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Isopogon and Petrophile

David Lightfoot

Isopogon and Petrophile are two closely related genera in the Protea family (Proteaceae). Both are endemic to Australia and both have their greatest diversity in south Western Australia. Although some members of both genera have been cultivated for many years, there is still much to be learned and this is the aim of the Isopogon and Petrophile Study Group, of which David is the leader. For further information on the Study Group, contact David at isopogons@iprimus.com.au


The name of the genus derives from the Greek isos - equal and pogon - a beard. This is thought to refer to the hairs surrounding the fruit.

Isopogon latifolius Isopogon trilobus  
Isopogon latifolius (left)
Isopogon trilobus (right)

Click for larger image

Isopogon is an endemic genus. Some species (e.g. I.buxifolius) seem to be closely related to South African Proteaceae and may be revised in the future after DNA work has been completed. There are 35 currently recognised species with 27 of these found in the south west of the country. The eastern varieties span the coast from southern Queensland into South Australia and the eastern Bass Strait islands. They don't make it to the Tasmanian "mainland".

Isopogons are mainly small to medium sized shrubs with a number being prostrate or near so. They are characterised by semi globular or ovoid inflorescences, packed with small flowers, which range in colour from cream to yellow in the east, with pink, and mauve added in the west. The flower heads are usually terminal and showy but in some species are axillary. Thus many have potential as cut flower varieties. The shape has lead to the common names of drumsticks and coneflowers. The foliage is extremely variable throughout the genus ranging from simple narrow to ovate, through to deeply and intricately divided. The new growth is often red and contributes to the horticultural desirability of some taxa.

Isopogon and Petrophile Seed
One thing I have noticed when collecting seed from Isopogon and Petrophile is that, in general, the seed set in Petrophile inflorescences is far greater than that in Isopogon. (I have yet to determine the rate of viable seed though.)

More often than not each Isopogon head contains only a few, if any, seed. The mature head will break apart in your hand revealing its components. The scales are the remnants of the protective outer layer. Within these will be lots of non-viable fruit, which is characteristically a hairy dot.

The seed is contained in a drop shaped fruit of 1-3mm again surrounded by hair. It will have left an indentation on the axis of the fruiting head. There may only be a few but once you have found one, then sorting the seed from the chaff becomes easy.

The fruiting cone is held for some time before breaking up with loss of the external bracts. It is the loss of these scales and the disintegration of the fruiting cone that distinguishes Isopogon from Petrophile (where the scales are retained). In general isopogons grow close to the coast and when they do appear inland they are not found in arid areas. They are found in heath and dry sclerophyll woodlands, generally on very well drained nutrient poor, sandy soil.

Isopogon species have been cultivated since 1791 but are still relatively rare in today's gardens.


The name of the genus derives from the Greek petra - a rock and phileo - to love (philos, beloved). The first species described were discovered in the Sydney region in the sandstone country and were therefore thought to be 'Rock Loving'.

Like Isopogon, Petrophile is an endemic genus. At the moment, there are 53 recognised species with 47 of them originating in the south west of Western Australia. One species is confined to Kangaroo Island, off South Australia, and 5 species are found in New South Wales and south eastern Queensland. There are a number of as yet unnamed taxa in Western Australia, and the Herbarium in Perth is currently working on descriptions of these.

Most of the western species are found growing in deep sand, and in full sun within the sandplain heaths. Although some are found in gravely soils, it is almost universally extremely well drained. The eastern species live more up to their generic name and are found predominantly in sandstone country, in dry sclerophyll forest or heath. Again they grow in lots of sun, often occurring where other vegetation is thin.

The western species are found in temperate and semi arid climatic zones. They can be found reasonably close to the coast throughout the South Western botanical zone. The eastern species are found close to the Great Dividing Range and down to the coast.

Petrophile biloba
Petrophile macrostachya
Petrophile sessilis  
Three Petrophile species, clockwise from top left:
Petrophile biloba
Petrophile sessilis
Petrophile macrostachya

Photos: Margaret Pieroni; Brian Walters
Click for larger image

The species within the genus vary greatly in their form. They are all woody, varying in size from low, almost ground hugging plants, to 3 m high medium shrubs.

