Tree of the Week: The purple-leaf acacia

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Purple-leaf acacia -- Acacia baileyana ‘Purpurea’’

The purple-leaf acacia is unapologetically not green. Many people love it while others find its soft, silvery blue-gray foliage tinged with purple too intense for their green garden color scheme. The Bailey acacia, either in purple-leaf or plain-gray variety, is also known as mimosa for the fragrant, tiny, yellow flower pompoms that appear all over the plant at the end of winter. They are often sold in small bouquets as harbingers of spring in sun-challenged climates.

This acacia is a member of the legume or pea family that manufactures its own nitrogen fertilizer. It originated in southeastern Australia where it goes by the name Cootamundra Wattle. In Australia, it has a tendency to naturalize beyond its native habitat, so it is looked at with some suspicion here, but unlike some other acacias, the purple-leaf acacia (at least for now) is not on the Cal-IPC invasive-plant watch list. The genus name refers to the fact that many acacias are thorny, while the species name honors the Australian botanist Frederick Manson Bailey.

A moderate-to-fast-growing small evergreen (evergray?) tree, the purple-leaf acacia reaches 20 to 30 feet tall and wide. Its dense, spreading canopy is easily shaped as a single- or multi-trunk tree. The tree is short-lived and may last less than 30 years, which makes it ideal for a short-term landscape. The bark is smooth gray and its branches are somewhat weeping. Its leaves are small and compound, like tiny feathers. The purple sheen is most pronounced on new leaves, so its branches are occasionally pruned back to stimulate new growth. Prominent stamens define the tiny, soft, bright yellow flowers that are grouped in clusters. Easy to grow, frost-hardy, drought- and disease-resistant and seaside-tolerant, the tree likes full sun to filtered shade and accepts most types of soil. It has non-invasive roots.

The purple-leaf acacia is often planted on hillsides. It has a tendency to look a little wild. Its appearance is better when it is kept somewhat open, with some branches removed all the way back to the trunk. The tree is very popular in the San Francisco Bay Area.

-- Pieter Severynen

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