Pimelea

The elegant white flowers and attractive foliage of New Zealand’s many species of Pimelea make this a group of plants that is particularly worthy of a place in our gardens. Pimelea prostrata is already well-known to gardeners and landscapers, but this carpeting groundcover is just one of around 34 species native to our shores¹. The variety of growth forms exhibited within Pimelea provide us with a useful range of small shrubs that may be put to different uses within plantings.

The carpeting form that is manifested in P. prostrata is a feature of other species, including P. urvilleana (an excellent species that is less common within cultivation), P. oreophila (a montane species of grasslands) and the diminutive P. microphylla.

Another common growth form is the very orderly, upright habit of several species, such as the extremely gardenworthy Pimelea traversii, the recently described P. orthia, and the so-called sand daphne, P. villosa (syn. P. arenaria). The critically endangered Pimelea actea also loosely conforms to this habit, although its finer leaves furnish it with a slightly less uniform appearance than the other species mentioned.

The jewel of the New Zealand species, P. longifolia (pictured, above), has a more conventional form than those mentioned above, as does the smaller P. tomentosa (a threatened species with a soft appearance). And finally, the mounding ‘pom-pom’ form of certain species such as P. aridula and P. pseudolyallii presents yet another face to the genus.

Pimelea is predominantly a genus of coastal and montane habitats, both of which are home to the kinds of open plant communities that these small shrubs thrive in. Many of the species are either naturally rare or threatened within nature (due to the pressures applied by human activities or browsing by introduced mammals).

Two species in which we have taken a special interest are Pimelea actea, a critically endangered shrub now confined to just two tiny populations on the Manawatu coast (pictured in fruit, above), and Pimelea mimosa, a new species that is only found naturally on Te Mata Peak in the Hawke’s Bay. We have also been impressed by the alpine South Island species, P. traversii, which has shown itself to be a good garden plant that is even amenable to being grown in the warmer north – on the condition that it is given an open situation in which to grow.

A fine plant from the Hauraki Gulf, classified as Pimelea urvilleana ssp. nesica, is another form that has impressed us greatly. Introduced to cultivation by Oratia Native Plant Nursery, this form looks set to outstrip P. prostrata as a groundcover plant for northern areas of New Zealand, due to its more robust nature, denser growth and suckering habit (the latter is a very good attribute in groundcovers).

In our experience, the most damaging pest that affects several species of Pimelea is mealybug, which can kill plants in heavy infestations. In addition to encouraging vigorous growth (which is the best form of protection), neem oil is an effective treatment for this potential problem. One last piece of information that is worth recounting is that some species are referred to as New Zealand daphnes – a common name that has merit, as Pimelea is part of the daphne family, Thymelaeaceae. Many gardeners would appreciate this relationship, upon comparing the dome-shaped form of Pimelea‘s flowerheads with that of the common garden shrub, Daphne odora.

In some cases, horticulture has the opportunity to contribute very significantly to the conservation of a species. By creating an awareness of a species’ plight (as well as providing the tangible benefit of a collective population outside the wild), gardeners and landscapers can help in preserving some of our rarest plants.

Pimelea actea is an excellent example of a plant that, quite apart from deserving a place in gardens for its attractive appearance, may be profoundly assisted by cultivation in its survival. This recently-described species is now confined to just one small stretch of the Manawatu coastline, where fewer than 60 plants endure. In the wild, its future is acutely threatened by the impact of people and animals, and the shifting nature of its coastal environment.

P. actea is a beautiful species with a delicate character, whose stems have a very directional feel to them, similar to the habit of its montane relative, P. traversii. It produces small white flowerheads (with vibrant orange anthers at their centres) sporadically over most of the year. These are followed by its most remarkable feature; translucent berries that adorn the stems over a very long period – and are particularly conspicuous on P. actea’s lightweight structure.

Formerly, this species was known from Castlecliff (near Wanganui), Foxton Beach and where the Turakina River meets the sea. Due to its occurrence at the latter, it was previously known as Pimelea “Turakina”, and is occasionally sold under that name.

On the steep, south-facing cliffs of Hawke’s Bay’s Te Mata Peak, the last remaining plants of this unique shrub cling to their existence. Contrary to the expectations that might be laid down by such an introductory statement, this is not a doom-and-gloom story. This form of Pimelea (which was recognised as being distinct by the distinguished botanist, Tony Druce) is now subject to an active replanting programme by a society that is devoted to revegetating, and conserving the special plant communities of, Te Mata Peak.

