Plant profiles > Dianella

Family : Xanthorrhoeaceae

Gardeners often look upon the efforts of taxonomists as a confounding inconvenience; re-naming a species here, creating another one there. However, in many instances, botanists' work in discerning the relationships between plants makes a real difference in gardens.

The recent re-classification of Dianella is an excellent case in point. From one all-encompassing entity (D. nigra), two additional species have been described - both of which (D. latissima and D. haematica) show tremendous potential as garden plants. Although these two species do not represent new discoveries in the traditional sense (they were previously considered to be part of an array of forms within the gamut of D. nigra), gardeners and landscapers were extremely unlikely to have come face-to-face with them. However, now that they have been separated out as distinct, nurseries are starting to stock material of both of them.

The tangible difference between the two new species and D. nigra (which has been retained) is the much larger size of the plants (as well as the width of the leaves in the case of D. latissima). Due to their greater stature, D. haematica and D. latissima can bring a more commanding presence to plantings. This is not to say that D. nigra is unworthy of a place in the garden. To the contrary, there are many forms of this variable species that make very good garden plants (such as the plant photographed below, growing naturally in an urban Auckland park).

The genus Dianella contains a large number of species¹, and is primarily distributed around the Pacific Rim (in Australasia, southeast Asia, tropical South America, and various Pacific Islands)². Its name is derived from Diana, the Roman goddess of hunting and the moon, and was bestowed upon it when the first specimens of the genus were grown in France³ (in the King's gardens) - where they were referred to as "dianelle des bois".

Dianella latissima

This understated, flax-like plant is one of the most exciting introductions to native horticulture of recent years. Its robust leaves, relatively large stature and graceful arching form have invited comparison with coastal flax (Phormium cookianum) - a similarity that is further reinforced by the vibrant hue of its leaves.

It has been found in a range of habitats in upper parts of the North Island, from cloud forest on the Waima Range (where the picture to the right was taken) to regenerating kauri forest and dry coastal forest (where it is often associated with kanuka)⁴.

In many places, it grows on shallow soils and disturbed ground (such as on rocky outcrops and slipfaces) - habitats that are analogous with the often difficult conditions presented by gardens. As testimony to this, Geoff Davidson has recounted to me how plants of this species cling to an exposed coastal bluff on Motu Kaikoura island, in the hardest conditions possible.

Within the garden, it looks to be a valuable plant for establishing a sense of contrast (as well as order) within plantings. Although it does grow in full sun, I think that it is wiser to provide a modicum of shade - so that plants may achieve their optimum appearance. I have seen a number of public plantings in which Dianella nigra has been expected to contend with exposed, open conditions that it might deal with in nature⁵ - only for plants to suffer and look scrappy. D. latissima requires a reasonably well-drained position within the garden; unlike its relative, D. haematica, which will grow in either swampy or normal conditions.

The specific epithet, latissima, refers to the wide leaves that help give this plant its distinguished appearance. It was described as a distinct species in 2007, by Peter Heenan and Peter de Lange, two botanical scientists who have played a significant role in the taxonomy of New Zealand plants over the last two decades.

Footnotes

1. Outdated counts within literature of 20 years ago (or more), place the number of species at c. 25, but Peter de Lange informs me that this count is likely to increase several times over, with revision of the genus in Australia (where a potentially huge number of species occur) and the Pacific Islands.
2. It is also native to islands of the Indian Ocean.
3. The plants originated from Mauritius (which at that time was called Ile de France) and Réunion Island (which was then known as Ile Bourbon), and the "East Indies"; and were assigned to Dianella nemorosa, according to Lamarck's description. I do not know whether the name Dianella nemorosa still applies for that species, or whether it has become a synonym.
4. Much of the information in this plant profile is adapted from the sole authoritative account of Dianella latissima - the paper in which Peter Heenan and Peter de Lange described it and D. haematica as new species (Heenan, P.; de Lange, P. 2007. Two new species of Dianella (Hemerocallidaceae) from New Zealand. New Zealand Journal of Botany 45 : 269 - 285).
5. Plants may thrive within challenging positions in the wild, because they have developed from seedlings in those spots, or because a habitat provided a level of protection during their establishment (that has later disappeared) - such as the cover of certain shrubs above. In addition, some natural variants within a species may be tougher than other forms. However, when nursery-grown plants are expected to deal with such difficult spots, there is often an element of transplant shock.