Throughout history people have tied many kinds of plant branches at the end of poles to make brooms. Some of the plant species best suited for broom making came to be known by the name of "broom" themselves. That's the case with one head-high, yellow-flowered bush blooming in our area right now, Scotch Broom, CYTISUS SCOPARIUS, a member of the Bean Family. That's it flowering so prettily above in a very typical environment. As that picture shows, Scotch Broom is pretty. Sometimes you see whole slopes or fields of it and the effect of so much yellow beneath a blue sky is just breathtaking.
The problem with Scotch Broom is that it's invasive, having been introduced from Europe. Its seedlings out-compete native tree seedlings and prevent reforestation. Those pretty thickets of broom have displaced native plants and animals. Moreover the plants burn like crazy, so a bunch of them is a real fire hazard. Finally, the plants contain quinolizidine alkaloids and can be toxic to livestock, though goats seem able to eat it.
Several species of prettily flowering, shrubby brooms are currently running amuck in California -- not only Scotch Broom but also French Broom and Spanish Broom -- all looking fairly similar.
That's a close-up of the ¾-inch flower above.