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 SYNONYMS:
kangaroothorn: prickly acacia, prickly wattle, hedge acacia, Acacia armata R.Br.
black acacia: blackwood acacia
GENERAL DESCRIPTION:
Acacias are grown as woody ornamentals and not typically weedy, but a few species have escaped cultivation in some coastal regions.
kangaroothorn: Bushy, spiny shrub to 3 m tall, 3.5 m wide. Noxious weed. Introduced from south and eastern Australia.
black acacia: Tree to 15 m tall, usually with a single trunk. Hard, dark wood makes high-quality lumber. Introduced from eastern Australia.
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SEEDLINGS:
Juvenile leaves even 2-pinnate compound.
kangaroothorn: No description available.
black acacia: Intermediate leaves resemble juvenile leaves, but with expanded petioles.
MATURE PLANT:
Leaves are actually simple, leaf-like phyllodes (expanded petioles). Phyllodes alternate, lanceolate to oblong, evergreen, leathery.
kangaroothorn: Stipules modified into spines ~ 1 cm long. Phyllodes light green, waxy, ~ 1-2(3) cm long, 2-8 mm wide, oblique, with wavy margins. Midvein off-center. New phyllodes covered with hairs, especially margins.
black acacia: Phyllodes 6-15 cm long, 1-3 cm wide, straight to sickle-shaped, sometimes broadest near the apex, smooth, dull dark green. Main veins 2-5, parallel. Young branches or branch tips often have juvenile or intermediate leaves.
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ROOTS and UNDERGROUND STRUCTURES:
Usually associated with nitrogen-fixing bacteria.
kangaroothorn: Taproot branched, with numerous lateral roots.
black acacia: Can produce new shoots from roots under certain conditions.
FLOWERS:
Round heads 8-10 mm diameter, consist of 30-50 flowers. Stamens separate, exserted, > 10 per flower. Petals, sepals inconspicuous.
kangaroothorn: February-April. Heads bright yellow, solitary on stalks in leaf axils. Heads and stalks about equal to the length of the phyllodes.
black acacia: March-June. Heads pale yellow to cream-colored, in axillary racemes of 2-8. Racemes shorter than the length of the phyllodes.
FRUITS and SEEDS:
Pods split open along both margins. Seeds attach to pod with a long, folded stalk.
kangaroothorn: Pods aligned, short-stalked, brown, straight or slightly curved, cylindrical, 4-7 cm long, ~ 0.5 cm wide, rounded over seeds, rarely narrowed between seeds, sparsely to densely covered with long, soft, white hairs. Seeds dark brown to black, oblong, 3-5 mm long, with the stalk folded 2-4 times and a small aril
(expanded end of seed stalk that partially covers seed).
black acacia: Pods in clusters, flattened, loosely to tightly spiraled, slightly constricted between seeds, pale to reddish-brown, leathery, 3-12 cm long, 0.4-1 cm wide, with thickened margins. Seeds oval, black, 3-5 mm long, with a long, flattened, crinkled bright pinkish to red stalk encircling the seed in a double-fold.
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HABITAT:
Disturbed areas, usually near cultivated plants.
DISTRIBUTION:
San Francisco Bay region, western South Coast Ranges, South Coast. To 200 m (650 ft).
kangaroothorn: Common. s Marin, w Solano, w Contra Costa, w Alameda, nw Santa Clara, e San Mateo, sw Santa Cruz, nw Monterey, se Santa Barbara, s Los Angeles, w San Diego.
black acacia: Uncommon. Also occurs on Santa Cruz Island.
PROPAGATION/PHENOLOGY:
Reproduce by seed. Seeds typically disperse near the parent plant or to greater distances by human activities or water. Seed is hard-coated and requires scarification or degradation to germinate. Seed is probably long-lived, but longevity is undocumented. Pods and seeds are not utilized by native wildlife.
kangaroothorn: Tolerates temperatures to – 7º C. Produces seed when young.
black acacia: In California, escaped populations appear to expand slowly from their origin. Fire stimulates germination. Reproduces vegetatively by shoots from roots in Hawaii.
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MANAGEMENT FAVORING/DISCOURAGING SURVIVAL:
kangaroothorn: Cutting stems just below ground level typically kills plants.
black acacia: Hand-pull seedlings before roots are well established. Cut trees appear resistant to most chemical treatments and often develop new shoots from the crown and possibly roots.
