(Editor's Note: This article was originally published on March 13, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your questions.)
This first picture is of the Silver Wattle - Acacia dealbata. This is one of the loveliest of all the Acacias. I first met it, in full flower in a conservatory in England. Where I live now, it is a very common tree in the wild. In some areas dominating large areas of forest, it can grow to a height of 30 metres and is a magnificent sight in full flower, with such dense masses of its bright yellow flowers that the foliage is almost concealed. The Silver Wattle is known as Florist's Mimosa and is a popular cut-flower. The flowers have a heavenly scent.
Silver Wattle is popular in cultivation in and outside Australia. I have a couple growing in my garden and they are lovely all the year round, with their silvery-grey ferny foliage. They are fairly hardy growing above the snow-line in mountain areas. We have seen ours covered with snow when in full flower and it still set seed well that year.
The greatest range of Acacias is found in Australia, but there are a large number of species elsewhere in the world, including in particular Africa and the Americas.
Here are two Acacias from outside Australia, one each from Africa and the Americas.
These Acacias share with the Silver Wattle the feathery foliage. The leaf form which is common at least some of the time to all Acacias, consists of bipinnate leaves. Bipinnate means that each leaf is divided into separate leaflets (pinnae), either side of the main axis, and that these leaflets are again divided into even smaller leaflets. Here is a typical bipinnate Acacia leaf. This one is from a Silver Wattle in my garden.
Unlike their African relatives, very few of the Australian Acacias, which retain their bipinnate leaves have thorns.
Another feature of Acacia plants is that they have very varied flowering seasons. It is possible to find an Acacia in flower in Australia in almost every month of the year. The Silver Wattle shown above flowers in early springtime, August to September.
Here are some Australian Acacias with feathery bipinnate leaves and with a variety of flowering seasons.
Black Wattle - Acacia mearnsii is very similar to Silver Wattle in many regards, but has a number of marked differences. It's leaf is dark green, not silvery and the flowers are cream-coloured not brilliant yellow. This is a summer-flowering species and like some other summer-flowering Acacias, takes a whole year to develop its seed pods. The picture shows flowers and unripe seed pods on the same tree, photographed in my garden in November.
The very dark brown bark of the Black Wattle was perhaps the most important source of Tannin, when Australia was first settled and the species is still used for Tannin production today although mostly from trees growing outside Australia.
The Black Wattle is a tree growing to 15 metres. The Sunshine Wattle - Acacia terminalis is a winter flowering species, producing its cream to yellow, tipped with gold, flowers in April and May. The time of flowering, combined with its size, usually a shrub of under 2 metres, makes this an attractive shrub to grow in the garden Another ferny-leaved Acacia that is popular in cultivation is the West Wyalong Wattle - Acacia cardiophylla. Pictured here in my garden with a covering of snow, this Acacia is a shrub of up to 3 metres. It grows naturally in New South Wales and would not normally encounter snow, but it survived without any sign of stress.
Snow is very rare in our garden, falling perhaps once in 5 to 10 years!
Like the Silver Wattle this one flowers in August, in the early spring.
Leaves Replaced with Phyllodes
All the Acacias shown so far have ferny bipinnate leaves. In Australia however the majority of species only have these for their first few juvenile leaves. The ancestral Acacia must have had a bipinnate leaf, but some species have found this to be a disadvantage and have attempted to evolve back to a simple leaf. Evolution does not go backwards however and a new way of growing a leaf has to evolve. With many Acacias, the leaf stalk has been expanded into a flat leaf-like form, and the bipinnate leaf on the end gets smaller and then disappears altogether. The leaf stalk then takes over the role of the leaf and in this new form, it is called a phyllode. Phyllodes look to all intents and purposes like a simple leaf, except that they are expanded in the same plane as the stem whereas a true leaf is usually at right angles to the stem.
Here is a succession of pictures of a Blackwood tree - Acacia melanoxylon. The first picture shows a full bipinnate leaf. A blackwood has many such leaves and may grow to nearly 2 metres, before the transition to phyllodes takes place. The second picture shows the transition. The leaf stalk is expanded to a leaf-like shape, but there is still a reduced bipinnate leaf on the end of it. The third picture shows the foliage of a mature blackwood tree, with no bipinnate leaves at all, only phyllodes taking over the function of the leaves.
It can be an attractive garden tree, but it tends to grow root suckers and become an ever-spreading thicket rather than a single tree.
Many species perform the transition from leaves to phyllodes much earlier in the life of the plant. Here are seedlings of three different Acacias. In each case, there are a few bipinnate leaves, and then the rapid transition to the adult leaf-form, the phyllodes, although these are differently shaped in the three species. The first is the Myrtle Wattle - Acacia myrtifolia, the second is the Golden Wattle - Acacia pycnantha and the third is the Varnish Wattle - Acacia verniciflua.The Myrtle Wattle - Acacia myrtifolia is one of the smaller shrubs. It is quite variable. The form in Tasmania only grows to 1 metre, usually less, and is an attractive little shrub to grow in a rockery. Here in Victoria it is a shrub of up to 3 metres, a nice garden shrub but rather too much for a rockery.