Foliage is incredibly variable. Leaves can be simple, divided, or multiply divided. They can be flat or terete, glabrous or hirsute. Indeed, Petrophile diversifolia has different shaped leaves (juvenile) close to the ground, compared with those at the top of the stems. Some species have red new growth, and these include P.fastigiata, P.seminuda, P.striata, P.plumosa, and P.circinata. Petrophiles can be stunning in flower, especially in areas where the plants are grouped together. The flowers are packed into usually cone shaped heads (although some are ovoid). The inflorescences are generally born terminally or less commonly at the leaf axils. Species with terminal flower heads often display them prominently above the foliage, and would be candidates for use as cut flower. The majority of species have flower colours in the range from cream to yellow. The eastern species are cream, and a couple of western species have grey/pink to pink flowers.

Each individual flower has a small bract that becomes woody after the flower has finished. They remain after the fruit has matured. As noted above, this feature distinguishes Petrophile from Isopogon, as the fruiting cones stay complete on the shrub, and gives rise to the common name of Conesticks. (In Isopogon, they break up and disperse when the fruit is mature).

The fruit is a small nut that is generally shaped like a heart or oval with a hairy border down both sides. Some species have a tail, which comes out of the apex. In general, the fruit are released when the plant or branch dies or when it is burnt in a fire. The hairs may act as a water repelling layer and prevent germination in all but the wettest of years.

Petrophile species were first introduced to cultivation in the eighteenth century, but these days are rarely seen outside specialist botanical and enthusiast's gardens.

Cultivation Requirements

The eastern species of both Isopogon and Petrophile appear to be the hardiest, especially where there is any humidity. The western species are very susceptible to root rot fungal attack from the root rot fungus Phytophthora cinnamomi. The wet summers of the east coast promotes the growth of this fungus and is part of the reason for the cultivation difficulty of western species.

Isopogon fletcheri Isopogon dubius  
Isopogon fletcheri (left)
Isopogon dubius (right)

Click for larger image

In virtually all cases the plants do best in summer dry areas in extremely well drained and slightly acidic soils. In order to maximise flowering excellent sun exposure is preferable. It has been said that most species will tolerate mild frosts especially once they are established.

Supplementary fertilizers are rarely needed, except for iron and trace elements if the plants show yellowing or signs of mineral deficiencies. As with other Proteaceae, these plants are very sensitive to phosphorus toxicity and therefore specialist native plant fertilizers are the only ones that should be used.

An acid pH and an underlying layer of limestone has been said to be advantageous.

Supplementary watering is not required, once established and should be avoided during the warmest periods (in order to reduce the chances of root rot fungus).


  Isopogon anemonifolius
  Isopogon anemonifolius is a widely cultivated and reliable plant for many areas

Both Isopogon and Petrophile can be grown from seed which germinates well if fresh, although germination times may be variable from the same batch and so seed trays should be kept for some time before being discarded. Germination in some species is improved by smoke treatment, but experimental work is lacking in many taxa. Another thought for improving germination may be to remove the hairs on the nuts that seems to repel water, or nick the nuts to allow water to get to the seed embryo (NB these are my own ideas and have not been explored experimentally to my knowledge).

Care must be taken with the new seedlings to avoid damping off. Therefore for best results sterile pots and mediums should be used, with an extremely well drained potting mix. Over watering needs to be avoided.

With Petrophile, seed cones over a year old have mature fruit. Once the seed cone is removed from the plant it should be kept in a warm dry place. After a few days the scales will begin to open and the nuts can be removed fairly easily with a pair of tweezers.

Propagation can be carried out from cuttings taken from semi-firm new growth, and treated in the regular way with the warning that the hairy leaved species should not be misted too much for fear of fungal disease. Very new growth should be removed. Rooting hormone and under-tray heating will improve results.

Grafting work is only just being explored with Isopgon - I have not seen commercially available grafted plants. I have not heard of results from grafting of Petrophile. The hardier eastern species or even some eastern Isopogon could be tried as rootstock.

From the newsletter of the Isopogon and Petrophile Study Group, November 2001 and May 2002.


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Australian Plants online - March 2003
Association of Societies for Growing Australian Plants