Hiking through America’s national parks offers breathtaking views and unforgettable experiences, but some green residents along those trails can turn your adventure into a painful ordeal. Knowing which plants to avoid can save you from uncomfortable rashes, severe allergic reactions, or even hospitalization.
Last summer, I watched a fellow hiker casually brush against what looked like ordinary foliage, only to develop angry red welts within minutes. The ranger who helped him mentioned that plant identification is one of the most overlooked survival skills for outdoor enthusiasts.
Whether you’re a weekend warrior or seasoned backpacker, memorizing these troublemakers will keep your outdoor adventures memorable for the right reasons. A quick glance at distinctive features can mean the difference between an enjoyable hike and an emergency room visit.
1. Poison Ivy
Look for the classic “leaves of three, let it be” pattern with shiny, reddish stems. Each leaf cluster has a longer middle stem, and leaf edges might be smooth or notched. The leaves appear glossy green in summer and turn reddish in fall.
Contact causes an itchy, blistering rash that can spread if the plant’s oils remain on skin, clothing, or pets. The reaction typically appears 12-72 hours after exposure.
Once mistook this for Virginia creeper on a Blue Ridge Parkway hike. The resulting rash lasted two miserable weeks, teaching me to always look twice at three-leaved plants.
2. Poison Oak
Sporting the deceptive “leaves of three” pattern similar to poison ivy, poison oak has lobed, oak-like leaves with scalloped edges. The leaves appear fuzzy rather than shiny and turn brilliant red-orange in autumn.
Brushing against it releases urushiol oil that causes painful, itchy rashes lasting up to three weeks. Even dead plants retain their toxic oils for years.
My first encounter happened in Yosemite when I cleared a small bush from our campsite. The welts appeared the next morning, turning a pleasant camping trip into an uncomfortable ordeal.
3. Poison Sumac
Unlike its three-leaved cousins, poison sumac features 7-13 smooth-edged leaflets arranged in pairs along a reddish stem. The compound leaves resemble feathers, and small, greenish-yellow berries hang in loose clusters.
Many consider this the most toxic plant in the family, causing severe rashes and blisters upon contact. It thrives in boggy, wet areas of eastern parks.
A ranger at Congaree National Park pointed this out during a swamp tour, mentioning that many visitors mistake it for harmless sumac varieties. The difference? Harmless sumac has red berries that stand upright.
4. Stinging Nettle
Standing 2-4 feet tall with opposite, heart-shaped leaves covered in tiny needle-like hairs. These hollow hairs act like hypodermic needles, injecting irritants when touched. The stems are square and the plant often grows in dense patches along trails and disturbed areas.
Brushing against nettles causes immediate burning, stinging, and itching that can last several hours. Tiny welts appear where the hairs contact skin.
During a rainy hike in Olympic National Park, I slipped off-trail into a nettle patch. The burning sensation was instant, though rubbing the juice from a nearby dock leaf provided surprising relief.
5. Giant Hogweed
This massive plant towers 8-14 feet tall with huge, deeply-lobed leaves spanning up to 5 feet across. White flower clusters form umbrella-shaped heads up to 2.5 feet in diameter. Purple blotches and coarse white hairs mark the thick, hollow stems.
Contact with sap causes phytophotodermatitis – painful blisters that worsen in sunlight and can leave permanent scarring. Even brushing against it can transfer the toxic sap.
Spotted this monster plant from a safe distance in Shenandoah. A fellow hiker mentioned her cousin needed hospital treatment after clearing one from his property without protection.
6. Manchineel Tree
Found primarily in Everglades National Park, these unassuming trees have shiny green leaves similar to apple trees and small, sweet-smelling apple-like fruits. Spanish explorers called them “little apples of death” for good reason.
Every part contains potent toxins that cause severe blistering on contact. Standing under it during rain can cause skin burns as water carries toxins from leaves. Eating the fruit can be fatal.
Park rangers often mark these trees with red paint and warning signs. One guide told me indigenous people historically used the sap for poison arrows – that’s how toxic this innocent-looking tree really is.
7. Jimsonweed
Recognizable by its trumpet-shaped white or purple flowers and spiny, egg-sized seed pods. The large, toothed leaves emit a foul odor when crushed. Growing 3-5 feet tall, it’s often found in disturbed soils and along trails.
All parts contain dangerous levels of tropane alkaloids. Even casual handling can transfer toxins through skin, causing hallucinations, elevated heart rate, and confusion.
Came across this during a Joshua Tree camping trip. The ranger mentioned that every year, curious hikers end up in emergency rooms after touching or smelling the distinctive flowers, mistakenly thinking it’s harmless.
8. Poodle-Dog Bush
Common in burned areas of California parks, this plant stands 3-6 feet tall with purple bell-shaped flowers and sticky, hairy stems. The fuzzy, lance-shaped leaves have a distinctive sweet smell some compare to marijuana.
Contains prenylated phenolics that cause severe dermatitis similar to poison oak but potentially more painful. The reaction often doesn’t appear until 12-36 hours after contact.
Hiking in Sequoia after a wildfire, my friend pointed this out just as I was about to grab it for support on a steep section. The ranger later explained it thrives after fires, making it a hidden danger in recently burned areas.
