There are both narrow-leaved and broad-leaved peppermint eucalypts in the Reserve: eucalyptus radiata (narrow-leaved) and eucalyptus dives (broad-leaved). Not all variants of both these types of trees contain the specific compound prized in producing medicinal eucalyptus oils but both are characterised by leaves with many oil glands which produce a ‘peppermint’ smell when rubbed or crushed (Doran et al, 2005).
E.dives is described as a small to medium sized tree (8-25m) commonly low-branching and spreading whereas e.radiata is described as being able to grow to 45m (Costermans, 2009). In the Reserve, there are some taller examples of around 25 -30m, particularly along with Coral Fern Walk but most seem to be small to middle-sized and they seem to have quite similar shapes. They live in similar habitat in the Reserve, with groups along the eastern edge next to the paddock and interspersed among the other eucalypts in the southern section. Costermans (2009) points out they do hybridise. This can occur, for example, in those cases where there has been human disturbance at the margins of cleared areas.
This could explain the difficulty in telling one peppermint from the other, especially in the case of those trees along the eastern border of the Reserve.
Identifying peppermints can start with the bark (see three examples below). The peppermint type is described as persistent, with long splits revealing interlaced bark fibres underneath (Brooker & Kleinig, 1999). In these cases, it is finely fibrous, has long fine splits and is coloured grey or grey-brown (although the third example is more brown than grey). Also in these cases, the bark is persistent, covering most of the tree, although, it comes away on the smaller branches, showing smooth underneath, as illustrated in the last image.
In common, both trees (Costermans, 2009; Brooker & Kleinig, 1999) have leaves that are stemmed, curved, asymmetrical and oblique where the leaf connects to the leaf stem. Their lateral veins tend to be at a small angle to the midrib. They are longer than wide and alternating. The flower buds are in clusters of seven or more around a single stem (peduncle), occur in the leaf axils, are club shaped and yellow with rounded or slightly pointed caps. These in the Reserve will be flowering soon- somewhat later than indicated in the field guides.
They flower at the same time (September/October to December/January) and their fruit is quite similar in shape: pear or cup-shaped , valves around or just below rim level. The fruit of e.dives is slightly larger (0.7 X 0.7) than e.radiatia (0.6 X 0.6) and both have three or four valves.
So, seeing the differences in the two trees can be difficult in the Reserve.
There are differences. One is in the size of the leaves. Narrow-leaved peppermints have slightly narrower leaves. The leaves of e.radiata are around 15 X 1.5cm (Costermans, 2009) or 8-16cm long and less than 2cm wide (Brooker & Kelinig, 1999). Those of e.dives are 10-15cm long and 2-3.5 cm wide. So, broad-leaved peppermints have slightly broader leaves. This might be difficult to appreciate when the leaves are some height above you, however.
The other is in differences in juvenile growth.
The juvenile growth of the narrow-leaved peppermint is opposite, non-stemmed, narrow and pointed, and green (Costermans, 2009) – see above.
On the broad-leaved peppermint, the juvenile growth is also opposite, non-stemmed but is grey-green, usually glaucous, ovate and larger (up to 15 X 7cm). The group of leaves illustrated above contains opposite and ovate leaves, and the veins are more distinct. In this case, the leaves do have an odour that is pepperminty, many of the leaves are long and narrow, although wider (between 2 and 3cm), and other features suggesting broad- leaved peppermint. They more probably belong to e.dives rather than e. radiata.
Of course, the tree could be a hybrid.
Brooker, M.I.H. & Kleinig, D.A. (1999) Field guide to eucalypts Volume 1 south-eastern Australia, Bloomings Books: Hawthorn.
Costermans, L. (2009) Native trees and shrubs of south-eastern Australia, Reed New Holland: Sydney
Doran, J.C., Kar, A.K, Larmour, J.S. & and Reid, N. (2005) Variation in frost tolerance of the 1,8-cineole-rich variants of the peppermint eucalypts, Eucalyptus radiata and E. dives, Australian Forestry, 68(2), 137–143