Most of us know the plants we want to steer clear of, those we don’t want to tangle with.
Experience and knowledge tells us there are some plants that fight back through either chemical or physical means. These defenses have evolved over time to protect the plants from their major enemies — most often grazing/browsing animals or insects that would eat the plant.
There are those of us who are lucky enough to have been taught at an early age to identify some of these plants. Sometimes, though, the knowledge of plant weaponry comes from firsthand experience.
I recently hosted a day camp for children where we learned about aspects of sustainability. We had the camp in a small public park, and the first thing that I noticed was the abundance of poison ivy all around the area. Since I’m one of those people who is highly sensitive to poison ivy (we’re talking weeping blisters that itch and burn), I quickly taught the kids how to identify it by looking for the characteristic three leaflets.
The plant — and others in its family: poison oak, poison sumac and, in tropical places, mango and cashews — produce a resin called urushiol. This resin is a means of chemical defense against predators (animals who would eat the plant).
The reaction to poison ivy is a true allergy, meaning that it only affects people who are truly allergic to it. Some people could roll in a patch of it and never be affected. But I’m so allergic, I think I can get it just by looking crossways at a nearby plant.
There are physical plant defenses as well. Thorns on briers protect plants from being eaten by animals by causing pain in sensitive mouth tissue.
Sometimes plants deploy both defenses, such as in the case of stinging nettle. The nettle plant is covered with tiny hairs called trichomes. These hairs act like tiny needles that deliver chemical irritants, including histamines, into the skin of anyone who just lightly brushes up against it. I can say from experience that running into stinging nettle is not a pleasant experience.
But what about plant defenses you don’t know about? What about plants you haven’t experienced? There’s been much media coverage lately about a new plant making its way across the country that is one nasty character to deal with — giant hogweed (Heracleum mantegazzianum). This plant can cause some serious damage, unlike many of its relatives in the carrot family Apiaceae.
If you haven’t seen the coverage on the news or on social media, giant hogweed is a monstrous plant (8 to 14 feet tall when mature) that produces a chemical called furocoumarin. This chemical causes hypersensitivity to sunlight in the skin, resulting in severe phytodermatitis. On contact with the skin, furocoumarin breaks down, forming derivatives that enter the cell, bond to DNA and cause cellular death. The result can be burns, blisters, scars and blindness. Since the reaction is delayed by hours, there have been cases where people have extensive contact with the plant that causes widespread damage.
There haven’t been any reported sightings of giant hogweed in our area, but it is a good idea to be on the lookout. With the plant being in the news recently, I was alarmed when I received a call from a homeowner this past week asking me to identify what she thought was possibly the vile hogweed on her property. Her inquiry started when her son had to be taken to the hospital with what looked like chemical burns on his body.
She had bought the suspected plant at a local market, thinking that it was the herb angelica. But could it be hogweed?
I made my house call and snapped a few pictures of the plant to study later and compare it with pictures of giant hogweed. The identification was a little harder since they had cut the plant down and it was coming back as new growth. The conclusion — it could possibly be hogweed, but I’m not sure enough to make a conclusive answer (I’m still waiting on a second opinion). It definitely wasn’t angelica.
Even if it wasn’t hogweed, the plant could have caused the son’s painful burns. It turns out that giant hogweed isn’t the only member of the carrot family that produces furocoumarin. In fact, many produce small amounts of it.
Angelica can truly produce a burn effect if exposure is great enough. Other plants in the family, such as wild parsnip and cow parsnip, can have the same effect. Even long-term exposure to carrot plants, such as long-term harvesting on large farms, can cause burns. Other family members have even more notorious protections. Just ask Socrates about family member poison hemlock.