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What is an Acacia?

Pronounced A'kay-sha, there is something mildly amusing about the name: maybe it's due to “Acacia Avenue” being the clichéd middle-class suburban address for retired colonels, writing complaint letters to their local paper? As always, there is some basis of reality in the joke, as there are over sixty Acacia Avenues in the UK, and they are mostly on suburban housing estates.

So what are Acacias? They are medium-sized trees, mostly deciduous, and variously known as Wattles, Thorntrees, or Mimosa. They fall roughly into two divisions: the majority of species are from Australasia and are thornless, whereas those from the African continent have serious thorns, which developed to protect them from grazing animals.

Most of the Acacias have finely divided foliage, but some have, instead of leaves, interesting structures which are called phyllodes: this is where the leaf stalk, or petiole, is flattened and widened and undertakes all the usual leaf activities of transpiration and photosynthesis.

In warmer parts of the UK, such as London, and the south coast, Acacias are popular street and ornamental trees, as they don't grow very large: most species only live for 20-30 years, and as well as very attractive foliage, they have spectacular flowers: each individual flower is small, but they are presented in dense inflorescences that may be either globular heads, or cylindrical spikes.

Why are they called Acacia?
The botanical name of Acacia comes from the Green word akakia which means “thorns” and clearly is most appropriate for the African species. There has been a movement in Australasia to reclassify their thornless Acacia as “Racosperma” but it has proved to be a bit of a political hot potato, so for now the whole genus remains in Acacia, whether they have thorns or not.

What are Acacia used for?
The wood is hard, strong and durable, and was widely used in ship-building in the past. Today, acacia timber is mostly used for the production of floorings, furniture, toys, jewellery and tools.

Acacias are renowned for producing sap which is processed into a natural gum, known as Gum Arabic, which has been used for over 2,000 years as glue, as a thickener in paints and watercolours, in cosmetics and in the food industry. It even has its own E number, which is E414! Originally, Gum Arabic was made from the hardened sap of Acacia nilotica (which is also known as Vachellia nilotica) but these days, is more likely to be extracted from Acacia senegal, and indeed this species is often called simply Gum Arabic.

As a plant, quite apart from the ornamental appeal of the delicate leaves and the colourful, fragrant flowers, certain species of Acacia have been used as windbreaks and as stabilising planting for the shifting sands of the sea coasts, in areas as far apart as Africa and California. Acacia saligna in particular is a rampant coloniser, seeding generously and germinating easily, such that hundreds of seedlings can appear below a parent plant.

Being part of the legume family, the seeds of many Acacia species have been harvested for centuries, both for animal fodder, and for human consumption - the seeds are high in protein and store well for long periods. When ground into flour, they can be eaten as a paste, or baked into cake or other foodstuffs.

Black wattle (Acacia mearnsii) bark is particularly rich in tannin, and is still used in the tanning industry to this day: this particular species is now cultivated in many countries, notably being grown in plantations in South Africa, having been exported from its native Australia.

The Acacia in popular culture
Acacia pycnantha (yes, that is the correct spelling - pronounced pike-nan-ther), the Golden Wattle, is Australia's national flower - they even have their own day, Wattle Day, which is celebrated each year on the first of September.

Not exactly popular culture, but going back to the Bible, it is thought that Acacia seyal is the “shittah” tree, which was used in the construction of the Ark of the Covenant. It is not possible to prove or disprove this, but certainly the timber of several Acacias is fragrant as well as hard, and beautiful, and is often used for ornamental purposes, so it is entirely possible that an important storage vessel would use such a wood.

Spanning the gap between ancient and modern, Acacia is an enduring Freemason icon, symbolising the immortality of the soul. It is also thought to symbolise innocence, as the original Greek name akakia also has the meaning of innocence, or being free from sin.

How do you know it's an Acacia?
Most Acacias have untoothed bi-pinnate leaves: this means that each leaf has a long central stalk or rachis (pronounced ray-kiss), from which project a large number of leaflets, rather like the oars in a galley. Each of these leaflets in turn has a central secondary rachis or rachii (presumably pronounced ray-kee-eye) with yet more leaflets upon it, giving the leaves a soft, feathery appearance. And if you look very closely at each leaflet, you will see that there are no teeth on the leaf edge: the margin is “entire”, or untoothed.

In early spring, they bear fragrant yellow pom-pom flowers in dense clusters, and later in the year these mature into flat pea-like pods, which is not surprising as they are part of the Leguminosae or Pea family.

The most frequently-planted (and hardiest) species is Acacia dealbata, which was reputedly brought into the UK from Australia by florists: it has no thorns at all - in fact, all parts are soft and fluffy, making it a very attractive, if not fully hardy, small tree.

So what about all those other Acacia lookalikes?
There are 48 plant records in our PlantFinder for Acacia, spread over three genera: Acacia, Robinia, and Albizia. As a broad generalisation, you can tell them apart by the shape of the flowers: Acacia have small, fluffy-looking pom-pom flowers, grouped together densely; Robinia have pea-like flowers in loose tumbling racemes; and Albizia have fine, silky-looking flowers.

Robinia are often known as False Acacia, and the main species is Robinia pseudoacacia, whose name really speaks for itself! It is also known as Black Locust, which creates confusion with another pinnate-leaved, spiny tree called Honey Locust, the proper name of which is Gleditsia triacanthos - these trees have truly wicked spines, but despite them, we have over a dozen cultivars of Gleditsia in our PlantFinder.

Getting back to Robinia, they have pinnate leaves with rounded leaflets, which are actually quite different from the finely divided, feather foliage of the true Acacia: in addition, most species of Robinia have what are called “paired thorns” at the buds: these are like rather large Rose thorns and are quite vicious. However, in recent years, thornless cultivars have been produced, specifically for street planting, and many of these include the cultivar name “inermis” which means “unarmed, defenceless” or - in our terms - thornless.

There are over a dozen cultivars of Robinia in our PlantFinder, including Robinia × slavinii ‘Hillieri’ or the Rose Acacia tree, whose pea-like flowers are an unusual dark pink.

Albizia are commonly known as Persian acacia, and they share the fine feathery untoothed bi-pinnate foliage of the true Acacia, but with the added advantage of showy, fluffy, flowers - usually pink - whose long fine stamens make them look like tassels of silk, hence the other common name of Silk Tree. The most popular species is Albizia julibrissin, a small deciduous tree, and this includes the cultivars Albizia julibrissin ‘Boubri ' with deep pink flowers, and Albizia julibrissin var. rosea, whose flowers seem to have started out white, but have been partly dipped in pink paint.

How to grow Acacia
Most Acacias are evergreen, but they are not fully hardy in the UK: so unless you have a sheltered garden, they are best grown in pots which can be moved out to a sunny deck or patio for the summer, then brought inside for the colder months, where the early flowers - appearing in February or March - can be enjoyed to the full.

However, if you have a sheltered spot, it may be worth trying to grow one of these trees - they need a lot of sun, and a well-drained, light soil as their roots won't enjoy a cold wet winter. If your tree should be caught by the frost, just cut it off at ground level and it will regrow in a shrubby form, which at least has the advantage of bringing the scented flowers down to nose level. So why are there so many Acacia Avenues, if the trees are not hardy enough to withstand the UK winters? Well, maybe they show the optimism of the town planners - or maybe they were looking ahead to a point where global warming allows them to flourish here!

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