Plant profiles > Olearia

Family: Asteraceae

When we think of daisies, we tend to imagine low-growing herbaceous flowers, similar to the kinds of daisies that emerge through meadows or are planted in flower gardens. However, the daisy family is a very large family of plants which also consists of trees and shrubs, of which New Zealand has many representatives.

One genus of woody daisies, Olearia, is especially well represented in a wide range of habitats throughout New Zealand. Olearia is an almost entirely Australasian genus, with a few stragglers north into New Guinea; and all of New Zealand's species of Olearia are exclusively found within this country.

The characteristic flower form of daisies is evident in all New Zealand Olearia, whether in large showy displays of usually white flowers (such as in Olearia angulata), or in smaller, almost indiscernable arrangements of flowers (such as in the scented coastal tree daisy, Olearia solandri). The daisy 'flower' form that most people will be familiar with is actually a collection of many flowers composed together within a group (a group of flowers is referred to as an inflorescence in botany).

To introduce a botanical note to this description, the parts that make up the inflorescence are called florets (effectively meaning small flowers). Within a traditional white ox-eye daisy form (such as that also exhibited in our native alpine daisies from the genus, Celmisia), there are two types of floret. The outer white florets (which look like petals) are called ray florets. The inner yellow florets (which, viewed together, look like a button in the middle of the inflorescence) are called disc florets.

Olearia adenocarpa

This relatively recent discovery from the Canterbury Plains is one of our most critically endangered plants. An overwhelming number of the remaining c. 650 specimens of this spreading, filiramulate1 shrub have the misfortune of living on heavily grazed sites - in which their futures look grim. Accordingly, this is one plant that should find some level of sanctuary in the gardens (and public plantings) of Cantabrians.

O. adenocarpa is one of a group of related small-leaved species of Olearia that occur over a wide range of the country, several of which are particularly associated with dry eastern areas of the South Island. For those who like their plants with insects, these Olearia spp. are notable in providing habitats for a wide variety of native moth species, some of which are entirely dependent on them. O. adenocarpa is most readily distinguishable from its closest relative, Olearia odorata, by its low, spreading growth habit - an unusual characteristic that was one of the main factors in alerting botanists to the distinctness of this species.

This spreading habit makes O. adenocarpa a useful species for gardens, as it can form a structural element within plantings that does not exceed 1.5m. The upward-reaching growth of its smaller stems have a directionality that makes a good contrast with the more rounded, 'centralised' form of other dryland shrubs, like Leonohebe cupressoides, Corokia cotoneaster and Olearia cymbifolia.

New main branches are often formed underneath the ground and, in the long term, as individual branches die off, they are renewed by new growth from the central crown. Bearing this in mind, within gardens, any old main stems that are diminishing in vigour should be viewed as a natural part of the plant's continuing growth (rather than a sign of poor health), and pruned back to maintain an orderly appearance2.

One of the other major characters that differentiates O. adenocarpa from O. odorata is the shape of the flower heads, which are more elongated, and contain less florets than those of O. odorata. They are similar to Olearia odorata in producing a strong perfume, another factor that weighs in favour of their use as a garden plant.

Olearia adenocarpa is a plant of the disturbed habitats that are formed by braided riverbeds, where it colonises abandoned channels (braided rivers regularly change their course). Its ability to withstand these harsh, drought-prone places makes it an ideal candidate for urban plantings within its home city of Christchurch, in a similar manner to the way in which the nationally endangered Muehlenbeckia astonii (shrubby tororaro) has been used extensively in Wellington's roundabouts and street plantings. Through such prominent use, its future can not only be assisted by increasing the number of plants in existence, but by making the public aware of its plight in the wild (where it is in desperate need of additional protection).

Olearia nummulariifolia

Certain colours are valuable for evoking particular feelings in gardens. The vibrant grass green hue of Olearia nummulariifolia is one such colour. It is an excellent species for brightening arid or dark environments, whilst its pseudo-formal growth form makes it a useful shrub for lending informal structure to plantings. Like the similar Olearia cymbifolia, O. nummulariifolia smothers itself in beautiful white flowers in summer, providing a valuable resource for native insects (such as our butterflies and native bees).

Due to the harshness of its subalpine habitat, O. nummulariifolia is extremely resilient to drought, cold and exposure. Surprisingly for a cold-climate species, a hybrid form of this species (O. nummulariifolia x coriacea) also adapts well to cultivation in warm, humid northern areas of New Zealand. In nature, it can grow to 3m, although in exposed positions it usually achieves a height of 1.6m or less. Plants of this species that I have observed on the Volcanic Plateau assumed remarkable 'bonsai' forms in response to constant battering from the elements.

We use this species for its structural characteristics, either as specimens within naturalistic plantings or as informal hedging. I have also seen it used to very good effect in the work of Dunedin landscape designer, Wayne Butson, who has employed it for informal structure (utilising the naturally compact form of this species) within plantings that successfully integrate native plants and exotic flowering perennials3. O. nummulariifolia derives its species name from the coin-like shape of the leaves (nummulus is Latin for coin).

 


Footnotes

  1. The term 'filiramulate' is an adjective that was adopted by botanists, as a superior alternative to the word 'divaricate'. It is considered to be a better term for this type of shrub, as 'filiramulate' refers to the internode length in relation to leaf size, rather than the literal meaning of 'divaricate', which is to branch widely (at roughly a 90 degree angle). Not all shrubs that fit within this group of plants branch at wide angles, therefore 'divaricate' does not describe this group of plants well enough.
  2. A similar 'renewal' process takes place in Teucridium parvifolium - another native shrub that grows from a central crown and sends branches from its base.
  3. In this case, the shrubs that were utilised may actually be the hybrid, Olearia nummulariifolia x coriacea, which is so close to the appearance of O. nummulariifolia that it is sometimes sold as that species. This form has been in cultivation for many years. The distinction is so minor that I consider them to be equivalent to O. nummulariifolia in their effect within gardens.