Silver Wattle

Acacia dealbata Link

Synonyms - Racosperma dealbatum, Racosperma dealbata.

Family: Fabaceae (was Mimosaceae)

Names:

Acacia was the name of a thorny Egyptian tree.
Wattle comes from British settlers making wattle and daub buildings using Callicoma serratifolia branches which was then called Black Wattle and is similar to the other Acacias of the area.
Dealbata refers to the whitish colour of the branchlets and foliage.
Silver Wattle because it has silvery leaves and branchlets and is a member of the Wattle or Acacia genus.

Other Names:

Wattle

Summary:

Silver Wattle (Acacia dealbata) is a large shrub or small tree, 4-30 m high with silvery, minutely hairy upper branchlets and grey to brown smooth bark that may become rough with age. The leaves are blue-grey to silvery when young and have tiny hairs. The main leaf axis is (10)40-120(170) mm long and has 6-30 pairs of branches, each of these side axes is divided into 10-68 pairs of small leaflets. The leaflets are (0.7)1.5-6 mm long and 0.4-1 mm wide and minutely hairy. The fragrant, pale yellow globular flower heads are 5 mm across and arranged in 10 to 40-headed sprays. The seed pods are flat, 5-10 cm long and 7-14 mm wide.
Native to New South Wales, Victoria and Tasmania, Silver Wattle has become a problem weed along roadsides and in Karri forest in the Porongurup Range. It flowers from July to November.

Description:

See the Weedy_Acacia_Key

Cotyledons:

Two.

First leaves:

Alternate. Many leaflets.

Leaves:

Compact, silvery to blue grey to dark green, alternate leaves with leaflets that have leaflets (bipinnate). The main axis is hairy, (10)40-120(170) mm long with 6-30 pairs of leaflets (pinnae) that are 25-40 mm long and these carry 10-68 pairs of sub leaflets (pinnules) that are (0.7)1.5-6 mm long x 0.4-1 mm wide, minutely hairy and set close together. There is 1 gland on the petiole and 1 at the base of each pinnae on the main axis (jugate)
Stipules - .
Petiole - Hairy. 1 gland.
Blade - of sub leaflets, silver grey green, 0.7-6 mm long x 0.4-1 mm wide, flat with parallel sides, straight, sparsely hairy and obtusely tipped. The mid nerve is difficult to see.
Young leaves are whitish yellow to whitish green.

Stems:

Branchlets are green to powdery silver grey, angular or flattened, usually hairy. Trunks are round and up to 15 m tall.
Bark is dark grey to almost black, smooth to deeply fissured.
Sapwood is diffuse porous.

Flower head:

Bright yellow and showy in long, branched sprays with 10-40 globular heads about 5 mm diameter with 20-40 small flowers. Borne at the ends of branches or from the leaf axils in a panicle like raceme. The axis is 60-100 mm long and the flower stalks (peduncles) are 4-6 mm long, whitish grey and hairy.

Flowers:

Golden yellow, sweetly scented, bisexual, small. Radially arranged floral segments of similar size (actinomorphic).
Ovary - Superior, usually stalkless, many ovules. Hairless.
Style - Threadlike.
Calyx - 5 short lobes, hairy.
Petals - 5. Free, prominent midrib, hairless.
Stamens - Many, free, stick out from the flower.
Anthers - All fertile, 2 celled, opening by a lengthwise slit.

Fruit:

Pod light purplish brown, oblong, almost straight, almost flat, with a whitish waxy bloom, 50-100 mm long x 7-14 mm wide, hairless, thickened edges and not constricted between the seeds. Sometime it has a white waxy bloom. Hairless. Seeds are lengthwise in the pod
Usually drops seed when ripe by opening 2 valves.

Seeds:

Usually somewhat flattened and oval, about 10 mm long. Funicle threadlike expanding near the seed into the aril.

Roots:

Taproot with many laterals. Suckers freely.

Key Characters:

Branchlets silvery grey, hoary pubescent, angled.
Leaves bipinnate, silver grey and hoary pubescent. More than 6 pairs of pinnae.
Mature pinnules less than 6 mm long and 1 mm wide, sparsely hairy and grey green.
Interjugary glands absent.
Jugary glands present
Ovary superior.
Stamen all free, more than 10, usually < 0.5 mm long, white, cream, yellow or orange yellow.
Filaments conspicuous.
Flower heads on extended axillary or terminal racemes or panicles.
Flower heads with 20-40 small flowers.
Flowers bisexual, actinomorphic, small, numerous, in a globular head or cylindrical spike.
Sepals more than 4 or 5.
Seeds in a pod.

Adapted from Nancy Burbidge, Flora of the ACT & Gwen Harden, Flora of NSW.

Biology:

Life cycle:

Perennial. Seedlings germinate in disturbed areas and grow quickly. In 2-5 years they flower. If damaged they will produce suckers profusely from the trunk and roots up to several metres away. Over time a thick mulch of seed pods is accumulated under the trees. Once they reach about 10 m tall the trunk breaks, usually within a metre of the base and the fallen trunk sends up vertical branches and forms roots where it touches the ground.