9. Wild Parsnip
Featuring flat-topped yellow flower clusters atop 4-foot grooved stems, wild parsnip resembles dill or Queen Anne’s lace. The compound leaves have saw-toothed edges and grow in an alternating pattern along the stem.
The sap contains furanocoumarins that cause painful, burning blisters when skin is subsequently exposed to sunlight. These chemical burns can leave dark scars lasting months or years.
Nearly brushed against a patch while photographing wildflowers in Rocky Mountain National Park. Another photographer warned me just in time, sharing that his previous encounter left scars that lasted over a year.
10. Death Camas
Growing from bulbs in meadows and grasslands of western parks, death camas produces clusters of cream-colored flowers on stems 1-2 feet tall. The grass-like leaves emerge from the base, and the plant resembles wild onions or edible camas.
All parts contain zygacine and other alkaloids that can cause severe illness or death if ingested. Even handling can transfer toxins that irritate skin and eyes.
Encountered this in Grand Teton’s alpine meadows where it mixed with edible plants. A wilderness guide emphasized that foraging without absolute certainty is never worth the risk, especially with look-alikes this dangerous.
11. Tree Of Heaven
This invasive tree grows rapidly to 80 feet with compound leaves containing 11-41 leaflets with smooth edges except for distinctive notches near the base. When crushed, the leaves emit a strong, unpleasant odor often described as rancid peanut butter.
Contact with sap can cause myocarditis (heart inflammation) and dermatitis in sensitive individuals. The tree also produces allelopathic chemicals that prevent other plants from growing nearby.
Found these throughout Shenandoah, where a ranger explained they’re aggressively removing them. The tree’s rapid growth and toxicity make it a double threat to native ecosystems and unwary hikers.
12. Water Hemlock
Often called “the most violently toxic plant in North America,” water hemlock grows 3-6 feet tall near streams and wetlands. Look for small white flowers in umbrella-shaped clusters, purple-streaked hollow stems, and twice-compound leaves with serrated edges.
Contains cicutoxin that attacks the central nervous system if ingested. Even small amounts can cause seizures and death within hours. Handling can transfer toxins through cuts or abrasions.
A naturalist at Acadia pointed this out during a wetland walk, mentioning its resemblance to edible Queen Anne’s lace. The key difference? Water hemlock’s leaf veins run to the notches between teeth, not to the tips.
13. Devil’s Club
Unmistakable in Pacific Northwest forests, devil’s club has large maple-like leaves up to 12 inches across on stems covered with vicious yellow spines and thorns. Small clusters of white flowers develop into bright red berries by late summer.
The spines easily break off under skin, causing painful infections that can last for days. Native to Olympic and Glacier National Parks, it forms dense thickets that are nearly impassable.
Made the mistake of grabbing a branch for balance on a slippery trail in Olympic National Park. Spent the evening removing tiny spines with tweezers while a park volunteer shared that indigenous peoples used it medicinally despite its defensive nature.
14. Pokeweed
Growing up to 10 feet tall with smooth purple-red stems and large, lance-shaped leaves. The most distinctive feature is the hanging clusters of dark purple-black berries on pink stalks that appear in late summer.
All parts contain phytolaccine and other toxins that cause severe gastrointestinal distress if ingested. The berries, though tempting, are particularly dangerous. Handling can cause skin irritation in sensitive individuals.
Found this growing along the edge of a campground in Great Smoky Mountains. A local mentioned that despite its toxicity, their grandparents used to prepare the young spring leaves (called “poke sallet”) after special processing to remove the toxins.
15. Cactus Glochids
These aren’t a specific plant but rather the tiny, barbed hair-like spines found on prickly pear and cholla cacti throughout southwestern parks. Almost invisible and deceptively soft-looking, they detach easily upon the slightest touch.
Glochids embed in skin with backward-facing barbs, causing intense irritation that worsens as you try to remove them. They can become infected if not properly removed with adhesive tape or tweezers.
Learned about these the hard way in Big Bend when I knelt to photograph a desert wildflower and brushed against a prickly pear. The park’s first aid station sees dozens of visitors each week with similar painful souvenirs.
16. Trumpet Creeper
This aggressive climbing vine features compound leaves with 7-11 toothed leaflets and spectacular trumpet-shaped orange-red flowers. The woody vine can climb 30-40 feet using aerial rootlets that cling to surfaces.
Contains compounds that cause contact dermatitis in about 1 in 5 people. The reaction varies from mild redness to painful blistering similar to poison ivy. Repeated exposure often increases sensitivity.
Noticed this beautiful vine covering trees in Mammoth Cave National Park. The ranger mentioned that while hummingbirds love the flowers, many gardeners develop mysterious rashes before realizing this ornamental plant is the culprit.
17. Cow Parsnip
Standing 4-9 feet tall with huge, deeply lobed leaves up to 2 feet across. White flower clusters form flat-topped umbels up to 8 inches in diameter. The hollow stems are ridged and covered with fine hairs.
Contains furanocoumarins that cause phytophotodermatitis – painful blisters that develop after touched skin is exposed to sunlight. The reaction can take 24-48 hours to appear and may leave dark scars.
Frequently seen along moist trails in Glacier National Park. A hiking guide explained that indigenous peoples used it medicinally and as food after careful preparation, but modern hikers should admire it only from a safe distance.