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A very important place is filled in gardens by the group of shrubs commonly classed together as 'brooms'. Of this group, which includes Genista, Spartium, etc., Cytisus is the most important genus. Its species are mostly quite deciduous, some are almost always leafless, and all are shrubby, varying in stature from 12 ft or more high, down to less than as many inches. The leaves are alternate, simple, or trifoliolate (sometimes both on the same plant). The flowers have the pea-flower shape characteristic of the family, and, with the exception of a few species, they are yellow. Fruit a pod. The genus is essentially a European one, but a few species extend eastward to Asia Minor, and others reach across the Mediterranean to N. Africa.
The affinities of the genus are with Laburnum, Ulex, and Genista. Laburnum is distinguished by the thickened or winged sutures (seams) of the pod; Ulex by the coloured calyx; but the distinctions between Genista and Cytisus are not so easily found. The most serviceable one is furnished by the seed. In Cytisus the outer coat of the seed has a wart-like excrescence near the hilum, which is technically known as the 'strophiole'. In Genista this is absent or rudimentary. A small group of species, of which the common broom C. scoparius is the best known, are by some botanists segregated as the genus sarothamnus. However, in addition to the taxonomic arguments against making this split, there is the practical one that many garden brooms derive from the crossing of the common broom with C. multiflorus. If the former were to be placed in Sarothamnus – and there is no obligation to do so – these brooms would have to be treated as bigeneric hybrids under the name × Cytothamnus. The section Tubocytisus (characterised by a tubular calyx) is also retained within Cytisus in the present revision, although by some authorities it is treated as a distinct genus under the name Chamaecytisus. For a further discussion of the taxonomy of the Cytisus-Genista group, see the introductory note to Genista in vol. 2.
The species of Cytisus are easily cultivated. They like a fairly good, but not rich soil, and abundant sunshine. Whenever possible, they should be raised from seeds, but if these are not available cuttings and grafts can be used. Cuttings should be taken in August when the wood has become firm. Pieces from 1[1/2] to 3 in. long may be used, always with a slight heel of older wood. They should be dibbled in very sandy soil under cloches, or in a frame, only uncovering to give water. They ought to push roots the following spring, and soon after can be potted in small pots, or, if vigorously rooted, planted out straight away in nursery beds, being careful to water and, if necessary, shade until established. The brooms do not transplant well after they have reached a good size, so it is wise to get them in their permanent places early.
Except for C. battandieri and C. ingramii no species of genuine importance or interest belonging to this genus has been introduced to gardens since the first publication of this work, the others now described having been in cultivation at or before that time. But a large number of varieties of garden origin have appeared, and continue to appear, which have added considerably to the importance of the genus as a contributor to ornamental hardy shrubs. Some of these are colour-forms of C. scoparius. The majority are members of a hybrid swarm stemming from C. × dallimorei. In recent years Dutch breeders have used C. × praecox as a parent: crossed with C. × dallimorei it has given 'Zeelandia' and, with C. 'Burkwoodii', 'Hollandia'; 'Dukaat' and 'Frisia' are second-generation seedlings from these crosses. C. × praecox has also been crossed with C. × beanii and C. ardoinii. The next few decades are likely to see a stream of new hybrids of ever-increasing complexity and perhaps of dwarfer habit than the older sorts.
The following is a selection from those available in commerce, provided by the Royal Horticultural Society:
'Burkwoodii'. – Cerise and maroon-red. Vigorous and bushy.
'C. E. Pearson'. – Rose, yellow and red.
'Cornish Cream'. – Cream and yellow. Bushy, open habit.
'Donard Seedling'. – Mauve-pink and flame-red. Rather loose habit.
'Enchantress'. – rose-pink and carmine. of spreading bushy habit.
'Firefly'. – Yellow and rich mahogany-crimson. Bushy.
'Goldfinch'. – Crimson and yellow; pink and yellow wings.
'Hookstone'. – Flowers lilac and orange.
'Johnson's Crimson'. – Clear crimson. Graceful arching habit. Free flowering.
'Killiney Red'. – Flowers bright red. Dwarf, compact habit.
'Knaphill Lemon'. – Lemon-yellow flowers.
'Lady Moore'. – Pinkish yellow and orange-flame. Loose, branching habit.
'Lord Lambourne'. – Creamy-yellow and maroon-crimson. Branching and spreading.
'Minstead'. – Fuchsia purple and white. Slender arching habit.
'Zeelandia'. – Pale lilac-pink and red. Long arching sprays.
Among the hybrids of C. × praecox mentioned in the introductory note on page 812 is 'Hollandia'. This is perhaps the best of the group, with flowers in blending shades of salmon-pink and lilac-pink.
The following should be added to the list of recommended broom cultivars on pages 812-13, both having received awards after trial at Wisley:
'Golden Sunlight'. – Rich golden yellow. Very free-flowering. Vigorous, arching habit.
'Lena'. – Of compact, comparatively dwarf habit, to 3 or 4 ft high. This is one of the most notable introductions of recent years. The flowers, very freely borne, have ruby-red standards and wings, and pale yellow keels. It was put into the Wisley trials by the Royal Botanic Gardens, Kew, and received a First Class Certificate.