Physiology:

Hardy and fast growing.

Reproduction:

By seed, suckers, coppicing, layering and regrowth from trunks of fallen trees.

Flowering times:

Late winter and spring in WA.
July to November in NSW.
Spring in Western NSW.
Late winter to spring in SE Australia.

Seed Biology and Germination:

Seed is often dormant and may require fire for full germination.

Vegetative Propagules:

Hybrids:

At least 2 forms or sub species. A high altitude, high rainfall form has smaller leaves, fewer heads and a shorter flowering structure.
A cultivar, Acacia dealbata “Kambah Karpet” is a prostrate ground cover.

Allelopathy:

Population Dynamics and Dispersal:

Spread by seed, root suckers and regrowth from fallen trunks . Regenerates quickly after fire. Larger trees often break near the base, smothering or breaking surrounding vegetation and then grow from the fallen trunk. The prolific production of pods builds up a thick mulch which prevents seedlings of most species establishing. This can produce thickets where little else survives. Seed is spread by birds, water flows and soil movement.

Origin and History:

Australia. Native to New South Wales, Victoria and Tasmania.
Introduced in South Australia. Escaped from gardens in other states.

Distribution:

ACT, NSW, SA, TAS, VIC, WA.
Main infestations in the Adelaide Hills in SA and the Porongurups in WA.
Occurs in the Jarrah forest, Swan coastal plain and Warren areas in WA.

Courtesy Australia's Virtual Herbarium.

Habitats:

Climate:

Prefers higher rainfall areas.

Soil:

Found on many soil types. Often on creek banks or in valleys. Often on clayey soils, loams, granitic soils and stony slopes.

Plant Associations:

Dry sclerophyll forests. River Red Gum. Jarrah and Karri forests.

Significance:

Beneficial:

Ornamentals, gums, timber, fuel, pollen, fibre and pulp.
Produces large amounts of pollen. A relatively poor source of tanning bark compared to other Acacias.

Detrimental:

Weed of bushland and roadsides. Especially around the Porongorup area in WA where it forms thickets excluding all other species. It forms many suckers if cut down or if the roots are damaged. Large trees snap off near the base and the fallen trunk forms roots underneath and new stems above invading the surrounding vegetation. It regenerates very well after fire often forming fire induced thickets.

Toxicity:

Generally, Acacias cause few problems to stock grazing amongst them.

Symptoms:

Treatment:

Remove stock if ill health due to grazing is suspected.

Legislation:

None.

Management and Control:

Picloram, Garlon® and glyphosate are used for chemical control as overall sprays, stem injection, cut stump or topical application to trunk.
Use Access® in diesel at 1:60 and paint or spray the lower 50 cm of trunk. Make sure the herbicide is applied all the way around the trunk or the untreated side of the tree may survive.
Use Garlon® 480 at 1:400 or Roundup® CT at 1:200 as an overall spray for control of juvenile trees and 2 L/ha Garlon® 480 for control of seedlings.
2 L/ha of Roundup® (360 g/L) controls seedlings in autumn, winter and spring but 4 L/ha was required for late summer applications.
A long term control plan is usually required for success.
Target areas that have been recently burnt because these will be more prone to invasion by regenerating trees, suckers and seedlings. The seed bank will also be reduced due to the fire induced germination of seed.

Thresholds:

Eradication strategies:

1)Apply herbicides in spring.
2) For mature or juvenile trees, apply a mixture of 1 L of Access® in 60 L of diesel to the lower 500 mm of the trunk or inject the stems with 1 mL Tordon® Timber Control herbicide per 1.5 metres of height. Repeat in 6 months if necessary.
3) For seedlings, apply 4 L/ha of glyphosate (450 g/L) or spray until just wet with a mixture of 100 mL of glyphosate (450 g/L) per 10 L of water. A concentrated mix of 1 part glyphosate with 2 parts water can also be applied using a window washer bottle. Apply about 3 mL of this solution per square metre of foliage. Lontrel®750 at 2 kg/ha may provide more selective control in some situations.
4) For juvenile trees, spray a mixture of 100 mL of glyphosate (450 g/L) plus 25 mL Pulse® Penetrant per 10 L of water onto the foliage until just wet.
5) Avoid burning or denuding the area as this will encourage seedling establishment.
6) Repeat treatment every second year to ensure that no trees reach an age where they can set seed.
7) Replant area with endemic seedlings.

Fire may be used to encourage seed to germinate but doesn't provide good control.
Bulldozing, felling and fire usually lead to greater infestations because they tend to sucker and coppice. Herbicides usually provide better control. If trees are cut down then use glyphosate, Garlon®, Grazon® Access® or Tordon® Timber Control to paint the stumps immediately after felling or inject herbicide into the trunk.
A large number of seedlings often emerge in the season after felling, burning or spraying. If these are left the infestation may become worse. Follow up every 2-3 years to ensure no trees reach an age where they set seed. If glyphosate is not providing good control then try Garlon®, Grazon®, Hotshot®, Starane® or clopyralid(300g/L). Test various times of treatment in your area. Avoid further burning or denuding of the area as this will encourage seedling establishment.
Don't buy or plant them in gardens outside their native range in New South Wales, Victoria and Tasmania.