The Tasmanian form has reddish stems and a suggestion of a red outline to the leaf. It has fewer flowers to a cluster than most wattles, having only about 5 flowers, but when fully open they still appear as little creamy-yellow spheres. The Golden Wattle - Acacia pycnantha is the floral emblem of Australia, emphasising the important place that the Acacias have in the Australian flora.
It is a large shrub or small tree, growing to 8 metres high. It is a magnificent sight when in full flower, covered with golden-yellow masses of flowers and has a wonderful scent.
This picture was taken in my garden. The Varnish Wattle - Acacia verniciflua is a shrub to small tree. It can grow to 8 metres high, but is more usually 2 to 3 metres.
The cream-coloured flowers are not in dense masses like some of the wattles, but it is still an attractive sight when in flower.
The leaves are coated with a sticky varnish-like substance as I well know from having collected seed from these plants for a revegetation project. Fingers end up very sticky indeed!
The leaves were apparently used by the Ganai people as a fish poison.
The Grey Mulga - Acacia brachybotrya, shows more variation in the phyllodes. These are much shorter with rounded ends, and they have an attractive glaucous colour.
The Bower Wattle - Acacia cognata is particularly graceful species, with slender phyllodes and generally a weeping form. It is ahrub to small tree and can grow to 10 metres. I have a lovely little tree in my garden, but I also have a very different form of this species. The left-hand image below shows the normal Bower Wattle. The right-hand picture shows a very special variety known as 'Limelight' also growing in my garden. It is a dwarf compact little shrub growing to just 1 metre and grown for its foliage makes a very attractive addition to the garden.
This species grows naturally in a fairly dry area and is therefore a drought-tolerant species.
It is a large shrub, growing up to 4 metres and is an attractive shrub for a large garden.
All the Acacias we have looked at so far have the flowers arranged in dense globular heads. This is far the commonest arrangement in the genus, but there are species that have the flowers in elongated heads or spikes, some dense, but others more disperse.The Narrow-leaf Wattle - Acacia mucronata, is wattle that has its flowers arranged in spikes, fairly compact, but the individual flowers are more readily distinguished than in the globular heads of most Acacias.
Its long slender phyllodes, make this an attractive shrub all year round, although it is rather large for many gardens, growing to 5 metres. Wattle phyllodes can show a great variety of shapes as well as sizes and colours. The Ovens Wattle - Acacia pravissima, has almost triangular phyllodes, with a hint of a spike on the outer corner, shown here at 2.5 times life size.
This is another large shrub capable of growing to 8 metres. It is particularly attractive in flower, covered with masses of rich golden-yellow flowers.
The Hedge Wattle - Acacia paradoxa is one of the prickly Acacias and its spines like those on the overseas species are stipules, which means that a spine develops at the base of each leaf (phyllode), but the phyllode itself is not spiny.
Acacias with Spines
All the Australian Acacias shown so far have been free of prickles unlike their overseas cousins, but there are Australian species that can be very prickly and there are two ways in which spines can develop.
The name Hedge Wattle probably implies that the spines together with the dense habit of this shrub, make it a formidable barrier and therefore a useful hedging species.
It is a dense spreading shrub, usually up to 2 metres high and is very colourful when in flower. The Spreading Wattle - Acacia genistifolia, also has spines, but this time it is the phyllodes themselves that are stiff and slender and end in a hard sharp point. There are many wattles that have spiny foliage like this and attractive as they can still be in flower, it makes them less popular as a garden plant.
The Spreading Wattle has cream flowers and grows to about 2 metres. Another very spiky wattle, Acacia verticillata is known as Prickly Moses or Prickly Mo, a corruption of Prickly Mimosa. Again the phyllodes are stiff little prickles, and there are a lot of them, they grow in whorls of 4 to 6. I have several times collected seed of Prickly Mo for revegetation works and it is a job where gloves are needed!
Usually only growing to 2 metres, in wet conditions this shrub can become a small tree to 9 metres. It has flowers in dense oval spikes rather than the spherical heads of most wattles.
Two more spikey wattles are small compact shrubs and despite their spines make quite attractive rockery shrubs. Pictured below, on the left is the Juniper Wattle - Acacia ulicifolia, which can grow a little above 1 metre and on the right the very similar Heath Wattle - Acacia brownii, which is usually less than a metre tall.One wattle species, known as the Winged Wattle - Acacia alata, has its phyllodes almost missing altogether. There is a continuous wing along both sides of the stem and the phyllodes are left as a protruding spiky angle along the wing.
As a low shrub, usually under 2 metres, its very unusual form makes it popular as an oddity for the garden
The Exception that Proves the Rule!
They took cuttings to the Royal Botanic Gardens in Melbourne, who successfully propagated it and it is now available in nurseries. It can only be propagated by cuttings, as seed grown plants revert to the normal pale yellow flowers.
This delightful cultivar has been given the name 'Scarlet Blaze' and it makes an attractive garden shrub. The image displayed is extracted from the image in PlantFiles contributed by AusDigi.