Until relatively recently, the species, Pimelea aridula, was considered an aggregation of several forms, some of which are geographically well separated from other populations – such as on Te Mata Peak, which stands like an island amidst the surrounding low hills and plains. A number of these were acknowledged as distinct within ‘Eagle’s Trees and Shrubs of New Zealand’ (2006), and these received official taxonomic consideration in a 2011 revision on Pimelea² – within which the Te Mata Pimelea was described as P. mimosa.

Contrary to the scrappy appearance of many of the plants that are remaining in the wild, Pimelea mimosa is a very attractive, mounding shrub, which will grow to 40cm high and 1m wide within the garden. A distinctive feature of this form is the particularly silver colour of the foliage. The white flowers are borne over an extended period during summer, and are at times present in great profusion.

The Te Mata Pimelea is well suited to Hawke’s Bay’s seasonally dry climate. Although the few extant wild plants are found in semi-shade on south-facing cliffs, it is equally capable of growing in hot, open conditions, making it a versatile garden plant. Being a local endemic, it should be planted more within the Hawke’s Bay. Surprisingly, the Te Mata Pimelea also grows well in other parts of the country, including Auckland (where we have planted it in multiple projects).

Pimelea mimosa grows naturally on limestone cliffs, within distinctive communities of plants, including an endemic form of snow tussock, Chionochloa flavicans forma temata, a distinct form of mountain daisy, Celmisia aff. gracilenta, and species that are typical of limestone (such as Scandia rosifolia and Clematis forsteri).

As with other threatened plants, the sites in which it endures do not represent its only habitat preferences – but rather places that were not converted to pasture, or are inaccessible to animal pests. Accordingly, the society in charge of replanting the Peak have planted specimens in a range of situations, including amongst the rough grass at the edges of pasture. Thanks to these volunteers, the effort to rejuvenate Pimelea mimosa is an example of the kind of good news story that we need more of in plant conservation.

The most widely cultivated of all the native Pimelea spp., this tough little character occurs in open places, within coastal to subalpine areas throughout the country. It is notable as a groundcover, for its capacity to thrive in dry conditions (a large percentage of groundhugging plants prefer moist ground). Other factors in its favour are the bluish-green colour of the foliage and the abundantly-produced, sweetly-scented white flowers that it bears over a long period from spring through to the beginning of autumn. P. prostrata normally grows to dimensions of around 5-10cm high and up to 1m wide.

As may be inferred by the common name, Strathmore Weed, P. prostrata is one of our native species that is able to colonise disturbed ground in human landscapes. I have observed this species growing in a small population on a roadside bank just south of Port Waikato, where it grew in association with Pomaderris amoena and bracken fern (Pteridium esculentum). This is an example of the kind of modified habitats that, in the absence of injudicious spraying by maintenance contractors, are regularly populated by native plants (some of which may be very interesting). These kinds of places can teach us a lot about suitable species for tough conditions, many of which we often expect plants to tolerate.

Another example of a habitat in which a similar level of resilience is a necessary trait is the ecosystem pictured above, at Kaitawa Point north of Wellington, where howling winds frequently scour out the banks facing the Tasman Sea. In this kind of scenario, plants like Pimelea prostrata and Raoulia hookeri benefit from the dynamic conditions that preclude other plants establishing here.

P. traversii is one of the finest species that this attractive genus has to offer, on account of its beautiful foliage, overall form, and the dome-shaped flowerheads that adorn its outer canopy in spring and summer. It is also one of the more adaptable of our montane species for cultivating in many parts of the country (including warmer areas where logic might tell us that it should not succeed).

Its erect stems and orderly leaf arrangement give well-grown specimens of Pimelea traversii a particularly refined appearance, an impression that is further reinforced by the bluish-grey colour of the leaves. Mature plants tend to assume a rounded, dome-like form in sheltered conditions; up to a height of around 60cm.

P. traversii is native to dry, often rocky habitats along the length of the South Island (from Marlborough to Central Otago), particularly on the drier eastern side of the island. As one might expect, well-drained conditions are necessary to successfully cultivate P. traversii, as is an open situation. It has recently been split into three subspecies (within the extensive revision of Pimelea by Colin Burrows), although gardeners are only likely to encounter the most widespread of these, P. traversii ssp. traversii.

Although one authoritative resource describes Pimelea traversii as fickle in cultivation³, this has not been my experience, nor that of colleagues from the nursery industry. Like Pimelea aridula, it is a species that should be embraced more by South Island gardeners and landscapers, as a relatively low-growing shrub for establishing structure in the lower tier of plantings.

The species was named in honour of W.T.L. Travers, a significant figure in botanical circles during the middle of the 19th Century who contributed much to the knowledge of the flora of Nelson and Marlborough (where this species was first discovered to science).