The ‘Iliahi tree (Sandalwood) is a remarkable, valuable, and fascinating plant that can play a vital role in native landscapes.
‘Iliahi trees (Santalum spp.) are hemi-parasitic and require a host plant to help them grow. Their shallow roots graft onto roots of other plants through a sucker-like organ called haustoria which enable them to take nourishment from the host (or multiple hosts). That would seem like a big disadvantage for the host plant, but the reality is more complex and there may be shared benefits. It could be that ‘Iliahi was an essential part of the mesic forests of Hawaii as a unifying element helping to balance resources.
Four species of Sandalwood are listed as endemic to Hawaii, including Santalum ellipticum, S. freycinetianum, S. haleakalae, and S. paniculatum. ‘Iliahi has few insect pests, is drought tolerant (particularly S. ellipticum), has attractive reddish new leaves and flowers (particularly S. freycinetianum), and has a slow to moderate growth rate with ultimate height varying between species and planting locations.
Historic records and other evidence shows that ‘Iliahi was a common mesic forest plant on all the main Hawaii islands from sea level to about 8,000 ft elevation, particularly on leeward and dryer slopes, but in wetter areas as well. Populations of ‘Iliahi had been decimated by clearing for agriculture, demand for fire wood, the voracious appetite of cattle and goats, decline of pollinators, and from seed predation by rats. But certainly the most devastating effect on ‘Iliahi was the Sandalwood trade of the 1800’s, which resulted in immense quantities of the trees being cut for trade to China.
Research with tropical Sandalwood species has shown that nitrogen-fixing legumes tend to make the best hosts. This indicates that native plants such as ‘Ohai, Maiapilo, Wiliwili, and Koa might be good candidates for bonding, although some growers in Hawaii have found that just about any shrub or tree species will do.
The uptake of nutrients from the host plant is very selective, meaning that ‘Iliahi only takes what it needs and apparently reduces its uptake if the host plant is overly stressed. It might also be possible that a host plant can get some nutrients and moisture from ‘Iliahi, in a reverse flow. Research has been directed at what hemi-parasitic plants take from their hosts, but very little is known about how the host plants or surrounding ecosystem might be benefiting from hemi-parasitic plants. For example, can ‘Iliahi transfer fluids and nutrients from a deep rooted Koa tree to a nearby shallow rooted Lama tree seedling that it is also attached to? The synergistic relationship between host and parasite and the implications of to a forest community is an important topic needing additional research and verification.
Even if there is not a transfer of nutrients from one host to another host, ‘Iliahi might still be helping its neighboring plants in subtle ways. Nitrogen fixing legumes like Koa and Wiliwili would have a distinct advantage in any plant community as they can out compete rival plants that cannot fix nitrogen. Researchers have suggested that a beneficial role of hemi-parasitic plants like ‘Iliahi is to help maintain species diversity in a forest community by reducing the competitive advantage of nitrogen fixing legumes by “relieving” them of some of their bounty thus reducing their vigor and enabling other plants to better compete. Also, perhaps ‘Iliahi can help attract pollinators and seed dispersers that can be of benefit to the host plants, for example by helping to support a more diverse bird population.
To grow ‘Iliahi from seed, recommendations vary and include de-pulping the seed, nicking the seed coat and soaking in water, or treating with 0.05 percent gibberellic acid. Hui Ku Maoli Ola, a native plant nursery on Oahu, reports good success with S. ellipticum by planting fresh untreated seed directly in a sterile potting mix, with germination within a month or two. Research from the University of Hawaii has shown that adding chelated iron can help significantly for the successful growth of ‘Iliahi in containers.
Richard Quinn, ASLA is a landscape architect at Helber Hastert & Fee and on the Board of Directors.