Callistemon - Background

Introduction

Callistemon is a genus of around 30 species in the Myrtle family (Myrtaceae). All except four species are endemic to Australia, the others occurring in New Caledonia. Callistemons are commonly known as "bottlebrushes" because of the cylindrical, brush-like shape of the flower spike. They are very popular for gardens and landscaping both in Australia and overseas and numerous cultivars have been brought into cultivation.

  
Goodbye Callistemon?
The problem with the classification of Callistemon and Melaleuca on the basis of the arrangement of the stamens is that this supposed difference is not clear cut and Callistemon tends to merge into Melaleuca rather than being unambiguously distinct. The well known Callistemon viminalis is one that has often been discussed as not easily fitting the accepted definition of Callistemon.

Over the years there have been suggestions that the differences between species of the two genera are not sufficient to warrant them being kept distinct. A paper by Lyn Craven of the Australian National Herbarium (Novon 16 468-475; December 2006 "New Combinations in Melaleuca for Australian Species of Callistemon (Myrtaceae)") argues that the differences between the two genera are insufficient to warrant them being retained separately and that they should be combined. As Melaleuca has precedence, adoption of Craven's work would transfer all species of Callistemon into Melaleuca. Some state herbaria have adopted this change but, at this stage, the re-classification has not been taken up in the Australian Plant Census, which ANPSA recognises as the authority on plant nomenclature. For this reason we have retained Callistemon and Melaleuca as separate genera.

Craven's re-classification has been adopted in a recent (2013) publication "Melaleucas: their botany, essential oils and uses" by Joseph J. Brophy, Lyndley A. Craven and John C. Doran.

In nature, callistemons are often found along watercourses or along the edges of swamps. They are generally plants of open forest or woodland in relatively high rainfall areas.

Characteristics

Callistemon is closely related to Melaleuca ("paperbarks" and "honey myrtles"). Basically, the main difference between the two genera is the manner in which the stamens are connected to the floral tube. The stamens are generally free in Callistemon but united into bundles in Melaleuca (see also "Melaleuca and Callistemon; Why are they Different?")

The showy parts of the flowers of Callistemon are the stamens, the petals being small and inconspicuous. The stamens are often brightly coloured with red being the most common, but a whole range of colours...white, green, yellow, pink, salmon, mauve and purple...occur on various species and cultivars. The Callistemon "flower" is really an inflorescence formed by a cluster of small flowers arranged linearly along and around the branches. Because of this arrangement, the familiar "bottlebrush shape" is formed by the colourful masses of stamens.

Peak flowering for most species and cultivars is late spring to early summer (October to early December in Australia), however, a second flowering in autumn is not unusual. The flower spikes occur terminally at the ends of branches with the foliage continuing to grow beyond the ends of the spikes.

Following flowering, three-celled woody seed capsules develop with each capsule containing many small seeds. The seed pods usually remain tightly closed unless stimulated to open by the death of the plant. In a few cases, however, the seed is released from the capsules when ripe (eg. C.viminalis).

Most callistemons are small to medium shrubs but some are prostrate and a few can become small to medium sized trees.


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