Superior, Arizona
- Sep 1, 2020
- 4 min read

Boyce Thompson Arboretum featured in Public Garden Magazine

Public Garden Magazine

Volume 35, Issue 3, 2020

Boyce Thompson Arboretum Eucalyptus Collection

by Lynne Nemeth

"Mr. Big," Eucalyptus camaldulensis, red-river gum

As Boyce Thompson Arboretum (BTA) in Arizona approaches its one hundredth anniversary, so too does its oldest collection, Eucalyptus, with the first accessions dating back to the early 1920s. The largest eucalyptus tree in the collection, now nicknamed “Mr. Big,” was a three-year-old, six-foot sapling in 1926. Today, “he” is 117 feet in height, with a circumference of more than 22 feet, and has been named an Arizona Champion Tree. Mr. Big is a red gum or longbeak eucalyptus (Eucalyptus camaldulensis), the largest known tree of its species in the United States according to the Arizona Forestry and Fire Management agency. Native to Australia, red gum can be found near water or in floodplains throughout Australia. BTA founder William Boyce Thompson planted the tree that was to become Mr. Big in the floodplain of the ephemeral Queen Creek, which runs through BTA.

Flowering Eucalyptus torquata, coral gum

The genus Eucalyptus, in the myrtle family (Myrtaceae), comprises over seven hundred species of flowering trees and shrubs. It is the preferred habitat and food of koalas. While most species are native to Australia and Tasmania, a few are also indigenous to Indonesia, the Philippines, and Papua New Guinea. More than two thirds of Australian forests are eucalypt forests; species are found in habitats ranging from snowy mountain ranges to tropical rainforests. Many eucalypt species are adapted to fire, resprouting after burns or bearing fire-resistant seeds—as demonstrated in their quick recovery after the latest Australian wildfires. All eucalyptus trees are evergreen and annually shed their bark. One of their most readily visible (and breathtaking) features are their blossoms, which have no petals. The entire flower consists of hundreds of fluffy stamens which may be white, bright red, vibrant orange, or fuchsia. The abundance of stamens is critical to the species’ selfpollination; the high concentration of cineole, the chemical that gives eucalyptus its fragrance, deters pollinators—and predators. Only the koala, ringtail possum, and a few insects can eat eucalyptus leaves and bark. Cineole, also known as eucalyptol—and the main ingredient in eucalyptus essential oil—is used in indigenous Australian medicine as an antibacterial, anti-fungal agent, and insect repellent.

View looking up through the canopy of the eucalyptus forest

While other gardens hold large Eucalyptus collections, BTA’s is unique, with a focus on xeric or desert species. Today, the collection numbers 428 individual plants and 103 species (111 taxa), with 307 accessions—all native to Australia. Given the size and focus of the collection, staff in 2017 applied for national accreditation through the Plant Collections Network. Two other BTA collections had been accredited previously, the Oak Multi-Site Collection in 2012 and the Fabaceae (Desert Legume) Collection in 2014.

The collection was provisionally accredited in 2018, with full accreditation awarded in 2020 on the basis of its broad taxonomic diversity. Of the species held, close to 50% are species of conservation concern. Thirteen are endangered, 21 are vulnerable, and 17 are near threatened (IUCN Red List), among them Eucalyptus camaldulensis—Mr. Big. Wild Eucalyptus face many threats, including deforestation, land alteration for agriculture, livestock grazing, global climate change, and wildfire. Three endangered species at BTA—Eucalyptus microcarpa, E. spathulata, and E. odorata—are all declining due to land clearing as the demand for more agricultural areas and livestock pastures increases in Australia. In 2019, the IUCN added many species to its Red List and increased the threat assessment rankings of others. While Eucalyptus are fire-adapted, the 2019 Australia wildfires, combined with many other human-imposed threats, may dramatically accelerate this downward trend. Consequently, the need for ex situ conservation will further increase. In addition, Myrtaceae, Australia’s dominant plant family, faces risk from myrtle rust (Austropuccinia psidii), first detected in 2010. The disease can cause deformed leaves, heavy defoliation of branches, reduced fertility, dieback, stunted growth, and plant death. However, recent studies indicate that climate change may gradually reduce the risk of the disease (doi.org/10.1016/j.biocon.2017.11.035).

Flowering Eucalyptus kruseana, book-leaf mallee

BTA’s acquisition of eucalyptus dates from the 1920s, when the first trees were planted, to the 1990s, when the focus shifted to wild-collected plants. Much of the older material came from Armstrong Nursery in California (now Armstrong Garden Centers, celebrating its 130th anniversary this year). Over the years, BTA has propagated eucalyptus on site as well as at the University of Arizona. Since the 1980s, enhancement of the collection has focused on wild-collected plants and seeds. To date, 41% of the collection is from the wild, 19% is cultivated, and 40% is of unknown origin.

Our ultimate goals for the collection are to increase the number of taxa we hold and to develop partnerships with our sister gardens who hold Eucalyptus collections, eventually developing a multi-site collection for education and interpretation.

Lynne Nemeth is Executive Director of Boyce Thompson Arboretum.