Isotoma axillaris

Rock Isotome, Showy Isotome, Blue Stars

Family: Campanulaceae

An erect to ascending perennial herb, to about 0.5 cm high, usually growing in clumps but forming a groundcover.

It is a very widespread species, growing from the coast to the inland of NSW, as far west as north-west of Ivanhoe. It extends into Queensland, through the coast, inland and central areas, as far west as near Augathella and as far north as Clermont and Rockhampton. It spreads into Victoria, occurring acorss much of the northern parts, from north of Orbost, down toward Traralgon and as far north-west as Swan Hill.

It is often found in on rocky cliffs, especially granite and sandstone outcrops; as well as in shallow sandy soils of slopes and around rocky creeklines, in open shrubland and dry sclerophyll woodland.

Isotoma spp. have simple and alternate leaves, which are often toothed or lobed. In this species, leaves have an overall obovate to oblong-ovate outline but are strongly pinnatisect-pinnatifid, to 15 cm long and about 2 cm wide, with segments on leaf edges about 5 mm wide, narrowing at the apices and with toothed margins.

Isotoma have 5-merous flowers which can be bisexual or unisexual with a 5-lobed calyx and a 5-petaled corolla, somewhat tubular, often produced solitarily in leaf axils, or in terminal racemes. In this species, flowers are solitary in leaf axils on pedicels to 17 cm long(!); with flowers having a tube to 35 mm long and a diameter to 35 mm wide at the opening; bright blue to mauve (rarely pink or white); with a yellow, white or green-ish throat, usually bisexual. Full-flowering plants are very striking.

The fruit is a capsule. In this species, it is about 18 mm long; generall hemispherical in shape.

In the garden

A very common plant in cultivation for native gardens. It is a species for which it is said: “one you have it established, you will always have it”. It can set seedlings readily.

Author’s notes:

I always have a few Isotoma axillaris growing in my garden as they are a great small plant, growing to about 40 cm high and about that wide, with a prolific display of blue star flowers that are about 3 cm in diameter.

The flowering period is quite long, starting in October, and they flower well into late summer if not allowed to dry out for too long.

I use them to define borders and they look great growing next to the blue flowering Brachyscome multifida.

They prefer to grow in full sun in lighter soils but I have found them quite adaptable in my northern Sydney garden. Isotoma axillaris grow much better if not allowed to dry out for too long and after a wet 2011 winter, they are growing and flowering the best for years in my garden.

Maintenance: After flowering the foliage will die off and I then cut the plant back hard, to encourage fresh new growth. Every two or three years Isotoma axillaris are eaten by little green caterpillars, 2 or 3 cm long, the exact same colour as the foliage, which can be removed easily by hand (once you know what to look for).

A word of warning: Be careful when cuttings stems as they exude white sap that produces a severe stinging sensation if rubbed on eyes and may cause irritation to sensitive skin. However, this should not stop you growing these plants as the flowering display is worth the care.


Many people propagate plants by letting them self-seed in the garden and then digging them up and moving them to desired spots.

If conditions are good, you will have just enough seedlings that can be dug up and replanted where you require them.

Other information

This species likely regenerates from seed after fire.

Warning is again given that this plant does cause allergic reactions in some people from handling the sap. Caution is advised.

Isotoma is a genus of 12 species, occurring in Australia and New Zealand. Australia has 10 species, all endemic. NSW currently has 6 species.

Isotoma from Greek Isos (ίσος) meaning ‘equal’, and tomos (τόμος) meaning a “section”, referring to the equal lengths of the corolla segments.

axillaris Latin – “axil-bearing”, referring to the position of the flowers in the axils of leaves.

This species is not considered to be at risk of extinction in the wild.

NSW Flora Online (PlantNET) – Isotoma axillaris profile page        https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Isotoma~axillaris

Australia National Botanic Gardens – Isotoma axillaris profile page    https://www.anbg.gov.au/gnp/interns-2008/isotoma-spp.html

Wrigley, J.W. & Fagg, M.I. (2001). Australian Native Plants – Propagation, cultivation and use in landscaping. 4th edition. New Holland Publishers, Pty. Ltd. Australia.

By Jeff Howes. Editing and additional text by Dan Clarke