My neck of the woods isn’t like the Chihuahuan Desert, where nearly everything that photosynthesizes seems like it evolved to grab, shred, tear, puncture, and stab you (just try an off trail hike at Carlsbad Caverns National Park if you want the experience and say hello to the lechuguilla while you do). Nor is my habitat like the poison-oak dominated slopes found in coastal California where a careless walk through brush can leave you itchy for weeks. No, not like that. Heck, I don’t even need to worry about ticks.
Along the Skagit River, devil’s club and a couple of species of invasive blackberry will stop you in your tracks with their numerous, stout thorns. Besides those few, the list of plants to avoid drops off fairly quickly, with a notable exception. One of the most ecologically interesting and menacing members of my plant community is a nondescript perennial that’s easy to ignore until it’s too late.
Lots of plants are fuzzy with fine hair. Some plants, like common mullein (Verbascum thapsus), utilize hairs on their leaves and stem like sunscreen and to make grazing just a little uncomfortable for herbivores. Some hair is just there, perhaps not serving a specific adaptive purpose, or not one that we know currently. But one plant in my forest, Urtica dioica or stinging nettle, has turned their hairs up to 11.
Nettle is rather inconspicuous. It has oppositely-arranged, coarsely-toothed, and heart shaped leaves. Its flowers grow in small, string-like clusters from the leaf axils and lack petals, typical for a wind pollinated plant, but what it lacks in showiness it makes up in its ability to inflict pain.
I learned about stinging nettle as a young teenager scrambling up a creek bank in Pennsylvania. The bank was steep and muddy. I needed just a little extra support to prevent me from sliding down. Lacking a tree to hold, I grabbed a group of herbaceous stems and immediately realized I had made a mistake. I made it up the bank, but the palms of my hands burned for the rest of the day. I was just introduced to nettle’s defense against mammalian herbivores.
Stinging nettle is equipped with tiny, but potent, stinging hairs. On the plants in my area, the hairs are particularly concentrated on the stems, flowers, petioles, and leaf undersides. Each hair is tipped with a small, fragile bulb that breaks off when contacted to expose a needle-like tip that, hardened by calcium carbonate and silica, readily injects a cocktail of chemicals into your skin. The stinging sensation is immediate and long lasting.
Among other chemicals, the juice inside a hair contains histamine, which is an inflammatory compound (we take antihistamines to inhibit the affects of allergic reactions), and serotonin, which constricts blood vessels and acts as a neurotransmitter. In sum, it is designed to irritate.
Why the need for this defense? Nettle leaves are nutritious and high in vitamins A and C as well as protein. They would likely be a sought after commodity by deer and other browsing mammals if it weren’t for their stinging hairs.
We can neutralize the sting by drying or steaming the leaves. Steamed, the leaves taste as mild as spinach and they make a decent pesto.
The rash you get from poison ivy is an accident of evolution. The oily liquid, urushiol, which causes the itchy dermatitis on us doesn’t affect other North American mammals or birds. Your dog won’t get it. Deer eat the leaves. Many bird species relish poison ivy fruits for food. The stinging hairs on nettles tell a different story. They are purposefully indiscriminate against all mammals.
Plants, like all life forms, experience a wide variety of limiting factors. Stinging nettle may have evolved one way to dissuade herbivorous mammals, but the same defense doesn’t deter insects or snails. The stinging hairs don’t work on parasitic fungus or microorganisms either, nor on anything that attacks and eats its perennial rhizome. But, its stinging hairs work, quite well in fact for their evolved purpose—discouraging mammals from eating it.
Despite the pain nettle can inflict, I look forward to seeing it sprout each spring. It gives me an opportunity to reflect upon why it needs to evoke such discomfort in mammals. Stinging nettle is a plant with teeth. It fights back.