That’s what the little sign said in the garden next to these flowers. One of the docents was explaining to someone something about holding a match near the flowers fumes and…foom! So I just had to Google it and look what I found.
Dictamnus – In the summer months, the whole plant is covered with a kind of flammable substance, which is gluey to the touch, and has a very fragrant, lemony aroma; but if it takes fire, it goes off with a flash all over the plant. The name “burning bush” derives from the volatile oils produced by the plant, which can catch fire readily in hot weather, leading to comparisons with the burning bush of the Bible, including the suggestion that this is the plant involved there. The daughter of Swedish botanist Carl Linnaeus is said to have ignited the air once, at the end of a particularly hot, windless summer day, above Dictamnus plants, using a simple matchstick.The volatile oils have a reputed component of isoprene. (wikipedia)