With the outdoor gardening season starting up, it may be useful to remind gardeners (and hikers!) to avoid, if possible, any contact with phototoxic plants, present in both our gardens and in fields and forests, or at least, to avoid them on sunny days.
A phototoxic plant is one that causes a cutaneous reaction in humans or animals when there has been contact with the skin followed by exposure to the sun. You only have to brush against one to be affected, although actually handling the plant will make things worse. So, unlike plants that cause skin reactions under any circumstance, such as poison ivy (Toxicodendron radicans) or stinging nettles (Urticaria spp. and Laportea spp.) and where the reaction can therefore take place at any time, even at night, phototoxic plants only wreak havoc on beautiful sunny days … when gardeners and hikers are most likely to be outdoors. And unlike allergic reactions, there is no need to have been exposed to the plant in the past.
The main chemical compounds that cause phototoxic reactions are furocoumarins, produced by plants to protect themselves from predators. They are mainly found in two plant families: the Apiaceae (ombellifers) and the Rutaceae (the orange family). Some are wildflowers or weeds, such as giant hogweed (Heracleum mantegazzianum) and wild parsnip (Pastinaca sativa), while others are garden plants, such as rue (Ruta graveolens), a medicinal and edible herb, and gas plant (Dictamnus albus), an ornamental perennial.
Susceptibility to phototoxic plants (called photosensitivity) varies greatly from one individual to the next. Some people can manipulate many phototoxic plants with no reaction whatsoever while others are only affected quite lightly, with a reaction the equivalent of a mild sunburn. However, in other people, the affected skin darkens or becomes covered in painful blisters that may even leave permanent scars. Extremely sensitive people sometimes end up in the hospital! Children tend to be more photosensitive than adults, but, as with allergies, some adults become more seriously affected after repeated contacts.
Which Plants to Watch Out For
Phototoxic plants were not all created equal. Some are much more dangerous than others. Giant hogweed is considered extremely phototoxic and most people need to avoid it, but some people react to plants that are rarely a problem for the average human, such as wild carrot (Daucus carota), a weed common in fields almost everywhere and to which only a small minority of people react. Several common garden herbs and even a few everyday vegetables can cause phototoxic reactions in very sensitive people, especially when they handle their foliage, although furocoumarins can be present in some roots as well. It you know you’re sensitive to furocoumarins, it might be wise to touching anise, angelica, celery, carrots, coriander, dill, fennel, lovage, parsley and – above all! – rue (about a third of all people are sensitive to the latter). Among tropical edibles, reactions to limes, lemons and sour oranges are not rare and some people develop lip blisters from eating figs (from another family, the Moraceae). It’s perhaps best to eat these in the shade or at night!
Even if you have never reacted in the past to plants known to be very phototoxic, such as giant hogweed and wild parsnip, it is always best to avoid contact with these plants, especially on a sunny day. Even on a gray one, it’s always wiser to wear gloves and long sleeves and pants if you’ll be handling potentially phototoxic plants … and to wash well with soapy water once back at home!