Who owns the land that you want to spray? There are different considerations depending upon ownership.

It's a mix of private, county, county parks, state, and federal. But the ecology is the same regardless, and this is an ecological question: what spray will kill scotch broom and least bother the ecology? Spray is needed on account of the mini-ecology in many places, as the scotch broom has provided windbreak and shade. But we don't want to hurt the plants that have grown up in the shelter of the bushes, or severely impact any soil ecology.

One of the most effective ways to kill Scotch Broom selectively is to use a "cut stump" method. The plant is cut off and within a few minutes (before the wound can dry out) an herbicide such as "Crossbow" is painted or sprayed on the cut stump. Make sure to read and follow the directions specifically for the method of application and follow all safety precautions. A foliar spray of a product such as "Crossbow" in the spring may also be fairly effective. Mature plants are less susceptible to herbicides. "Crossbow" is a selective herbicide that kills broadleaf plants, but not grasses.

It is illegal to use an herbicide on land that you do not own. Anyone applying herbicides to public land must have a public applicator's license or a commercial applicator's license.