Page banner image

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.

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 (further information can be found under the “Melaleuca and Callistemon” tab)

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.

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.

Melaleuca and Callistemon: Why are they Different?

Melaleuca and Callistemon are two of the best known Australian members of the Myrtle family. All of the Callistemons and many of the Melaleucas have flowers arranged in “Bottlebrush” fashion clustered together in cylindrically shaped spikes. But only Callistemons are commonly called “Bottlebrushes” ; Melaleucas are usually called “Paperbarks” or “Honey Myrtles” or sometimes “Tea Trees” although that name is more appropriate to another related genus, Leptospermum.

So what makes Melaleuca and Callistemon different?

The main difference has to do with the way in which the stamens (the male parts of the flowers) join to the floral tube. Figure 1 shows a cross section of a single Callistemon flower. This shows that each stamen joins the floral tube independently of every other stamen (this is referred to as the stamens being “free”).

Figure 1

In Figure 2, however, which is a cross section of a single Melaleuca flower, the stamens are joined together into groups with each group joining the floral tube as a unit (this is referred to as the stamens being “united”). Each Melaleuca flower contains five of these groups or “staminal bundles”.

In most cases this difference can be easily seen by examining the flowers with the naked eye. However, the problem with the current classification 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.

Figure 2

While all Callistemons have their flowers arranged in a “bottlebrush” shape the inflorescences of Melaleuca may also have a globular or irregular shape. It should also be remembered that there are other genera in the myrtle family which may have free or united stamens combined with “bottlebrush” flowers. Botany was never meant to be easy! The other common genus with free stamens is Kunzea which differs from Callistemon in having seed capsules which are not woody and which shed seed annually. Apart from Melaleuca there are several genera which also have united stamens. These include Calothamnus, Beaufortia, Eremaea and Regelia. The distinction between these and Melaleuca requires examination of the arrangement of the anthers and other floral structures.

 


Botanical Terms

Anthers
Pollen-bearing structures at the tips of the stamens
Filament
A stalk which connects the floral tube and the anthers (see stamen)
Floral Tube
A tube formed by the fusion of the basal portions of the stamens and the sepals.
Ovary
That part of the female portion of the flower containing the ovules. The ovary may be divided into a number of cells.
Ovules
Structures inside the ovary which become seeds after fertilisation.
Petals
One of two whorls of leaf-like structures in a flower, the other being the sepals.
Sepals
One of two whorls of leaf-like structures in a flower, the other being the petals. Sepals are the outermost whorl and are usually green in colour.
Stamens
The male parts of the flower comprising the filament and the anthers
Stigma
That part at the tip of the style which is receptive to pollen
Style
The female part of the flower which connects the stigma to the ovary

Callistemon – Propagation

Introduction

Callistemons can be propagated by either seed or cuttings. However, to maintain desirable characteristics of a particular plant vegetative propagation (eg. cuttings) must be used. This also applies to propagation of named cultivars.

Seed

Callistemon seed
Callistemons have very fine seed – there is enough here to produce a forest!
Photo: Brian Walters

After flowering, callistemons produce seed capsules in rows along the branches and in the majority of cases the seed remains within the capsules indefinitely and can easily be collected at any time. There are, however, a few species that release seed once it is mature. The popular Callistemon viminalis (weeping bottlebrush) is in the latter category.

In the case of species which retain the seed indefinitely on the plant, the capsules need to collected and placed in an open container in a warm position until the fine seed is released. This should occur in 2-3 days. For best results, the capsules should be at least 12 month’s old (i.e. the most recently formed capsules are best avoided). With those species which release the ripe seed annually, the plant needs to be kept under observation and seed capsules collected when the capsules commence to open.

Bog Method Diagram

Germination of the seed of Callistemon species is usually quite easy by normal seed raising methods. No special pretreatment is needed. Germination should occur in 14 to 30 days, depending on the species.

A common method used for germination of Callistemon and related plants is the “bog method” where the pot containing the seeds is placed into a saucer of water until germination occurs. This results in moisture reaching the seeds by capillary action and ensures that the seeds do not dry out.

