Parkinsonia aculeata L.
Family: Fabaceae (was Caesalpiniaceae or Caesalpinioideae)
Parkinsonia recognises the English botanist, John Parkinson (1567-1650).
Aculeata is Latin meaning provided with thorns and refers to the thorny habit.
Other names:Barbados Flower Fence
Blue Palo Verde
Horse Bean Tree
Mexican Palo Verde
Summary:A hairless, spiny perennial shrub or tree with smooth green bark and zigzagged drooping branches. It grows up to 20 m tall (usually 5-7 m tall) or can have a shrub form in drier areas and form dense thickets. It has distinctive, pendulous pinnate leaves with up to 100 pairs of leaflets that fall off easily leaving the main axis. Clusters of yellow flowers are produced in the leaf axils.
Leaves:Very sharp spine, 5-15 mm long where the leaf joins the stem.
Oblong or parallel sided in outline. Divided twice (bipinnate).
The leaf consists of a long, flattened central axis that is 200-400 mm long and 2-3 mm wide and ends in a short spine. This carries 1-4 pairs of primary leaflets. On each of side of these leaflets, up to 100 pairs of oblong secondary leaflets that are 4-10 mm long occur. The primary and secondary leaflets fall off easily leaving the main axis which is green, flattened, ridged and 200-400 mm long. Stiff, very sharp spines, 5-15 mm long are formed at the base of each leaf and these persist on older branches.
Petiole - short.
Stems: Up to 20,000 mm tall with trunks up to 400 mm thick but usually much smaller at 5000-7000 mm tall. Smooth green bark, branched with stiff spines up to 12 mm long on branches. Branches often slender, zig zag and drooping. Single or multi-stemmed with steeply ascending main branches.
Flower head:Loose spike like racemes, 75-150 mm long of 8-12 flowers arising from leaf axils. Pods at the base of the infructescence mature first.
Flowers: Yellow and orange. 5 petalled. Fragrant. On a long drooping stalk. 20 mm diameter.
Petals - 5 petals of which 4 are yellow, 6-15 mm long and bent back and 1 is erect with orange spots or orange-yellow.
The flag petal is UV absorbent and is an important recognition cue for pollinating bees.
Fruit:A light brown pod, 30-150 mm long x 7 mm wide, cylindrical and tapered to a sharp point, on a short stalk. Constricted between seeds. 1-11 seeds per pod and typically 1-3 seeds with an average of 1.6. Pods are indehiscent and must decay or be physically damaged to release the seed. Pods have limited nutritional value.
Seeds:Oblong, dark green to brown or black. 8-12 mm long x 3-7 mm wide. Testa is green and soft when young and mottled brown and hard when mature. The seed weight is 85(56-103) mg in Australia and heavier (161 mg) in some American populations (van Klinken et al., 2009). The cotyledons are light yellow and the endosperm is translucent, tough and initially plastic but becoming brittle at maturity. The embryo is 27-34%, the endosperm 25-33% and the seed coat 40% of the seed weight (Woods, 1988).
Roots:Shallow taproot and many, very shallow laterals.
Key Characters:Zig zag branches. Spiny. Yellow, 5 petalled flowers with one orange or spotted petal.
Photosynthetic green bark. Characteristic leaf structure. Large flowers with open petals.
Most seed germinates after the first storm for the wet season but will germinate at any time of the year. The seedling is a single thorny stem. Top growth is usually slow in Australia as the extensive root system develops. In other tropical countries seedling can reach 1-2 m in 1-2 years. Recruitment levels are generally low with an average of 0.1% in Australia. Competition with grass reduces establishment significantly. Plants may flower in their second or third summer but often take longer and usually occurs when the plant is about 1500 mm tall. Leaves are produced all year usually with a flush of growth as day length starts to increase. Pinnules are short lived and usually only survive for a few months. A flush of flowering occurs in August to September in hot areas (and a month or two later in cooler climates) for mature plants with sporadic flowering at other times. Pod maturation occurs about 3 months after bud formation. Most pods drop from the tree within 6 weeks of reaching maturity with a few remaining on the tree for up to a year. It takes 6 weeks to over a year for the pod to decay and release the seed.
It can probably live for more than 30 years but rarely lives for more than 20 years in Australia.
It can form dense inpenetrable thickets but these are usually localised (less than 1 Ha) and it more commonly exists in the landscape as isolated mature trees.
Physiology:A very hardy plant able to grow under extreme conditions.
It tolerates a very wide range of acidities.
Waterlogging tolerant once established.
Plant size is not a good predictor of plant age.
Stem cross sectional area is a good predictor of plant size.
Xerophytic adaptations include sunken stomata, heavy cutinisation of the leaf, hypodermal layer of water containing cells, photo synthetic bark and shedding of leaves in drought.
Prefers full sunlight and growth is reduced in shaded situations.
Doesn't fix nitrogen (Sprent and Sutherland 1990, Stewart et al 1992).
As a fodder it has a high fibre content, poor digestibility and low protein content of 3.7%.