Herbicide resistance:

None reported.

Biological Control:

Unlikely because it is an Australian native species.

Related plants:

See the Weedy_Acacia_Key
There are more than 500 native Acacia species in WA.
Weedy and look-alike species include
Acacia Hedge (Acacia paradoxa). Noxious weed.
Black Wattle (Acacia decurrens) Bark used for tanning.
Blackwood (Acacia melanoxylon) Bark used in tanning.
Blakely's Wattle (Acacia blakelyi) Used in revegetation.
Brigalow (Acacia harpophylla)
Burrow's Wattle (Acacia burrowii)
Caterpillar Wattle (Acacia lasiocalyx) Used in revegetation.
Cedar Wattle (Acacia elata) Ornamental
Chisholm's Wattle (Acacia chisholmii)
Coast Myall (Acacia binervia) is toxic to stock.
Cootamundra Wattle (Acacia baileyana) Ornamental
Curracabah (Acacia concurrens)
Currawong (Acacia sparsiflora)
Cutch Tree (Acacia cutechu) Noxious weed.

Deane's Wattle (Acacia deanei)
Dwarf Silver Wattle (Acacia nano-dealbata)
Flinders Ranges Wattle (Acacia iteaphylla)
Gidgee (Acacia cambagei)
Georgina Gidgee (Acacia georginae) is toxic to stock.
Golden Wattle (Acacia pycnantha) Bark used in tanning.
Golden-wreath Wattle (Acacia saligna) Used in revegetation.
Gosford Wattle (Acacia prominens) Ornamental
Green Wattle (Acacia mearnsii) Bark used in tanning.
Hop Mulga (Acacia craspedocarpa)
Manna Wattle (Acacia microbotrya) Used in revegetation.
Mimosa bush (Acacia farnesiana was Vachellia farnesiana)
Motherumbah (Acacia cheelii)
Mountain Cedar Wattle (Acacia elata)
Mulga (Acacia aneura) Used for fodder.
Prickly Acacia (Acacia nilotica). Noxious weed.
Prickly Moses (Acacia pulchella)
Queensland Silver Wattle (Acacia podalyriifolia)
Red Wattle (Acacia sylvestris)
Silver Wattle (Acacia dealbata)
Snowy River Wattle (Acacia boormanii) Ornamental
Sweet Wattle (Acacia suaveolens)
Sydney Golden Wattle (Acacia longifolia)
White Sally (Acacia floribunda)
Acacia glaucescens is toxic to stock.
Acacia jucunda
Racosperma species.

Plants of similar appearance:

Other bipinnate Wattles;
Albizia (Paraserianthes lophantha) differs in its darker green foliage with larger leaflets at 5-10 mm long x 1.5-3 mm wide, and also in its much larger cylindric flower heads that are 3-6 cm long.
Green Wattle (Acacia mearnsii) is very similar but has golden hairy flower head stalks (peduncles) and interjugary gland on the main axis (rachis).
Dwarf Silver Wattle (Acacia nano-dealbata) is very similar but smaller with smaller pinnules.
Red Wattle (Acacia silvestris) is very similar but has interjugary glands, low lying hairs and larger pinnules.
Karri Wattle (Acacia pentadenia) is similar but has dark green foliage, 2-4 globular flower heads in each spray, young growth is green rather than silvery and pods narrower at 3-4 mm wide.
Cootamundra Wattle (Acacia baileyana) has blue-green leaves that are much smaller.

References:

Bodkin, F. (1986). Encyclopaedia Botanica. (Angus and Robertson, Australia).

Burbidge, N.T. and Gray, M. (1970). Flora of the Australian Capital Territory. (Australian National University Press, Canberra). P204. Diagram.

Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P359. Photo.

Everist, S.L. (1974). Poisonous Plants of Australia. (Angus and Robertson, Sydney).

Harden, Gwen J. (1991). Flora of NSW. (Royal Botanic Gardens, Sydney). P391. Diagram.

Hussey, B.M.J., Keighery, G.J., Cousens, R.D., Dodd, J. and Lloyd, S.G. (1997). Western Weeds. A guide to the weeds of Western Australia. (Plant Protection Society of Western Australia, Perth, Western Australia). P178. Photo.

Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).

Lazarides, M. and Cowley, K. and Hohnen, P. (1997). CSIRO handbook of Australian Weeds. (CSIRO, Melbourne). #3.7.

Moore, J.H. and Wheeler, J.R. (2008). Southern Weeds and their Control. (Second Edition). Department of Agriculture and Food Western Australia. P163. Photos.

Paczkowska, G. and Chapman, A. (2000). The Western Australia flora: a descriptive catalogue. (Wildflower Society of Western Australia (Inc), the Western Australian Herbarium, CALM and the Botanic Gardens & Parks Authority). P304.

Tame, T.(1992). Acacias of Southeast Australia. Kangaroo Press, Kenthurst, NSW, Australia. P190. Diagram.

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.