When I first moved to former home I noticed the lovely field of yellow flowers next door. From mid-July to mid-August St. John's Wort blooms in our dry sandy soil. If you crush the flowers between your fingers, it will leave a rusty reddish stain. Some people soak the flowers in a vegetable oil base to make what is believed to be a healing oil. I found out the hard way that it is not a good way to treat a sunburn on a horse's nose--it actually makes it worse. St. John's Wort causes photo sensitivity--whether it is ingested or as an oil applied to the skin.
If you suspect poisoning from any plant or substance call your veterinarian immediately.