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Cytisus scoparius (L.) Link (= Sarcothamnus scoparius (L.)
W.D.J. Koch), Broom or Scotch Broom

Account Summary

Native, frequent. European temperate, but widely naturalised including in N America and New Zealand.

1892; Praeger, R.Ll.; Inish Doney island, Lower Lough Erne.

Throughout the year.

Growth form and preferred habitats

The bright lemon-yellow flowers, deciduous, both simple and trifoliate leaves, borne alternately on erect, evergreen, pliable, 5-angled, unarmed photosynthetic stems, 2.5-3.0 m tall, that become woody as the plant matures, together make this variable shrub easily and instantly recognisable. It is present in Fermanagh in a wide variety of open, sunny or lightly-shaded, more-or-less disturbed ground sites and in unmanaged grassland habitats, on suitably acidic, low-nutrient, infertile, lime-deficient soils. C. scoparius thrives on a wide range of acid to neutral soils and multiplies on disturbed sites, preferring nitrogen-medium substrates and strongly drained, water-shedding sites (Peterson & Prasad 1998). It is practically impossible to exaggerate the importance of disturbance, natural or generated by man, to the spread and colonising ability of a plant like Broom.

Typically, C. scoparius is strongly calcifuge and avoids lime which puts a limit on its distribution (Clapham et al. 1987), but in Fermanagh it is recorded at Knockmore in the heart of the local limestone district and also in the Claddagh River Glen (otherwise known as the Marble Arch), where regionally famous limestone caves are situated. It is presumed the plant must have located pockets of acidic, low-lime conditions in these areas, but they are the exception in terms of the species distribution.

The shrub develops a deep taproot that allows it to tolerate considerable levels of drought. Its Rhizobium root nodules fix nitrogen efficiently and while its leaves are deciduous, in mild climates the evergreen stems continue to photosynthesise in winter, facilitating year-round nitrogen fixation and prolonging the growing season. Nitrogenase activity ceases, however, if there is heavy frost and severe levels of frost can kill the plant outright (Peterson & Prasad 1998).

Flowering reproduction

Flowering takes place in May and June; plants flower from their second or third year of growth and continue doing so for the 10-15 year lifespan of the typical individual. Pollination is carried out by both honey- bees and bumble-bees in a mechanism first described by Müller (1883). Essentially, pollination is an explosive mechanism which depends for its effectiveness on the bursting of the anthers and the shedding of their pollen over the immature style and stigma while the flower is still in bud. The style then elongates so that when the flower is completely open, it is held under tension inside the filament tube between the fused keel petals. The pollen covered style is suddenly released by the weight of the visiting bee, bursting the sutures of the keel, throwing out a cloud of pollen above the insect and dusting its hairy back with it.

The stigma then rapidly describes a spiral path and in doing so first strikes the insect's back, where it may pick up pollen from the previous flower visited. It then continues in its path to strike the underside of the insect's abdomen and possibly pick up pollen deposited there by the short stamens of its own flower (Gill & Walker 1971; Proctor & Yeo 1973, pp. 200-1).

The legume pod is 25-50 × 10-12 mm, oblong and strongly compressed and becomes dry and dark brownish grey when ripe. It is composed of two valves that have long brown or white hairs on the sutures. When fully ripe, which can be as early as mid-July, the pod audibly and explosively splits apart, the two valves twisting and ejecting the numerous (up to nine) blackish brown seeds into the air and onto the ground around the parent plant.

Secondary seed dispersal

The seed coat has an edible protein elaiosome outgrowth attached to the hilum scar, similar to that found in the genus Viola (see the Viola species accounts on this website). The elaiosome attracts ants that carry the seeds towards their nests and assist in their dispersal. Experimental elaiosome removal had no significant effect on seed germination of C. scoparius in a Californian study (Bossard 1993). Other studies along riverbanks in New Zealand strongly suggest seed is also dispersed by flowing water (Williams 1981).

