Scotch broom is a noxious weed in the NW US. It is hardy to zone 5, performing better in zone 6 with better survival. The production of seed is when it becomes a problem. Currently, it is not on any list/law of noxious or exotic weeds in Illinois, so keeping it is up to you. If you do keep it and want to minimize seed dispersal, the plants should be cut back after flowering.