Giant hogweed does the worst damage of anything you can brush against. Sap contact has caused blistering rashes, permanent scarring and even blindness. Poison ivy and poison oak hurt far more people in a year, though, so the ranking of plants poisonous to touch depends on what you mean by worst. One plant does the deepest single injury. The other group does the most harm in total, and you are much more likely to find that group in your own yard.
A shaded trail edge in the humid Ohio River valley, and the vines along it had a faint shine I took for morning damp. That shine is urushiol, a colorless resin sitting on the stems and leaves of poison ivy, oak and sumac, where it helps the plants hold water. I left both vines standing and went back for gloves before I pulled anything off that path. I never touched a leaf that day. My own eyes were not going to catch that resin anyway, because it carries no color and no smell.
The two hazards do not work the same way, and that changes what you should do after contact. Urushiol sets off an allergic reaction in your skin, so the damage comes from your own immune system rather than from a burn. Hogweed and wild parsnip work through sunlight on the exposed skin, which means the sap alone is not the whole story.
Giant hogweed is hard to miss once you know the shape. The plant regularly clears 6 feet (1.8 m) and sometimes reaches 18 feet (5.5 m), on thick hollow stems that are ridged and marked with purple spots. Giant hogweed sap is the part that hurts you, not the leaves or the flower head.
Wild parsnip is the smaller cousin you are far more likely to mow into. The plant stands 2 to 5 feet (0.6 to 1.5 m) tall with hairless grooved stems and yellow flowers, and turns up along the roadsides and field edges you mow past. The sap works the same way hogweed sap does.
The common hazard needs almost nothing to set you off. According to CDC and NIOSH, an amount of urushiol smaller than a grain of salt is enough to raise a rash in 80% to 90% of adults. That is why one careless handful of weeds ruins your weekend, and why a urushiol rash is the outcome most gardeners meet at some point.
Cover the skin before yardwork. Long sleeves, gloves and long pants keep resin and sap off you, and they cost nothing but a warm afternoon. If sap does land on you, rinse the skin right away with rubbing alcohol or dish soap and plenty of water, and scrub under your nails where the resin hides.
Poison hemlock does not burn skin. It causes no skin irritation at all and is toxic only when eaten, so the roadside plant people warn each other about is not the touch hazard they think it is.
Keep skin that met hogweed or parsnip sap out of sunlight after you wash, since light is half of what causes the burn. And learn the two stem patterns rather than the leaves. Purple spots on a hollow ridged stalk, or a smooth grooved stem under yellow flowers, tell you more than any leaf count ever will.
Read the full article: Poisonous Plants: Which Ones Are Truly Deadly
