The genus Eucalyptus, commonly known as gum trees, encompasses over 700 species and exhibits one of the widest size ranges in the plant kingdom. This extreme variability, from small shrubs to the world’s tallest flowering trees, often confuses those considering planting one. Understanding the potential size of a Eucalyptus depends entirely on knowing the specific species and the environmental conditions it is growing in. The size a plant ultimately reaches is a complex interplay between its inherent genetic programming and external factors that encourage or limit its growth.
The Vast Size Range of Eucalyptus Species
The genetic blueprint of a Eucalyptus species sets its maximum potential height, creating distinct categories from giants to shrubs. At the top end of the spectrum are the Giant Forest Trees, such as the Mountain Ash (Eucalyptus regnans), the tallest flowering plant on Earth. Living specimens currently reach heights of over 100 meters (330 feet) under ideal conditions. Several other species, including Eucalyptus globulus (Tasmanian Blue Gum) and Eucalyptus nitens, also regularly exceed 80 meters.
Moving down the scale are the medium to large tree species, which are often utilized in commercial forestry and large landscapes. These trees typically mature in the range of 30 to 45 meters (100 to 150 feet), a category that includes the widely planted River Red Gum (Eucalyptus camaldulensis) and various Blue Gums. Even these mid-sized trees can quickly outgrow a typical residential setting due to their rapid growth rate.
A significantly smaller group are the Mallees, characterized by multiple thin stems arising from a large, woody underground storage organ called a lignotuber. Mallee species, such as Eucalyptus oleosa and Eucalyptus erythrocorys, rarely exceed 10 meters (33 feet) in height. This multi-stemmed form is an adaptation to fire and drought, making them much more manageable for smaller gardens.
The smallest varieties are the dwarf cultivars and true shrubs, suited for restricted spaces. Cultivars like Eucalyptus leucoxylon ‘Euky Dwarf’ are bred to remain between 3 and 10 meters (10 to 33 feet). In extremely harsh, high-altitude or arid environments, some species like Eucalyptus vernicosa exist as true low shrubs, often less than one meter tall.
Environmental Factors Determining Maximum Height
The maximum height determined by genetics is rarely achieved in cultivation because external conditions impose physical limits on growth. The most significant constraint is the tree’s water-conducting system, governed by the laws of physics and the “hydraulic limitation hypothesis.” As a tree grows taller, it must expend more energy to pull water against gravity, creating perpetual water stress in the leaves at the crown.
This constant hydraulic strain forces the stomata (leaf pores) to close to prevent fatal water loss, which simultaneously reduces carbon dioxide intake and photosynthesis. The resulting decline in the tree’s ability to produce sugars limits its ability to invest in new growth, causing the rate of height increase to slow dramatically as it approaches its theoretical maximum.
Soil conditions also impose a physical ceiling on growth, particularly soil depth and water availability. Although Eucalyptus species are drought-tolerant, they are prodigious water users when water is available, and their fine roots can extend for many meters. If a tree with a large potential height is planted in shallow or poor-quality soil, the limited root volume restricts the overall biomass the plant can support. Trees planted outside their native climatic range, enduring high frost or low light levels, will have their growth severely checked, preventing them from reaching optimal height.
Controlling Eucalyptus Size in Cultivation
Managing the size of a fast-growing Eucalyptus requires intentional and specific pruning techniques. The tree’s ability to recover from fire or damage via its lignotuber makes it suitable for these methods. Coppicing is the most radical form of size control, involving cutting the entire trunk down to a low stump, typically 12 to 45 centimeters (5 to 18 inches) above the ground.
This severe cut stimulates the growth of multiple new shoots from the base, creating a dense, multi-stemmed shrub form. This process must be repeated every few years to maintain the desired size. Coppicing is favored because the new growth often retains the attractive, rounded juvenile foliage, which is preferred by florists over the adult leaves. Pollarding is a similar technique where the main trunk is cut back to a higher, permanent point (around 2 meters or 6.5 feet), forcing the crown to regenerate at that fixed height.
Another effective method for limiting size is growing the plant in a container, which restricts the root system. The volume of the container directly correlates with the maximum height the plant can achieve. Dwarf or mallee species are the best candidates for long-term container growth, such as Eucalyptus ‘Azura’, which can be maintained at about 3 meters (10 feet) in a large pot with regular pruning. The container must have excellent drainage, and the plant requires consistent watering, as the confined root ball is highly susceptible to drying out.