Shortly after silver wattle finishes blooming up high, any of four species of broom begin blooming down low. Brooms and silver wattle often naturalize together. All bloom with the same delightfully brilliant yellow. The four brooms are French broom – Cytisus monspessulana, Scotch broom – Cytisus scoparius, Portuguese broom – Cytisus striatus and Spanish broom – Spartium junceum.
Sadly, none are desirable species. All are exotic weeds. They are only a topic for gardening because they are so aggressively invasive. Not only do they overwhelm and displace native species, but they also enhance soil nitrogen to promote the growth of other exotic weeds! They are unpalatable to deer, and are not bothered by insects or disease. Furthermore, brooms are combustible!
It is best to enjoy their cheery bloom from a distance, where they grow wild where they really should not. The various species tend to dominate distinct regions, with some degree of mingling. Big specimens can get eight feet tall, but do not live long as they are replaced by herds of seedlings. French broom is the only evergreen species; but any can defoliate in response to hot dry weather.
Tony Tomeo 
I remember seeing broom in Scotland and thinking how lovely it was, but it’s wet there; it seems odd that it would be invasive in California, but we’re probably talking microclimates here again!
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There are a few different species. At least three are naturalized here. One does better in damp coastal regions. One does better down South. One does better in chaparral regions. I suspect that the one in Scotland is adapted to that climate. There Is likely at least one that is native, but I do not know. Gorst likely lives there too. It looks like a low growing broom.
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