Cuttings

Propagation of callistemons from cuttings is generally a reliable method. Cuttings about 75-100 mm in length with the leaves carefully removed from the lower half to two-thirds seem to be satisfactory. “Wounding” the lower stem by removing a sliver of bark and treating with a “root promoting” hormone both seem to improve the success rate.

General Propagation

Further details on general plant propagation can be found in the Plant Propagation section of the website.

Callistemon – Cultivation

Callistemons are so common in cultivation and seem to be present in even the most weed infested and poorly maintained gardens, it could be said that they thrive on neglect. However, that is not quite true. What is true is that many will survive with little or no attention but this is often accompanied by sparse foliage and few flowers. To get the best out of them it is useful to consider the way they grow naturally.

Pruned bottlebrush plant
Callistemons usually respond well to severe pruning
Photo: Brian Walters

Given that callistemons are usually found on moist sites, it should be no surprise to find that they enjoy a reasonable amount of moisture under garden conditions. This does not mean that they require anything like daily watering…once established, they will grow happily with just the occasional watering to help them through dry periods. A sunny position will usually produce the best flowering but plants will tolerate anything except total shade.

Apart from watering, the main concerns for successful bottlebrush growing are pruning, pest control and fertilising.

The woody seed capsules that form along the stems after each flowering season can look unsightly particularly as the new capsules follow the previous ones in succession on the same stem. To prevent this, it is usually recommended that plants be pruned annually just behind the spent flowers. This pruning also has the added advantage of stimulating branching leading to a greater profusion of flowers in subsequent years. If a bottlebrush plant has become too large or sparsely branched, it can usually be rejuvinated by severe pruning almost to ground level. The accompanying photo shows a plant of Callistemon ‘Captain Cook” producing new foliage after a severe cut back. However, pruning those species which have a weeping growth habit can destroy their shape (e.g.C.viminalis and its cultivars such as “Hannah Ray” and “Dawson River”).

Sawfly larvae
Sawfly larvae on the foliage of Callistemon citrinus
Photo: Brian Walters

There are several pests which can attack callistemons although healthy plants can usually cope without human intervention. Sawfly larvae are an exception – they are common pests. They are bronzy green in colour with a pointed tail and, because they occur in groups, they can inflict a great deal of damage to the foliage quickly. They are best controlled by physically removing them either by hand (using gloves!) or with a jet of water from a hose.

Scale is another pest that can be removed by a strong jet of water but this may need to be carried out several times. If this is not successful, the traditional treatment with white oil is usually effective.

Another pest that can be troublesome is webbing caterpillar. These grubs more commonly attack related genera such as Melaleuca and Leptospermum but can cause damage to certain callistemons (the cultivar C.”Little John” seems particularly prone). Again, a jet of water is effective treatment.

Callistemons are fairly tolerant of fertilisers, unlike some other genera of Australian plants. The use of a slow release fertiliser after flowering will usually be sufficient.

Callistemon – Early History of Cultivation

Colin Cornford

The late Colin Cornford was the leader of the Society’s Melaleuca and Allied Genera Study Group. The following article is reproduced from the March 1992 issue of the Group’s Newsletter.


European botanists and collectors of the late 18th century showed considerable interest in the plants of the remote southern continent of Australia. Callistemon citrinus was among the plants collected by Joseph Banks and Daniel Solander in 1770 during the discovery of the east coast of Australia. By 1788, three species from the Sydney region, C.citrinus, C.linearis and C.salignus, were available to English horticulturists.

Callistemon lineraris
Callistemon lineraris
Photo: Brian Walters

The convict artist, Thomas Walling, produced detailed illustrations of several Port Jackson (Sydney) species during the 1790s. An engraving of C.speciosus is published in a book published in France in 1813 which featured the plants growing in Empress Josephine’s garden at Malmaison and was probably introduced by French botanists Leschenault and Labillardiere who collected seed, including C.speciosus, in Western Australia in the 1790s and early 1800s. C.speciosus was introduced to English horticulture in 1823. C.rigidus was introduced to English horticulture in 1815. Further introductions to English horticulture were:

