Eucalyptus trees species were brought from Australia to the US in the mid 1800′s and presumably around that time to Arizona. The are common and ubiquitous in Tucson. The one in the photograph is in Agua Caliente Park and is a favorite of mine. It is a beautiful tree with great presence.
Eucalytus trees are biologically remarkable. Oil extracted from eucalyptus leaves and gum produced by the trees contains compounds that are powerful natural disinfectants and insect repellants, protecting the trees from insects, and also have medicinal qualities. The nectar from their flowers makes a fine honey. Because they are fast growers they are useful in the lumber industry and as landscape trees. However, in non-native habitats – and they are now found on all the continents – they can be destructive to native species as a competitor, in plantations they do not support native wildlife and when grown in places with high fire risk, such as California, they are a problem because they are highly flammable.