Hawaii’s green landscapes are composed of relatively few tree species. A drive down Kalakaua Avenue in Waikiki reveals mostly coconut palms (Cocos nucifera) and rainbow shower trees (Cassia × nealiae) with occasional kukui (Aleurites moluccana), Clusia rosea, satinleaf (Chrysophyllum oliviforme) and variegated hau (Hibiscus tileaceus) trees. Elsewhere, Honolulu’s street trees include Chinese banyan (Ficus microcarpa), Hong Kong orchid tree (Bauhinia × blakeana), rainbow shower trees, monkeypod (Samanea saman), fiddlewood (Citharexylum spinosum), royal poinciana (Delonix regia), fern tree (Filicium decipiens), and perhaps a dozen others.

From 2005 to 2006, the common coral tree (Erythrina variegata) was mostly eliminated from parks and streets by the Erythrina gall wasp (Quadrastichus erythrinae). Periodically, monkeypod trees come under attack from a caterpillar that defoliates them and damages growing points until parasitic and predatory insects bring it under control. Another insect pest, the spiraling whitefly, also causes defoliation and unsightly masses of white fibers on a number of tree species, fiddlewood being one of the more severely attacked. In 2007, plumeria (Plumeria obtusa, P. rubra) trees were attacked by the papaya mealybug. More recently, the lobate lac scale (Paratachardina lobata lobata = Paratachardina pseudolobata) has been found on a number of ornamental plants, sometimes with devastating effects, and a new stem borer (Josephiella sp.) is affecting Chinese banyans.

As a result of a limited palette of tree species used in Hawaii landscapes, we are vulnerable to loss of green canopies when an epidemic of insects causes losses in the landscape. Hawaii has been fortunate to have escaped, so far, the loss of coconut palms due to the lethal yellowing disease that has all but eliminated coconut palms in Florida and Texas landscapes. Texas has recently issued quarantines for a number of other palms susceptible to this disease. New insect pests are being identified in southern California landscapes that could have severe effects in Hawaii should (when) they arrive here. If an insect were to enter Hawaii that attacks the rainbow shower tree, it would have a very great impact, particularly in Honolulu, where the cultivar Wilhelmina Tenney is a principal street and park tree.

Stemming the loss

While Hawaii enjoys a green landscape, there is room to expand the palette of tree species employed in residential and public landscapes and street and highway plantings. Urban tree trends include a call for columnar and native trees, smaller trees, adaptability to varied urban growing conditions, and trees that can modify the urban environment. Large trees are recommended by scores of researchers for reducing stormwater runoff, improving air quality and maximizing shade along streets, in parking lots and in commercial areas.

Producers look for new plants to keep up with consumer demand for unique landscape plants. A number of underutilized and exotic species have been identified, but information on their production (propagation, growth rates), environmental requirements/tolerances and functional uses needs to be developed. At present, we are working on a project designed to fulfill these needs by identifying 30 new (to Hawaii landscape use) or underutilized species and providing this information. Bringing a new urban tree to market may take 20 years or more, including the time that it takes for success “in the landscape setting” to stimulate demand for it; thus, this project has a long timeline.

Landscape architects and designers tend to specify plants that they are familiar with, and the result has been hotel landscapes with much the same package of plants and residential developments with the same trees used throughout. They are less likely to try new trees because their firms’ reputation depends on success of their plantings. While the movement toward using native plants in landscapes is growing, there are few truly native trees employed in local landscapes. The “Expanding Tree Diversity in Hawaii’s Landscapes” project has enabled the evaluation of species beyond the few identified in Bornhorst (2005) and Lilleeng-Rosenberger (2005), especially their propagation.

The Tropical Plant and Soil Sciences Department at the University of Hawaii has composed a list of suitable trees to help expand urban tree diversity in Hawaii. These trees have been selected with special attention to natives, although some trees are introduced. The trees have a wide range of use in the landscape.

The following trees are the first group to be released while others are being worked on to meet the goal of 30 trees. As we identify and release more, we’ll follow up with their introductions.

Afrocarpus gracilior (weeping afrocarpus)

Colvillea racemosa (Colville’s glory)

Ficus binnendijkii (alii, Amstel king)

Pittosporum hosmeri (Hawaiian name = Ho’awa)

Pseudobombax ellipticum (shaving-brush tree)

Sapindus oahuensis (Hawaiian soapberry; Hawaiian name = lonomea)

Wallaceodendron celebicum (banuyo)