Acacia

Plant Characteristics
Lifespan:	⌛	perennial

Cultivation

Features:	✓	evergreen, deciduous

Scientific Names
Fabaceae >	Acacia >

Acacia (pronounced /əˈkeɪʃə/) is a genus of shrubs and trees belonging to the family Fabaceae. Acacias are also known as thorntrees, whistling thorns or wattles, including the yellow-fever acacia and umbrella acacias.

Until 2005, there were thought to be roughly 1300 species of acacia worldwide, about 960 of them native to Australia, with the remainder spread around the tropical to warm-temperate regions of both hemispheres, including Europe, Africa, southern Asia, and the Americas. However, the genus was then divided into five, with the name Acacia retained for the Australian species, and most of the species outside Australia divided into Vachellia and Senegalia.

Acacias are also known as thorntrees or wattles, including the yellow-fever acacia and umbrella acacias.

A few species are widely grown as ornamentals in gardens; the most popular perhaps is Acacia dealbata (Silver Wattle), with its attractive glaucous to silvery leaves and bright yellow flowers; it is erroneously known as "mimosa" in some areas where it is cultivated, through confusion with the related genus Mimosa.

The leaves of acacias are compound pinnate in general. In some species, however, more especially in the Australian and Pacific islands species, the leaflets are suppressed, and the leaf-stalks (petioles) become vertically flattened, and serve the purpose of leaves. These are known as phyllodes. The vertical orientation of the phyllodes protects them from intense sunlight, as with their edges towards the sky and earth they do not intercept light so fully as horizontally placed leaves. A few species (such as Acacia glaucoptera) lack leaves or phyllodes altogether, but possess instead cladodes, modified leaf-like photosynthetic stems functioning as leaves.

The small flowers have five very small petals, almost hidden by the long stamens, and are arranged in dense globular or cylindrical clusters; they are yellow or cream-colored in most species, whitish in some, even purple (Acacia purpureapetala) or red (Acacia leprosa Scarlet Blaze). Acacia flowers can be distinguished from those of a large related genus, Albizia, by their stamens which are not joined at the base. Also, unlike individual Mimosa flowers, those of Acacia have more than 10 stamens..^[1]

The plants often bear spines, especially those species growing in arid regions. These sometimes represent branches which have become short, hard and pungent, or sometimes leaf-stipules. Acacia armata is the Kangaroo-thorn of Australia and Acacia erioloba is the Camelthorn of Africa.

Read about Acacia in the Standard Cyclopedia of Horticulture
Acacia (from word meaning a point or thorn, referring to the stipules often spinescent). Leguminosae, tribe Mimoseae. Trees or shrubs grown out-of-doors in warmer parts of the United States and some of the species as cool greenhouse plants for the showy yellow bloom. Leaves bipinnate or reduced to phyllodia with vertical edges (i.e., lf.-like petioles): fls. regular, orange-yellow, occasionally lemon-yellow or white, in cylindrical spikes or globular heads, solitary, or in pairs or clusters, or in axillary racemes; sepals and petals 5, 4 or 3, free or united; stamens many, long: pod a legume, opening by two valves (occasionally indehiscent): funicle of the seed filiform or ending in club-shaped aril, either twice encircling the seed or simply bent back upon itself. (The difference between Acacia and Albizzia lies in the stamens, which are free in the former and united at base in the latter.) A very large genus (said to be 450 species) dispersed throughout the tropical parts of the earth and even pushing their way into parts of the temperate zones. The phyllodine series is confined almost entirely to Australia and the Pacific Isls., while the bipinnate series is scattered over the warm parts of the remainder of the globe. The number of species reported from Amer. is large (about 70), of which at least 30 are Mexican. Acacias are said to be natives of the following states: Ariz., Ark., Calif., Fla., Okla., Kan., La.. Mo., Nev., New Mex. and Texas. Those in the following account are Australian, unless otherwise stated. This list will undoubtedly be modified as botanists segregate the other genera from the Mimoseae group. Acacias vary greatly under cult., the variation affecting infl., size and shape of lf., and even the funicle. One should not expect to determine an unfamiliar species without lvs., fls. and fr., with its seeds in place. Other species interesting because of their ant-inhabited thorns are described under Bull-horn Acacias. Other species referred to Acacia are to be found under Albizzia. Acacias are quick-growing plants and are short-lived. Various kinds have been known to grow from 11 to 12 feet in four months and 25 feet in six years. These trees are thus in their full maturity at thirty years of age, and shortly afterward begin to deteriorate. While they may thus be used as street trees to secure immediate effects, more permanent trees should also be planted to take their places. The leaves of some species are used in cookery, the flowers of A. Farnesiana for perfumery, the bark of various species for tanning; A. Senegal furnishes most of the gum arabic of commerce; a drug or medicine is made from the wood of A. Catechu; a soap or hair-wash from A. concinna; several are used as forage plants, others for dyes, and still others for fiber. Many of them have scented wood, others make fine furniture wood, and are used for cabinetwork and fence-posts or fuel, still others for street trees (as A. melanoxylon and A. dealbata), and all are more or less ornamental. The Ark of the Covenant, as well as the furniture of the Tabernacle, are said to have been made from timber of A. Seyal, which yields the Shittim wood of the Bible. Also on account of its incorruptible wood, this species for ages was used by the Egyptians to make coffins for the burial of their kings. ^CH The above text is from the Standard Cyclopedia of Horticulture. It may be out of date, but still contains valuable and interesting information which can be incorporated into the remainder of the article. Click on "Collapse" in the header to hide this text.