Trees can reshoot from the main trunk down to about 200 mm below ground after mechanical damage or fire.
Large trees can produce 2500 seeds/m2.
Flowers are pollinated by bees.
The chromosome number is 2n = 28.
Flowering times:Any time of year.
Summer to early autumn in western NSW.
May to June mainly elsewhere.
Seed Biology and Germination:The seed has a hard seed coat and over 80-99% is dormant at maturity and nearly 100% is viable. There is little or no physiological dormancy and nearly all seeds germinate if the seed coat is breached. The seed coat is densely packed palisade cells impregnated with water repellent material. There appears to be no secondary dormancy.
Germination of seeds is improved by intense dry heat, passage through animals, fire, scarification, boiling or acid treatment. Exposure to wet, warm to hot heat appears to be the primary method of dormancy release in the field.
Most dormancy release and imbibition occurs within a day of wetting and few seeds germinate more than 4 days after inundation. There is a strong relation with temperature at the time of wetting and germination (van Klinken and Flack, 2005).
Germinates over a wide range of temperatures from 15-35oC with most of the response from 25-350C (see graph). Imbibition is not affected up to 600C. Temperatures above 350C reduce germination due to heat stress and below 200C germination is reduced due to cold stress.
Optimum germination is from 250C to 350C with 85% of non dormant seeds germinating in a median time of 2.1-2.9 days.
Light is not required for germination.
Seeds need to be exposed to air for germination to occur.
Tolerates a wide range of acidities from pH 3-11.
Seedlings are very sensitive to water stress. There is no germination when osmotic potential are greater than 1.4 MPa.
Germination and establishment is greatest when planted at 10-70 mm deep.
Seeds swell to about twice their size as they imbibe water.
128oC for 8 minutes kills seed. 3000C for one minute releases dormancy by affecting the seed coat. Fire can kill seeds at the soil surface, release dormancy of seeds up to 2 cm and has little effect on deeper seeds.
Germination can occur at any time of the year but tends to peak in the summer set period when conditions are more likely to be favourable. Establishment tends to be greatest as the water recedes because early germinating seedlings often drown.
Seed will remain viable for long periods in cool dry conditions. In the field seed bank longevity can be variable. 97% of seeds germinated within 35 days of being buried at upland sites in the wet season in Victoria whilst 9% of seeds were still dormant after 4 years in wetland sites. 57% of seeds laying exposed on the soil surface were dormant after 2 years. The degree of wet heat seems to be the major factor determining dormancy.
Environmental seed refuges can become a source of new infestations when conditions such as canopy cover, moisture conditions, heat or burial depth change.
High recruitment can occur especially after floods but these are rare.
After Klinken and Flack, 2005.
Reshoots readily when damaged. Even seedlings cut of below the cotyledonary node can reshoot and survive.
Mature plants can reshoot from up to 20 cm below ground level.
It can be propagated by root and shoot cuttings.
Hybrids:No hybrids have been recorded in Australia.
Several genetically distinct populations occur in the Americas and it hybridizes with other Parkinsonia species elsewhere.
The nursery trade has developed a 3 way hybrid called “Desert Museum”.
Allelopathy:Population Dynamics and Dispersal:
Spread initially by intentional planting.
Now the main spread is by flood waters carrying the buoyant pods or animals carrying them in mud attached to them. Pods can float for up to 14 days. Seedlings are often seen on flood lines or may be spread by tidal movements.
The seeds are large and most drop close to the parent plant which generally prevents its spread in areas where flooding doesn't occur.
Some spread by machinery carrying contaminated soil.
It has the potential to infest much greater areas than it now occupies.
Animals do not appear to be a significant in dispersal. Pods are rarely eaten and seedlings are rarely seen in dung pads. Seed will survive passage through the gut when accidentally consumed. In pigs, seeds took 3-8 days to pass through the gut and had a 50% viability. Similarly birds do not appear to be a significant in dispersal (van Klinken et al., 2009).
Origin and History:Central America. Caribbean. Mexico. West Indies.
Introduced for shade and gardens in the late 1800's. Probably introduced to central Queensland in the 1860's.
First recorded in the Sydney Botanic Gardens in 1901.
Australian specimens appear to be related to those from Venezuela.
Distribution:NSW, NT, QLD, SA, WA.
Courtesy Australia's Virtual Herbarium.
Occurs over 3.3 million ha but usually at low levels.
Usually follows fairly linear bands along water courses.
USA, Africa, India Indonesia, Middle East
Warm temperate. Semi-arid to sub-humid tropics and sub-tropics.
Prefers areas with distinct wet and dry season and those receiving run-off or supplemental water such as from bores or rivers.
Generally in areas with 250-1400 mm annual rainfall.
Mildly frost tolerant. Severe frosts (-60C) tend to kill it.
Hot and dry regions elsewhere in the world.
Soil:Sandplains to self mulching clays. Skeletal soils and rocky gullies. Black clay soils. Deep black vertisols. Often on saline or fresh water, high pH seasonally waterlogged areas. It can survive 9 months of inundation but tends to die out of these areas if it is inundated regularly.
