Acacias are gaining in popularity with garden designers and landscapers as rapid-growing evergreen shrubs and small trees, often serving as screens. They provide eye-catching foliage and prolific fluffy, yellow inflorescences that often appear in winter.
Commonly known as wattles or mimosas, acacias are part of the pea family, Leguminosae. Most acacias available in the UK originate from Australia and Tasmania. The taxonomy has been hotly debated of late, and the names of many species from Africa and the Americas are about to change, while those of almost 1,000 Acacia species from Australia will remain unchanged.
The distinctive foliage is of two types: feather-like, bipinnate leaves or strap-like phyllodes — enlarged, flattened leaf stalks that function as leaves. Colours range from downy white to green, blue or silver-grey, with juvenile growth sometimes tinged red, purple or bronze. In some species, stipules form woody spines.
In sheltered sites they produce a mass of vivid flowers, often in late winter to early spring. A selection of Acacia species will provide colour throughout the year.
The individual flowers occur as globular, pompom-like heads arranged in racemes or cylindrical spikes. The inflorescences vary in colour from cream and pale yellow to gold. A number of species are wonderfully fragrant, while characteristic pea-like pods add ornamental value.
Species such as A. saligna are resistant to wind and salt spray. Some such as A. retinodes Award of Garden Merit (AGM), A. longifolia and A. cyclops will tolerate lime, but most species will become chlorotic in limey conditions. Most species should be planted in well-drained, neutral to slightly acidic soil in full sun.
Hardiness is key to their suitability. A. dealbata AGM, A. longifolia and A. baileyana AGM will tolerate temperatures down to –10°C, while A. saligna can handle temperatures down to –5°C — though buyers should enquire about provenance as this will have a bearing here. Other species will grow in milder areas, protected by south- or south-west-facing walls.
Pests include caterpillars, root mealybug, red spider mite and oleander and brown scale. In parts of Africa and the US, acacias that have been introduced have become invasive but are managed using biological controls.
What the specialists say
John Eddy, mail order manager, Trevena Cross Nurseries, Cornwall Usefully, acacias are evergreen, they grow quickly and can be cut back to size. Most plants we grow can tolerate conditions down to –6°C or –7°C without any problems.
A. baileyana ‘Purpurea’ Award of Garden Merit (AGM) is absolutely beautiful. There is a brilliant contrast between foliage and flowers. A. verticillata riverine form is also a real beauty — it is more compact, it flowers profusely, and it’s very hardy and tolerant of strong winds. A. dealbata AGM grows rapidly and you quickly have a tree of 7m to 10m — not a tree for the smaller garden.
A. pravissima AGM has a beautiful weeping, spreading habit and an unusual triangular leaf, and is better suited for small gardens. A. melanoxylon is another big tree that grows quickly and is extremely wind-tolerant, so useful for a shelter belt. A. retinodes AGM has red bark and grows quickly, more than a metre in its first year, flowering in its second year.
Rupert Eley, partner, The Place for Plants, Suffolk The best Acacia by a long way is A. pravissima AGM. It is a real ‘wow factor’ plant. It’s a fast-growing evergreen that flowers profusely at Easter. Many of our customers are inspired by one in our planted border.
Another favourite is A. dealbata AGM, which is a very attractive tree, as are A. baileyana AGM and the smaller cultivar A. baileyana ‘Purpurea’ AGM. These tend to be bought for leaf colour but are not reliably hardy for us.
All our acacias are vigorous growers. Acacias are not leaf droppers or untidy and we have not found the roots to be problematic. The ‘wow factor’ lasts for about 10 years, but they are not long-lived and will succumb in harsh winters.
Doug Hammond, commercial manager, Goddards Nurseries, Surrey, and former landscape designer I have found that A. dealbata AGM is the most commonly planted species, being the most versatile and tolerating most soils except shallow soil over chalk.
It is a superb tree when grown as a standard, reaching up to 25m, -although in this country it will rarely achieve more than 15m to 20m. More commonly, I have planted it as an open bush where it can be more easily managed and contained, responding well to heavy pruning in the spring.
A. pravissima AGM is a good alternative to A. dealbata AGM, with small, grey-green triangular leaves. It offers a slower growth rate, only reaching 3m to 8m, and should only be planted in a sheltered area because it is not 100 per cent hardy.
A. baileyana AGM is becoming more popular as it is quite different from the more common varieties. It reaches about 7m tall and is quite vigorous. The fern-like foliage is its attraction and it is ideal for screening or where mid-winter colour is required. A. baileyana ‘Purpurea’ AGM, with a subtle purple hue to its foliage, is another great selection.
A. retinodes AGM, with its narrow, pointed -foliage, is hardy and reaches a good height for most situations of over 6m. It is not widely grown but is a valuable species as an alternative screening plant to other evergreen plants such as Eucalyptus or x Cupressocyparis.
While Acacia is a great landscape plant, it should be used with caution. It doesn’t like prolonged wet conditions and has a low tolerance of alkaline soils. And, like many vigorous species, the rootzone can be troublesome when planted near buildings and services. Acacias will also interfere with planting schemes where they are planted alongside smaller under-planting.
Species and cultivars
• A. baileyana Award of Garden Merit (AGM) is a shrub or small tree growing to 8m, with arching branches and silver to grey-green bipinnate leaves. The inflorescences are a bright golden yellow and appear in late winter to early spring. It has a tendency to naturalise.
• A. baileyana ‘Purpurea’ AGM has young foliage that is deep purple, in contrast to the blue-green older leaves and golden inflorescences.
• A. dealbata AGM is a bushy shrub or tree that has dense, silvery green, bipinnate leaves. The golden, pom-pom-like, fragrant inflorescences flower in late winter to early spring.
• A. karroo is the emblematic thorny species from South Africa. It is a shrub or tree growing to 15m with bipinnate leaves. The tree usually has a round crown. The fragrant, bright yellow inflorescences flower in the summer.
• A. longifolia is a spreading shrub or small tree with long, dark green phyllodes. The bright yellow spike-like inflorescences flower in mid- to late winter. It is fairly lime tolerant.
• A. melanoxylon is a fast-growing tree to 30m. The bark is charcoal grey and the phyllodes dark green. The creamy yellow inflorescences flower in the spring.
• A. pravissima AGM is a bushy shrub or small tree to 8m tall. Branches are slender, almost pendulous. The phyllodes are distinctly triangular and a dull blue-green colour on both sides. Small, prolific clusters of golden-yellow inflorescences flower in early spring.
• A. retinodes AGM is a shrub or small tree to 6m with crowded, narrow, willow-like phyllodes that are grey-green. The fragrant inflorescences are pale yellow and flower year-round, but mainly in summer. Fast growing but short-lived, it is the most lime tolerant of the species.
• A. riceana is a shrub or graceful tree to 10m tall with slender, weeping shoots and pungent, dark green phyllodes. The pale yellow, drooping, spike-like inflorescences flower in the spring.
• A. rubida is a shrub or small tree to 12m tall with green to blue-green phyllodes that are often tinged red. Bipinnate leaves may persist with phyllodes. The light golden inflorescences flower in the spring.
• A. saligna is a bushy shrub or tree to 6m tall with green to grey-blue phyllodes. The golden yellow inflorescences flower in the spring. It is fast growing and has a tendency to sucker.
• A. verticillata is a thorny shrub or small tree to 9m tall with dark green to olive, needle-like phyllodes in whorls of six. The crushed root smells of garlic. The lemon-yellow, spike-like inflorescences flower in spring.