstrating that organisms can evolve in the blink of an eyeāin just a few decades. Some of them have even turned into new species.
And of course many of us have heard about herbicide resistant weeds. Chemicals like Glyphosate (Roundup) were thought to be a miracle for revolutionizing agriculture, but weeds are rapidly evolving resistance to herbicides, and every passing year they get stronger and more effective at resisting these chemicals. Amaranthus palmeri, a species of Amaranth, has been found resistant to SIX different herbicide families AT ONCE.
The most accomplished of all herbicide survivors is a little Poa annua found on a golf course, which resisted seven families of herbicide.
Sometimes they change the chemical mechanisms the herbicides target. Sometimes they release enzymes that neutralize the herbicide. Sometimes they make a hundred duplicates of the gene that codes for the protein targeted by the herbicide. Sometimes they create little storage closets in their cells where they stuff all the herbicide away where it can't hurt them. Sometimes they strengthen the protective coating of their leaves so the herbicide can't absorb. It used to be that these adaptations had a fitness cost, but the plants are getting better at it. There's Amaranth now that is completely unaffected by doses of herbicide 18 times what is recommended on the label.
Did you know Amaranth is edible? Just a fun fact...
Weeds are such an awesome, almost spiritually potent facet of nature. They are liminal creatures, always existing on the margins, defying the boundary between wild and tame. They are the food and medicine that sustains our bodies, essential to our civilizations from the very beginning, but they destroy our ambitions of control.
They are plants of the immigrant and enslaved, the friend of the impoverished and marginalized, essential to the lives of the poor all throughout history, and the scourge and ruiner of hollow aesthetics of wealth. They take care of us. Yarrow which dressed the wounds of Achilles, Kudzu which clothed the people of China for thousands of years and fed them through many famines, Dandelion which is good for food and has myriad medicinal properties, Pokeweed which is poison if prepared incorrectly, but important food for the impoverished, Amaranth which fed the great empires of Mesoamerica...
Wherever we are, weeds are too. They evolve rapidly and continuously, eternally transforming themselves so they can never be destroyed. They sustain and heal us, but we are powerless to control them. Weeds are inevitable, indomitable, and an inherent consequence of Nature's movement and powerful striving to LIVE.
And they are innately tied to our existence. We made weeds. Weeds made us. They remind us that we are part of Nature's movementāhowever we alter and shape the world around us, something adapts, stitching the gap shut as soon as it is opened. We exist on this earth, therefore a great diversity of life DEPENDS on our existence.
Is this not the coolest, most thrilling thing ever