Every tree on our property seems to be supporting a luxurious wrapping of poison ivy - poison oak.
The word "toxic" leads the way in the official Latin with good reason;
"dendron" comes from the Greek word for tree;
"radicans" means the plant roots from its stem;
"diversilobum" means many shaped lobes.
Radicans is the usually more vine-like version and diversilobum is the usually more shrub-like poison oak.
This photograph from Wikipedia of a mature poison ivy vine, thick as a child's arm, shows those clinging roots very clearly.
I ripped down a lot of these uglies from the trees surrounding our house in Maryland, before realizing what they were, because the leaves I had been taught to look for were hidden high above in the foliage of the supporting tree.
I discovered the hard way that "Leaves of three, don't touch me" is not warning enough. Poison ivy & poison oak, North American natives, have adapted themselves to a wide variety of locations and conditions and forms.
There is the leafless hairy-monster vine, and the pretty little vine with delicate red leaflets, the dark green shrub three or more feet high, and the sprawling light green ground cover.
The leaves may look slightly shiny, or not.
The leaves may have toothed edges, or not.
The leaves may have a mitten shape, a spoon shape or an oak leaf shape.
They may grow in the woods, or in exposed sunny areas, or between rocks, or in freshly turned soil.
They do not mind brackish water, or season flooding, and are satisfied with acidic or alkaline soil.
Here is a vine clinging to the hackberry in the front courtyard area.
Notice the groups of three leaves.
The terminal leaf on the end hangs down farther than the other two.
The two side leaves are exactly opposite each other and closly attached to the stem.
Notice the varying & irregular lobes.
If the leaves appear to have teeth, they will be irregular, not perfectly spaced around the entire leaf.
No thorns, no hairy stems.
The "poison" is a sap called urushiol.
It bonds to the skin and causes an allergic reaction: itching, rash & blisters that can last for weeks.
Gently washing the skin within ten minutes of exposure to the sap can sometimes prevent an allergic reaction.
The sap can be transferred to clothing, or dog fur, or shoes and cause an allergic reaction in someone who has not touched the plant.
But the rash itself is not contagious, any more than hay fever is contagious.
Each new exposure may cause a more severe reaction than the last.
These plants are becoming MORE toxic!
Why? Carbon dioxide.
Carbon dioxide, a greenhouse gas, is increasing in our atmosphere.
And plants love it because they convert that gas to food, to the sugar and carbs that help them grow big and strong.
For some reason this is especially true of Toxicodendron, and recent studies indicate that it is growing significantly bigger, and it's sap significantly stronger (by 50% or more) than 100 years ago.
Nobody likes a rash, so weed carefully!