The genus Eucalyptus encompasses over 700 species of evergreen trees and shrubs native primarily to Australia. Often grouped under the common name “gum tree,” these plants display a wide range of characteristics. The perception of one species representing the entire genus often leads to planting mistakes regarding mature size and cold tolerance. Selecting the right Eucalyptus depends entirely on the specific conditions and goals of the individual gardener.
Defining Your Needs: Criteria for Selection
The most limiting factor when selecting a Eucalyptus is its cold tolerance, which relates directly to your location’s USDA Plant Hardiness Zone. Many species are native to frost-free regions and will not survive freezing temperatures. However, others, known as “hardy eucs,” can tolerate significant cold snaps. This climatic constraint immediately narrows the choices down to a handful suitable for temperate climates.
The potential mature size is another defining parameter. Some Eucalyptus species are among the tallest hardwood trees in the world, capable of exceeding 200 feet. Gardeners must differentiate between these massive varieties and smaller, shrub-like forms that can be maintained through pruning or grown in large containers. Understanding a species’ natural growth habit prevents selecting a towering forest tree for a small suburban yard.
The tree’s primary purpose should also guide selection. Some varieties are sought after for their silvery, juvenile foliage popular in floral arrangements. Others are planted for rapid growth to serve as a quick privacy screen or windbreak. Certain species are cultivated specifically for the high concentration of cineole in their leaves for essential oil production.
Top Eucalyptus Varieties and Their Key Attributes
For gardeners in cooler regions, Eucalyptus gunnii, known as the Cider Gum, is the most suitable option due to its superior cold hardiness. This Tasmanian native can tolerate temperatures down to approximately 0°F (USDA Zone 7) once established. The Cider Gum develops into a large, upright tree ranging from 30 to 70 feet tall. It features striking blue-silver, rounded juvenile leaves that transition to blue-green, lance-shaped adult foliage.
If the primary goal is ornamental foliage for cutting, Eucalyptus cinerea, the Silver Dollar Tree, is an excellent choice. This species is highly valued because it retains its attractive, waxy, blue-gray, coin-shaped juvenile leaves for many years. While capable of reaching heights of 20 to 50 feet (Zone 8), it responds well to coppicing. This technique is used to maintain a bushy, shrub-like form that constantly produces the desirable juvenile growth.
For fast biomass production or essential oil harvesting, Eucalyptus globulus, the Blue Gum, is one of the most widely planted species globally. This tree exhibits a phenomenal growth rate, often growing over 10 feet per year in optimal conditions, and can eventually reach heights of 150 to 300 feet. Its hardiness is limited, generally restricted to USDA Zone 9 and warmer regions, as it is easily damaged by temperatures below 20°F.
A unique aesthetic choice is Eucalyptus deglupta, the Rainbow Eucalyptus, which is the only species native to the Northern Hemisphere. This tropical giant is prized for its spectacularly multicolored bark that peels away to reveal vibrant streaks of green, blue, purple, orange, and maroon. It is extremely frost-intolerant, requiring a warm, humid environment suitable only for USDA Zones 10–11, and can grow quickly to between 100 and 250 feet tall.
Essential Care and Site Placement
Proper site preparation is necessary for the long-term health of any Eucalyptus, starting with ensuring excellent drainage. These plants dislike saturated soil, which can lead to root rot despite their eventual drought tolerance once established. When planting, select a location that receives full sun, as they are not adapted to growing in shaded conditions.
The aggressive nature of the root system requires careful placement, especially for larger tree varieties. The shallow, lateral roots of many Eucalyptus can spread up to 100 feet from the trunk. These roots can cause damage to foundations, pipes, and paved areas in their search for water. Planting the tree a substantial distance from any built structure is a preventative measure against potential infrastructure damage.
Managing the size and form of the tree is often achieved through pruning. Coppicing is a common technique used to control height and encourage juvenile foliage. This involves cutting the main trunk back close to the ground, stimulating new shoots to emerge from the protective lignotuber at the base. This method is effective for ornamental species to maintain a dense, bushy appearance.
Safety Considerations and Potential Drawbacks
A significant consideration before planting is the inherent toxicity of the foliage and essential oils to most pets, particularly dogs and cats. The leaves contain the compound eucalyptol, a neurotoxin that, if ingested, can cause symptoms ranging from vomiting and diarrhea to seizures and coma. Pure eucalyptus oil is highly concentrated and is particularly dangerous to ingest, with even small amounts potentially being fatal to pets and humans.
The high flammability of most Eucalyptus species presents a substantial fire risk, especially in dry climates. The leaves are loaded with volatile oils that can vaporize in high heat. The trees also shed large amounts of highly combustible litter, including oily leaves and strips of bark. This combination of fuel and flammable vapors causes fires in eucalyptus groves to burn hotter and spread rapidly into the canopy.
Another concern is the potential for certain species, such as Eucalyptus globulus, to become invasive outside of their native range. Their rapid growth and prolific seed production allow them to outcompete native vegetation, disrupting local ecosystems. Furthermore, their root systems are voracious water consumers, which can quickly dehydrate the surrounding soil, making it difficult for other plants to thrive nearby.