Tolerates a wide range of soil types including extremely saline soils and soils with ph from 3 to 11.
Plant Associations:Often associated with the “prickle bushes” Mesquite (Prosopis species), Mimosa Bush (Acacia farnesiana), Prickly Acacia (Acacia nilotica)
Ornamental, hedge plant, shade plant, windbreak, fuel, paper making and low quality fodder.
The wood is brittle and of dubious durability for carpentry.
A very hardy plant able to grow under extreme conditions.
The leaves are eaten by sheep, goats and camels. It is generally avoided by cattle but they may eat it late in the dry season when little else is available.
Used in rehabilitation. It was thought to fix nitrogen and consequently planted for this reason but probably doesn't fix nitrogen.
Used as a herbal medicine for fevers, epilepsy and vomiting. Seeds are a minor food source and children eat flowers in West Africa.
Commercial exploitation of products from Parkinsonia appear to be very limited.
Detrimental:It can form dense thickets, blocking access to water, hindering mustering and crowding out more desirable vegetation. The thorns can injure stock, hooves and car tyres.
Provides refuge for feral pigs.
Can cause hay fever when in flower.
It chokes irrigation channels in Mexico.
Declared a weed of national significance in Australia in 1999. A weed of New Caledonia but rarely a troublesome weed elsewhere in the world.
It probably has a relatively low impact on agricultural production and biodiversity with the main cost being for control of thickets and around water points.
Toxicity:Not recorded as toxic.
Legislation:Noxious weed of NSW, NT, QLD, SA, TAS, VIC, WA.
Weed of National Significance (WONS).
Management and Control:All spread is by seed.
Mechanical removal is effective if seedlings are controlled by cultivation and perennial pastures especially grasses are encouraged to compete with it.
Blade ploughing, grubbing, stick raking, cutter barring and chain pulling have been used as mechanical methods of control. Blade ploughing typically provides more than 90% control for around $150/ha whilst bulldozing is a similar cost but only 45% control usually. Chaining is cheaper but typically only provides 30% control. There is often a large recruitment of seedlings after the disturbance caused mechanical control that need to be controlled by ploughing, spraying or burning.
Fire provides variable levels of control and usually assists with the rundown of the seed bank by destroying surface seed and reducing dormancy of shallowly buried seed. Flaming around the base of trees with a 8200C flame thrower for 10 seconds provided 83% kill (Vitelli and Madigan, 2004) and cost 7.5 cents/plant (140/ha) and is best suited to sparse infestations.
Triclopyr is used to spot spray remnants. Basal bark and cut stump treatments using triclopyr or picloram plus 2,4D are effective on larger plants. Basal bark spraying of dense infestations (2000 plants/ha) costs about $500/ha in 2009.
Basal bark spraying is most effective when the plants are actively growing.
Cut stump applications are effective at most times of the year.
Basal bark spraying and cut stump applications are the preferred method for sparse populations.
Foliar and aerial spraying is preferred on young dense infestations that are less than 2 m tall. Kill rates of around 66% have been reported and this is mainly due to poor control of larger plants in the stand.
Application of the residual herbicides, tebuthiuron and hexazinone by air or ground provides high levels of control (81-99%) for less than $200/ha.
Hexazinone granules are also effective.
Goats and camels could provide some control. Camels selectively browse fresh growth, flowers, young pods and leaves and can reduce seed set to very low levels. They tend to avoid mature pods but when they are consumed only low levels of viable seed is passed in the dung. If possible stock should be confined to infested areas to reduce the risk of spreading seed.
Cattle can eat plants with stems up to 2 cm thick when little other forage is available and this will kill some plants.
Mowing is ineffective as it reshoots readily.
Mechanically remove old plants and burn on site. Control seedlings by cultivation in the first few years to encourage germination of dormant seed. Plant grass species and use residual herbicides to keep the area clean.
Herbicide resistance:Biological Control:
Biological control agents are under evaluation, several have been released and none appear to be having much impact to date.
Goats, camels and Agile Wallabies (Macropus agilis) appear to prevent establishment where their numbers are high as they selectively graze young Parkinsonia. Cattle are less effective. Locusts occasionally cause death of seedlings. Termites may kill mature Parkinsonia. A soil borne dieback disease also seems to be a significant in restricting populations and recruitment, but this doesn't appear to be present in WA. In other states it may restrict the impact of Parkinsonia.
Apart from termites no other insects appear to have much effect in Australia.
Related plants:None in the same genus in Australia.
Cercidium spp., Delonix spp., Peltophorum spp., Lemuropisum spp.
Plants of similar appearance:Mesquite (Prosopis species)
Mimosa Bush (Acacia farnesiana)
Prickly Acacia (Acacia nilotica)
References:A.P.B. Advisory Leaflet No. 37 (1978)
Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P382.
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). P124. Photo.
Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).
Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #933.1.
Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P461-463. Photos.
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