Cultivation

Read about Acacia in the Standard Cyclopedia of Horticulture
Acacias out-of-doors will not endure much frost. They seem to thrive very well in localities in which the winter temperature is as low as 20° F., or even 18°, and it is to be hoped that by careful selection strains may be secured that will withstand an even lower temperature. Some species are alkali-resistant as A. cyclops, A. retinodes and perhaps others. After the trees are once well established they do not ordinarily require further irrigation since they have a faculty of seeking water. However, if they are given plenty of water and good soil their growth is very rapid. Weevils sometimes ruin acacia seeds by laying their eggs in the flower-buds and appearing later in the pod. The cottony cushion scale and the black scale are also found to a limited extent, but so far have not proved troublesome. Cultivation in greenhouses as florists' plants is confined to few species, perhaps not more than a dozen being commercially valuable. All of this most important section thrive in a winter temperature ranging from 40° to 50°; in fact, little above the freezing point is sufficient. They do not like heat, and consequently are not adapted for forcing. If wintered cool and allowed to come along naturally with the increasing heat and light of the spring, they will flower in March and April, a season when their graceful beauty is appreciated in the private conservatory or is valuable to the commercial florist. The prevailing color of all the Australian species is yellow, varying from pale lemon to deep orange. The tall-growing kinds, or rather those inclined to make long, straight shoots, make excellent subjects for planting permanently against a glass partition of a conservatory, or against a pillar. There is scarcely a more beautiful plant than A. pubescens, with its slightly drooping, yellow racemes. It deserves a favored place in every cool conservatory. The acacias are of easy culture. If planted permanently in the border, provision for drainage should be made. A good, coarse, turfy loam, of not too heavy texture, is all they want, with the addition of a fifth part of leaf-mold or well-rotted spent hops. Few of our greenhouse pests trouble them. Water in abundance they like at all times, and in their growing season, which is the early summer months, a daily syringing is necessary. Several of the species of bushy habit are very much grown as pot-plants in Europe, and are now largely imported and sold for the eastern trade. A. armata and A. Drummondii are good species for this purpose. With our hot summers, the commercial man will do better to import than to attempt to grow them from cuttings. The acacias need pruning, or they will soon grow straggling and unshapely; more especially is this true of those grown in pots. After flowering, cut back the leading shoots rather severely. Shift into a larger pot if roots demand it, and encourage growth by a genial heat and syringing, giving at same time abundance of light and air. They should be plunged out-of-doors as soon as danger of frost is past, and removed to the greenhouse before any danger of early fall frosts. Cuttings root surely but not quickly. The best material is the side shoots from a main stem in the condition that florists call half-ripened —that is, not green and succulent as for a verbena, nor as firm and hard as the wood of a hybrid perpetual rose in November. The wood or shoot will be in about the right condition in June. No bottom heat is needed, but the cuttings should be covered with a close frame and kept moderately moist and cool by shading. The following spring these young plants can be either planted out-of-doors, where there is a good chance to keep them well watered, or grown on in pots, as described above. A few of the finest species are A. pubescens, suitable for training on pillars; A. Riceana makes a bush or can be trained; A. longifolia, an erect species, deserves a permanent position in the greenhouse border. Of all the species best adapted for medium-sized, compact pot-plants, A. armata and A. Drummondii are the best. The former has small, simple, dark green leaves and globular, pure yellow flowers. A. Drummondii has drooping, cylindrical, pale lemon flowers. As both these flower in March without any forcing in our northern greenhouses, they are very valuable acquisitions to our Easter plants. The acacia has two distinctive charms: the foliage is either small, simple and glaucous, as in A. armata, or much divided, graceful and fern-like, as in A. pubescent. All the acacias are among the freest-flowering of our hard-wooded plants. ^CH The above text is from the Standard Cyclopedia of Horticulture. It may be out of date, but still contains valuable and interesting information which can be incorporated into the remainder of the article. Click on "Collapse" in the header to hide this text.