Seed germination and survival in soil

Seed production is prolific and the hard seed coat (or testa) allows buried survival for 30 years or longer. A study in California concluded that 66% of seed from one year's crop germinated during the first year. The same study showed that at least 7% of seeds remained un-germinated after three years in the soil, allowing the development of a large seed bank. Variability in the duration between seed deposition and germination provides C. scoparius with considerable flexibility for coping with the intra- and inter-yearly fluctuations in precipitation and temperature typical of California's Mediterranean climate. Such climate factors also affected the degree and duration of hard-seeded-ness and thus helped determine levels of dormancy. However, no genetic basis for environmental differences in seed germination characteristics between widely separated countries should be assumed to exist without further research (Bossard 1993).

Lack of vegetative reproduction

There are no reports of vegetative reproduction occurring in C. scoparius (Peterson & Prasad 1998).

Variation

This species has been through several generic name changes in the not too distant past, synonyms including Spartium scoparium L., Genista scoparia (L.) Lam. and Sarothamnus scoparius (L.) W.D.J. Koch. Today, two subspecies are recognised: the erect, non-littoral shrubby form is subsp. scoparius, the most commonly quoted chromosome count for it is 2n=46, but other sources suggest 2n=48 (Morton 1955; Peterson & Prasad 1998). The prostrate or procumbent dwarf alternative coastal form of the plant is subsp. maritimus (Rouy) Heywood grows only up to 10 cm above ground and it has been variously listed as 2n=24, 46 & 48 (Gill & Walker 1971).

Morton (1955) found the prostrate plant had 2n=24 and the metaphase chromosomes were of twelve types, indicating a base number of 12 in the genus. He suggested that since the species had 48 chromosomes in the same twelve chromosome types in quadruplicate as the prostrate form, it might have arisen as an autotetraploid from the subspecies that he called Sarothamnus scoparius subsp. prostratus, now called Cytisus scoparius subsp. maritimus (Morton 1955).

Gill & Walker (1971) found most chromosome counts they made from material across England and Wales gave 2n=46 and they could not reconcile this with Morton's count of 2n=24. Not only was his count at variance with all other counts published, but the chromosomes he illustrated were much larger than those found by Gill & Walker, raising doubts about the identification of Morton's material.

There are also over 80 named garden cultivars and hybrids of C. scoparius, the crosses often involving C. multiflorus (L'Hér. ex Ait.) Sweet, White Broom, or C. × dallimorei Rolfe. (C. multiflorus × C. scoparius) (Griffiths 1994).

Fermanagh occurrence

In Fermanagh, C. scoparius has been recorded in 95 tetrads, 18.0% of those in the VC. Eight of these tetrads contain only pre-1975 records, which suggest some decline in available habitats. As the tetrad map indicates, Broom is scattered quite widely across Fermanagh, but the great majority of its sites are found on the better drained, non-calcareous soils in the east of the VC. Habitats include dry or well-drained heathy banks or slopes on acid, often sandy or stony soils. It also invades open woods or their margins, roadsides, river banks, quarries, old railway lines or embankments and other forms of disturbed ground. Broom is sometimes very prevalent on estate land, where it is probably derived from planted material.

In the wild, Broom is generally found as solitary bushes, seldom in quantity, but C. scoparius is well known to be an opportunistic pioneer colonist of open soils and is especially successful when there is little or no grazing pressure. Thus Broom can occasionally form large, dense, temporarily dominant, pure stands, as it does locally at the neglected sand pit near Pubble Bridge on the Tempo River, an open site which this leguminous shrub has rapidly colonised and where it currently comprises a very large, dominant stand. It remains to be seen if it will be capable of regeneration and of keeping at bay aggressive secondary colonists such as Birch and other larger trees and shrubs that could shade it out.

Irish occurrence

The New Atlas hectad map shows C. scoparius is common in lowland areas throughout NI and again further south in coastal counties from Dublin to Kerry, but is very much more scattered and occasional in C & W regions of Ireland. It is presumed that soils in the areas where it is scarce are principally limestone, or wet and peaty, neither of which would be suitable to support Broom. The Central Plain of Ireland also suffers the coldest winters in the country and a single very cold night can destroy Broom populations.

British occurrence

Broom is widespread and common throughout lowland Britain, but scarcer further north in Scotland and in calcareous areas of the country for the reasons already mentioned (Preston et al. 2002). In the distant past, C. scoparius must have been much more abundant than now since it features so commonly in English place names. Broom place-names can be counted by the score. A familiar one is Bromley in Essex, the name meaning 'broom clearing'. Another example is Brompton in Middlesex, the 'broom tun' or 'farmstead'.

