Its common name is Scotch broom (Spanish broom is a kindred species). It is highly visible in the spring, in many parts of the world : expanses of land with poor agricultural value, siliceous soils typically, in deforested areas, on hillsides where heather also grows, are populated by these bright yellow shrubs.
1-3 m high (3-9 ft), the plant is profusely ramified. Leaves are tiny, 5-15 mm long (1/5- 3/5 in). The yellow flowers, 15-20 mm wide and 20-30 mm long (an inch maximum in each dimension) are numerous. Their yellow pigment, luteolin, belongs to the chemical family of flavonoids. Weavers of the past used it, for tapestries for instance, before the advent of synthetic dyes in the mid-nineteenth century.
Pollination of these flowers amounts to rape, since flowers are fused shut. Forced ‟tripping‟ by a pollinator, such as a honey bee or a bumble bee, is required.
One hot summer day, my wife and I were walking in an area with quite a few Scotch broom, and were startled to hear the pop and bang of small explosions. It was the dried black oblong pods bursting open and disseminating their seeds. It was quite an interesting walk, to say the least.
Originating in northwestern Europe, the plant is now taking over the world. It is officially considered an invasive species in California and the American Northwest, in Australia, New Zealand, etc.
In the not-so- remote past, in addition to providing a natural dye, the main use was to make brooms, to which the plant owes its common name. Nowadays, two widely different applications coexist again.
The first comes from the ability of all legume species in the Fabaceae family to retrieve nitrogen from the air and to enrich the soil with it. Accordingly, Scotch brooms have become a resource to ecologically-minded farmers, who practice permaculture for instance — permaculture aims at emulating natural ecosystems.
The second stems — an intended pun — from the high toxicity of the branches. Yesteryear, they were used in abortive brews. Witches were believed to magically ride Scotch brooms to the end of the world.
Nowadays? Make sure your pet avoids these shrubs. A dog ingesting a mouthful of Cytisus may die. Such accidents when they occur can be expensive. The veterinarian’s fee is about US $ 500.
One of the toxic alkaloids involved is known as sparteine. It is precious to synthetic organic chemists as an auxiliary in — a mouthful — enantioselective reactions, i.e., transformations privileging a single optical isomer. In particular, sparteine strongly binds lithium ions, a use that has spawned a cottage industry of such enantioselective transformations. I knew of this in the Sixties already and should credit my colleague Gideon K. Fraenkel, at Ohio State University, for this piece of information.
Of late however, during recent years (2014-2017), sparteine has mysteriously disappeared from catalogs of suppliers of chemicals. The cause is unknown, at the time of writing. Is some corporation somewhere sitting on a hoard of sparteine in order to constitute a monopoly and thrive from it?
Finally, an historical far from minor point. In the French language, Scotch broom is known as genêt. In the twelth century, the French ruler Geoffrey the Fifth, from the Anjou region, became surnamed plantagenêt for his habit of sticking a branch of broom in his cap. Did he do it to ensure respect from witches magic? In any case, an entire dynasty of English kings, the Plantagenêts, was borne of him!