SIMILAR SPECIES:
There are at least 11 other Acacia species that occasionally escape cultivation in California. The following 4 species are more widespread or likely to be problematic. Unlike kangaroothorn and black acacia, they lack phyllodes and have even 2-pinnate compound leaves with more than 2 pairs of pinnae (first divisions of
2-pinnate leaves) at maturity. Flowers are in bright yellow heads and spines are absent, except where noted. For a quick comparison, refer to the table Comparison of selected Acacia species. In addition, the mesquites [Prosopis spp.] are distinguished by having 10 stamens per flower and 1-2 pairs of pinnae.
Cootamundra wattle [Acacia baileyana F. Muell.]: Tree, with 3-6 pairs of pinnae and silverish blue leaflets 5-7 mm long that touch at the margins. Roadsides, disturbed areas. San Francisco Bay region, western South Coast Ranges, western Transverse Ranges. To 300 m (1000 ft).
Silver wattle [Acacia dealbata Link]: Tree to 12 m tall, with 8-25 pairs of overlapped pinnae and 20-70 pairs of overlapped leaflets 2-5 mm long. Twigs angled, but lack wing-like ridges. Leaflets and twigs covered with minute silvery hairs. Disturbed sites, roadsides. Western North Coast Ranges, San Francisco Bay region, western South
Coast Ranges, South Coast. To 500 m (1650 ft). Often confused with green wattle.
Green wattle [Acacia decurrens (Wendl.) Willd.]: Tree to 15 m tall, similar to silver wattle. Pairs of pinnae 5-15, not overlapped. Pairs leaflets 15-35, overlapped, +/- glabrous. Twigs with prominent wing-like ridges, +/- glabrous. Reports of naturalization unconfirmed.
Sweet acacia [Acacia farnesiana (L.) Willd. var. farnesiana]: Shrub, with straight, paired stipular spines that turn white over time and gold to dull orange flower heads. A worldwide, often troublesome weed. Chaparral in southern San Diego Co. To 300 m (1000 ft).
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CONTROL METHODS:
Prevention: Kangaroo thorn is a spreading, prickly shrub 2-3 m high and 3-4 m across. It is native to the temperate regions of Australia and has been used in ornamental and hedgerow plantings. It was likely introduced to California for the same purposes. Little is known regarding the biology of kangaroo thorn. Established plants
are long-lived and are able to tolerate drought and frost. Seed are produced in pods that burst open in dry, warm conditions. Seed may be spread by vehicles, humans, and possibly animals. In California, kangaroo thorn has spread by ornamental and hedgerow plantings. Kangaroo thorn should be replaced with native California shrubs
native to the infested areas.
Mechanical: Physical removal of plants has been an effective control method. Plants must be cut or pulled below the soil surface. This may be accomplished by chaining, dozing or sawing. To prevent seed dispersal, plants should be removed before fruiting pods are produced. There will likely be a seedling flush following removal of
mature plants. First year seedlings may be controlled with mowing before becoming established.
Biological: There are no registered biocontrol agents for kangaroo thorn. However, grazing with goats has been effective in controlling smaller plants. It is unknown whether seed viability is retained through animal digestive systems. Therefore, animals grazing in infested areas during pod production should be held prior to moving
to other areas.
Kangaroo thorn seedling growth is generally slow in the first year. The establishment of competitive vegetation will reduce but not eliminate seeding recruitment. Areas where physical removal has been utilized should be replanted with desired vegetation. Otherwise, reinvasion by kangaroo thorn or other weeds is highly probable.
Chemical: Where chemical control is an option, triclopyr and clopyralid have been effective for controlling kangaroo thorn. Both should be applied as a foliar treatment when plants are actively growing. An integrated approach of physical removal of large plants followed by a herbicide treatment of new seedlings is effective and
reduces competition for grass establishment.
References
Lane, D. and K. Turnbull. 1979. The significance of noxious weeds on roadsides in agricultural
areas of Victoria, Australia. Weed Research 19:151-156.
Parsons, W.T. and E.G. Cuthbertson. 1992. Noxious weeds of Australia. Inkata Press,
Melbourne and Sidney. pp. 439-440.
Source: Plant Health and Pest Prevention Services (CDFA)
Florists Directory
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