Many cultivars of this ornamental shrub are very commonly planted in gardens, although it is not long-lived, tending to flower itself 'to death' within 10-20 years at most. Forms of it are often planted along new roadways to help stabilise steep embankments, or it is used as a nurse to young trees in plantations, as is, or was, the case in India where it quickly became a significant problem weed.

European and world occurrence

C. scoparius is native and widespread throughout lowland temperate Europe northwards to S Sweden and eastwards to C Ukraine – although not mapped in most of the Balkans (Hultén & Fries 1986, Map 1180; Sell & Murrell 2009). Originally it was restricted to Europe reaching south to Sicily and including the Azores and the Macronesian Islands, but it has been introduced to N America, New Zealand and India. Abroad, the vigorous colonising ability of the species is such that it has become a widespread and troublesome woody weed in waste land, river beds, native grasslands and previously forested hillsides across New Zealand and parts of Canada and California. It does not survive in more inland continental conditions in N America, being ± confined to lowland Oceanic or Mediterranean climatic regions by its sensitivity to frost and winter drought. The shrub thrives and grows rapidly in the conditions found in New Zealand, however, and is more vigorous there than in Europe, partly because of the absence of its major invertebrate predators (Williams 1981).

Toxicity

Broom contains several toxic alkaloids, but it is not dangerously poisonous. It contains small amounts of the toxic quinolizidine alkaloids sparteine and isosparteine. Other toxins present include cytosine, genistein, lupinidine and sarothamnine, although some of these names may refer to the same compound (Cooper & Johnson 1998). The alkaloids can depress the heart and nervous system, sometimes with paralysis of motor nerve endings. It is unlikely Broom would cause threatening poisoning as insufficient amounts of the toxins are present. The leaves being small and the stems wiry, sheep are not attracted to it, although goats will browse even this level of herbage. A horse would need to consume a very large amount of Broom (over 11 kg) to be at risk (Cooper & Johnson 1998; Peterson & Prasad 1998).

Names

The majority of the English common names in numerous, varied dialects refer to 'Broom' as in the brush used to sweep floors and other areas, eg 'Basom' in Cornwall and the W of England, 'Beeson' in Devon and 'Brushes' in Dorset. Grigson (1955, 1987) describes it as, "one of the great landscape plants" and, as it flowers early in the year in May and June, it is considered one of the plants indicative of love and romance in European poetry. He also refers to it as one of the best of all sweeping plants along with birch and heather. The long, slender, tough yet flexible branches, carried in tight fascicles make it ± ideal for sweeping. The English regal house the Plantagenets take their name from the broom, the Planta genista, a fact that Grigson thinks "everyone knows!" The Plantagenet family originated in Brittany and in Anjou on the Loire, where C. scoparius is a common shrub to this day.

Uses

Apart from its use to sweep floors, the tips of Broom branches have diuretic and cathartic properties that are widely and frequently used in herbal medicine. The sparteine they contain is a powerful diuretic which triples renal elimination (ie urine flow) and has been used for dropsy and kidney ailments of all kinds. In modern herbalism, Broom is used to slow and regulate the heart rate. While the list of other recorded uses from around B & I is long, varied, very local and minor, the two other principal uses in folk medicine are for the treatment of rheumatic complaints and as a purgative for liver troubles, jaundice and piles (Allen & Hatfield 2004).

In times of fodder shortage, Broom branches were resorted to as winter feed for sheep in the same way that Gorse was used, and it was allowed to grow around the margins of fields in preparation for this eventuality (Grigson 1955, 1987). The green branches were also supposed to prevent rot and dropsy in the sheep that ate them (Grieve 1931).

Young flower buds are edible and once were a favourite delicacy as they appeared on three separate tables at the Coronation banquet of James II. Grieve (1931) added that the flower buds served the double purpose of an appetizer and a corrective. Buds were collected and laid in pickle or salt until required, "which afterwards being washed or boiled are used in sallads as capers be and be eaten with no less delight" (Gerard 1633).

Threats

None.