  • C.brachyandus – Introduced 1843, flowered, 1848. It was grown in the gardens of the Royal Horticultural Society from seed provided by Governor Grey of South Australia.
  • C.citrinus (previously known as C.lanceolatus) – Introduced 1788.
  • C.phoeniceus – Introduced 1843 from seed supplied by James Drummond from the Swan River District of Western Australia.
  • C.pinifolius – Introduced from seed supplied by Allan Cunningham some time during 1820-1830.
  • C.rigidus – Introduced to English horticulture in 1800. The original plant was collected by Robert Brown in 1800. Brown’s description in 1819 was the first detailed taxonomic description of a Callistemon.
  • C.lanceolatus (now C.citrinus) – Appears to have been introduced to Kew Gardens by Joseph Banks in 1788. Curtis commented that C.citrinus was common in nurseries around England. The report states that the original plant was grown from a “root sent from Botany Bay”. It was popular in France and had been flowered there by 1800.
  • C.linearifolius – Introduced in 1820 from seed supplied by Allan Cunningham.
  • C.linearis – Introduced by Banks in 1788 when the species was first described as Melaleuca linearis.
  • C.macropunctatus (now C.rugulosus) – Introduced in 1811 possibly from seed collected by one of the French expeditions to Australia.
  • C.pallidus – Introduced in 1813.
  • C.salignus – Introduced by Banks in 1788.
  • C.viridiflorus – Introduced to England in 1818-1820. C.viridiflorus flowered in June 1824 and is still being cultivated as a greenhouse plant but there are reports that the plant is presently being grown outdoors by some Australian plant enthusiasts in England.

In 1889 J.H. Maiden in his book The Useful Native Plants of Australia described two bottlebrush species in the chapter on local plants utilised for timber. C.lanceolatus (now C.citrinus) was described as having hard and heavy wood suitable for ship-building and wheel-wright’s work and for implements such as mallets. C.salignus was described as having hard, close-grained wood suitable for use underground. He also states that “it has a pretty grain which looks well under polish”. Two slabs of C.salignus were exhibited at the London International Exhibition of 1862. C.viminalis lasts quite well in the ground. It has a dense, closely-grained wood which polishes well with a rich, red colour.

Callistemon Study Group

This Study Group was formed originally with the aim of studying the cultivation and propagation of the genus Callistemon, a member of the large myrtle family of plants (the Myrtaceae), so that more species could be seen in gardens.

Although the Group is now closed, newsletters and other publications that were produced by the group can be found at the following link.

***Click here to go to the Callistemon Study Group***

Callistemon – Further Information

Most books dealing with Australian native plants will contain useful information on the botany and horticulture of callistemons. There are also a number of Callistemon resources on the internet. Some of the most detailed references are listed below.

Books:

  • Brophy, J J, Craven, L A and Doran, J C (2013), “Melaleucas: Their Botany, Essential Oils and Uses”. Australian Centre for International Agricultural Research and Rural Industries Research and Development Corporation. This publication adopts the re-classification of Callistemon into the genus Melaleuca.
  • Cornford, C (1999), “Callistemon, Melaleuca, Kunzea, Leptospermum and Other Genera Within the Myrtaceae Family”, ANPSA Melaleuca and Allied Genera Study Group. A checklist of species and their performance under cultivation in a range of districts.
  • Elliot, W. R and Jones D (1982), “The Encyclopaedia of Australian Plants”, Vol.2, Lothian Publishing Company Pty Ltd, Melbourne.
  • Wrigley, J and Fagg, M (1993), “Bottlebrushes, Paperbarks and Tea Trees”, Angus and Robertson, Australia.

Journals:

Several issues of the Society’s journal “Australian Plants” are particularly useful for those interested in Callistemon and related plants.

  • Vol 9, Nos.69 and 70 December 1976 and March 1977; A two-part, detailed description of Callistemon, Melaleuca and related plants including a botanical outline of the features of the Myrtaceae.
  • Vol 11, No.89 December 1981; Detailed descriptions of most Callistemon species and many cultivars.
  • Vol 16, No.125 December 1990; Details of botany of Callistemon and its relatives, propagation, information on cultivars and many photographs.
  • Vol 16, No.127 June 1991; Information on related genera (particularly Calothamnus and Melaleuca) with photographs.
  • Vol 21, No.168 September 2001; Information on Callistemon and related genera; Growing Callistemon in large pots.

Internet: