Xerochrysum bracteatum


Xerochrysum bracteatum, commonly known as the golden everlasting or strawflower, is a flowering plant in the family Asteraceae native to Australia. Described by Étienne Pierre Ventenat in 1803, it was known as Helichrysum bracteatum for many years before being transferred to a new genus Xerochrysum in 1990. It grows as a woody or herbaceous perennial or annual shrub up to a metre tall with green or grey leafy foliage. Golden yellow or white flower heads are produced from spring to autumn; their distinctive feature is the papery bracts that resemble petals. The species is widespread, growing in a variety of habitats across the country, from rainforest margins to deserts and subalpine areas. The golden everlasting serves as food for various larvae of lepidopterans, and adult butterflies, hoverflies, native bees, small beetles, and grasshoppers visit the flower heads.
The golden everlasting has proven very adaptable to cultivation. It was propagated and developed in Germany in the 1850s, and annual cultivars in a host of colour forms from white to bronze to purple flowers became available. Many of these are still sold in mixed seed packs. In Australia, many cultivars are perennial shrubs, which have become popular garden plants. Sturdier, long-stemmed forms are used commercially in the cut flower industry.

Taxonomy

French botanist Étienne Pierre Ventenat described the golden everlasting as Xeranthemum bracteatum in his 1803 work Jardin de Malmaison, a book commissioned by Napoleon's first wife Joséphine de Beauharnais to catalogue rare plants that she had collected and grown at the Château de Malmaison. The Latin species name bracteatum refers to the papery bracts of the flower heads. Henry Charles Andrews transferred it to the genus Helichrysum based on the morphology of its receptacle in 1805, and it was known as Helichrysum bracteatum for many years. Leo Henckel von Donnersmarck described it as Helichrysum lucidum in 1806, and Christiaan Hendrik Persoon as Helichrysum chrysanthum in 1807. It was given the name Bracteantha bracteata in 1991, when Arne Anderberg and Laurie Haegi placed the members that are known as strawflowers of the large genus Helichrysum into a new genus Bracteantha, and designated B. bracteata as the type species. However, they were unaware that Russian botanist Nikolai Tzvelev had already placed X. bracteatum in the new, and at the time monotypic, genus Xerochrysum the previous year. This name was derived from the Greek words xeros "dry", and chrysum "golden", likely relating to the nature of the distinctive bracts. Confusion existed for a decade, with Bracteantha appearing in literature and the horticultural trade until it was clarified in 2002 that the latter name took precedence. Strawflower is the popular name for X. bracteatum in Europe, while in Australia it is known as an everlasting or paper daisy. An alternate name in 19th-century Europe was immortelle. X. bracteatum itself is very variable and may represent several cryptic species. Alternately, the Tasmanian species Xerochrysum bicolor may be combined with it in future taxonomic revisions.
X. bracteatum and its relatives belong to the Gnaphalieae or paper daisies, a large tribe within the daisy family, Asteraceae. However, a 2002 molecular study of the Gnaphalieae has indicated the genus Xerochrysum is probably polyphyletic, as the two species sampled, X. bracteatum and X. viscosum, were not closely related to each other. X. bracteatum has been recorded hybridising with X. viscosum and X. papillosum in cultivation, and possibly also Coronidium elatum and C. boormanii.

Description

The plant is an erect perennial, or occasionally annual, herb that is simple or rarely branched at its base. It generally reaches in height, but can have a prostrate habit in exposed areas such as coastal cliffs. The green stems are rough and covered with fine hairs, and are robust compared with those of other members of the genus. The leaves are,, or in shape and measure long and from wide. They are also covered with cobwebby hairs. Sitting atop tall stems above the foliage, the flower heads range from in diameter. Occasionally, multiple heads arise from the one stem. Like the flowers of all Asteraceae, they are composed of a central disc which contains a number of tiny individual flowers, known as florets; these sit directly on an enlarged part of the stem known as the receptacle.
Around the disc is an involucre of modified leaves, the bracts, which in Xerochrysum, as in most Gnaphalieae, are petal-like, stiff, and papery. Arranged in rows, these bracts curl over and enclose the florets, shielding them before flowering. They create the impression of a shiny and yellow corolla around the disc. The intermediate bracts are sometimes white, while the outer ones are paler and often streaked reddish or brown. These bracts are papery and dry, or 'scarious', with a low water content, unlike leaves or flower parts of other plants. They are made up of dead cells, which are unusual in that they have a thin primary and a thick secondary cell wall, a feature only found in sclerenchyma, or structural, cells, not cells of flowers or leaves.
The individual florets are yellow. Those on the outer regions of the disc are female, while those in the centre are bisexual. Female flowers lack stamens and have only a very short, tube-shaped corolla surrounding a pistil that splits to form two stigmas, while bisexual or hermaphrodite flowers have a longer corolla, and five stamens fused at the anthers, with the pistil emerging from the center. The yellow corolla and pistil are located above an ovary with a single ovule, and surrounded by the pappus, the highly modified calyx of Asteraceae. It comprises a number of bristles radiating around the florets. Yellow in colour, they persist and are thought to aid in the wind dispersal of the long fruit. The smooth brown fruit, known as a cypsela, is 2 to 3 mm long with the pappus radiating from one end.
In the wild, X. bracteatum can be distinguished from X. bicolor in Tasmania by its broader leaves and cobwebby hairs on the stems, and from X. macranthum in Western Australia by the flower head colour; the latter species has white flower heads whereas those of X. bracteatum are golden yellow. Xerochrysum subundulatum from alpine and subalpine areas of New South Wales, Victoria, and Tasmania is rhizomatous, and has markedly pointed orange bracts. The eastern Australian species X. viscosum may be distinguished by its rough and sticky leaves.

Distribution and habitat

Xerochrysum bracteatum occurs in all Australian mainland states and territories, as well as Tasmania. Widespread, it is found from North Queensland across to Western Australia, and in all habitats excluding densely shaded areas. It grows as an annual in patches of red sand in Central Australia, responding rapidly to bouts of rainfall to complete its lifecycle. It is common among granite outcrops in southwest Western Australia, and is found on heavier and more fertile soils in the Sydney region, such as basalt-, shale-, or limestone-based soils, generally in areas with a high water table. Associated species in the Sydney Basin include blackbutt in open forest, and the shrubs Empodisma minus and Baloskion australe in swampy areas. It has been reported growing in disturbed soil, along roadsides and in fields in the New England region in the United States.

Ecology

The brightly coloured bracts act as petals to attract insects such as hoverflies, native bees, and small beetles that pollinate the florets. Grasshoppers also visit the flower heads. The caterpillars of Tebenna micalis have been recorded on this species, as have those of the Australian painted lady. The tiny fruits are dispersed by wind, and germinate and grow after fire or on disturbed ground.
Flower production is related to increasing day length, and in general, plants produced the most flowers from December to March. Varying planting times or artificially changing light levels might be ways to increase production of flowers outside these months.
The water mould Bremia lactucae has infected commercial crops in Italy and California. In 2002 on the Ligurian coast, widespread infection of several cultivars, most severely 'Florabella Pink' and to a lesser extent 'Florabella Gold' and 'Florabella White', resulted in leaf blistering and the development of chlorotic lesions on the leaves, and white patches on the undersides, particularly in areas of poor ventilation. There was an outbreak of downy mildew in a cultivated crop of Xerochrysum bracteatum in San Mateo County, California in 2006, in which the leaves developed large chlorotic lesions. A Phytoplasma infection damaged X. bracteatum crops in the Czech Republic between 1994 and 2001, causing poor growth, bronzing of foliage, and malformation of flower heads. Genetically, the pathogen was indistinguishable from the agent of aster yellows. The root-knot nematode attacks and forms galls on the roots, which leads to the morbidity or death of the plant.

Cultivation

X. bracteatum had been introduced to cultivation in England by 1791. German horticulturist Herren Ebritsch obtained material and developed it at his nursery in Arnstadt near Erfurt in Germany. He bred and sold cultivars of many colours from bronze to white to purple, which spread across Europe in the 1850s. The bracts of these early forms tended to remain cupped around the flower head rather than flatten out like the native Australian forms. These were also annual rather than perennial forms. Many were given cultivar names such as 'atrococcineum', 'atrosanguineum', 'aureum', 'bicolor', 'compositum', 'macranthum', and 'monstrosum', although today they are generally sold in mixed seed for growing as annuals. Some coloured forms of South African Helichrysum are thought to have been introduced to the breeding program, which resulted in the huge array of colours. X. bracteatum was one of several species that became popular with European royalty and nobility from the early 19th century, yet were little noticed in Australia until the 1860s, when they became more prominent in Australian gardens.
Most of the cultivars brought into cultivation in Australia in the latter part of the 20th century are perennials. 'Dargan Hill Monarch' was the first of these, and many more have followed. Profusely flowering, these grow in many colours including white, yellow, orange, bronze, pink, and red. Their commercial lifespans are generally around three years. Queensland-based company Aussie Winners has a range of compact plants ranging from orange to white known as Sundaze. Plants of this series usually have larger leaves. This range won the Gran premio d'oro at the Euroflora exposition in Geneva in 2001, for the best new plant series in the previous three years. 'Florabella Gold', a member of the Florabella series, won the award for best new pot plant in the Society of American Florists' competition of 1999. The Wallaby cultivars are range of taller forms with narrow leaves and white, yellow or pink flowers. Other commercial ranges include the Nullarbor series, and Queensland Federation daisies, including 'Wanetta Sunshine' and 'Golden Nuggets'.
X. bracteatum is easy to grow both from seeds and from cuttings, although named cultivars only grow true from cuttings. Plants benefit from pruning of old growth in winter to allow for new growth in spring. Dead-heading, or pruning off old flower heads, promotes the production of more flowers. Fresh seeds germinate in 3 to 20 days and require no special treatment. Plants grow best in acid, well-aerated soils of pH 5.5 to 6.3, with low levels of phosphorus. They are sensitive to iron deficiency, which presents as yellowing of the youngest leaves while the leaf veins remain green.
X. bracteatum can be grown in large pots or window boxes, and is a good pioneer plant in the garden until other plants become more established. Lower-growing cultivars are suitable for hanging baskets and border plantings. The flowers attract butterflies to the garden. Dried flowers are long lasting—up to some years—and are used in floral arrangements and the cut flower industry. More robust longer stemmed forms are used for commercial cut flowers. The main factor limiting lifespan of dried flowers is the wilting of stems, so flowers are sometimes wired into arrangements. Immersing flowers in glycerol or polyethylene glycol also lengthens lifespan.

Cultivars