Propagation

Acacia seeds can be difficult to germinate. Research has found that immersing the seeds in various temperatures (usually around 80 °C) and manual seed coat chipping can improve yields to approximately 80 percent.^[2]

Read about Acacia in the Standard Cyclopedia of Horticulture
Propagation is by seeds sown either under glass or out in the open ground, or by cuttings from half-ripened wood, taken with a heel. Seeds may be prepared for planting in two ways: First, place them in ashes among the dying embers of a fire and allow them to remain until cool. These do not require immediate sowing, but if they are sown they will not perish if rain does not fall very soon afterwards. Second, pour hot water over seed, let cool and soak from twelve to forty-eight hours. Sow without allowing seeds to become dry. Either method softens the hard seed-coats and hastens germination. They will then usually germinate in about seven days to three or four weeks, depending upon the species and the season in which they are sown. Seed may be sown in the propagating-house at any time throughout the year, though early spring is the natural time. For open ground, sow in March or April. After germination, the plants are pricked off into flats or pots and shifted into larger ones as occasion requires. They are thus kept in pots until they are ready to be transplanted to their permanent quarters, since if placed in the open ground at once the tap-roots will grow with too great rapidity and the tree will either have to be balled or transplanted with the greatest care to prevent its receiving a shock, from which it will take at least a year to recover. When buying seedlings from a nursery, therefore, reject all those whose roots have penetrated the pot. While several species (A. pycnantha, A. melanoxylon, A. decurrens var. dealbata, etc.) have been known to resow themselves in California, there is no danger of their becoming a pest (such as A. armata in Australia), since the seedlings are seldom able to live through the dry season without irrigation. Cuttings should be made from the half-ripened wood, of which the best are from the side shoots of the main stem, taken with a heel. No bottom heat is required, or very little, but they should be covered with a light frame and kept moist and cool by shading. They root slowly but freely and should be potted immediately after rooting, but should not be planted in the open soil until they have developed good roots. ^CH The above text is from the Standard Cyclopedia of Horticulture. It may be out of date, but still contains valuable and interesting information which can be incorporated into the remainder of the article. Click on "Collapse" in the header to hide this text.

Pests and diseases

In Australia, Acacia species are sometimes used as food plants by the larvae of hepialid moths of the genus Aenetus including A. ligniveren. These burrow horizontally into the trunk then vertically down. Other Lepidoptera larvae which have been recorded feeding on Acacia include Brown-tail, Endoclita malabaricus and Turnip Moth. The leaf-mining larvae of some bucculatricid moths also feed on Acacia: Bucculatrix agilis feeds exclusively on Acacia horrida and Bucculatrix flexuosa feeds exclusively on Acacia nilotica.

Species

There are over 1,300 species of Acacia. See List of Acacia species for a complete listing. The genus however is apparently not monophyletic. This discovery has led to the breaking up of Acacia into five new genera as discussed in list of Acacia species.