Stinging Nettle, Urtica dioica, hosts the Satyr Comma, Milbert's Tortoiseshell, and others. This plant was common streamside at Icebox Canyon below Mt. San Antonio in the San Gabriels.
Don't touch the stems! I made this mistake, barely touching one; the pain in my finger lasted about twelve hours. These "hairs" are like little hypodermic needles, and deliver a dose of formic acid, histamine, and more.