Argemone ochroleuca Sweet
Synonyms - Argemone mexicana. Argemone ochroleuca is broken into two subspecies Argemone ochroleuca ssp. ochroleuca and Argemone ochroleuca ssp. mexicana by some authors.
Argemone is from the Greek word for eye cataracts because the juice was supposed to cure cataracts, however eating it causes glaucoma.
Ochroleuca is Greek for yellow and white in reference to the flower colour.
Mexican Poppy - because it is a poppy from Mexico.
Other Names:Chicolate (Mexico)
Golden Thistle of Peru
Mexican Pricklepoppy (USA)
Mexican Thistle (Jamaica)
Summary:A blue green, erect, spiny, annual thistle with yellow sap and large yellow-orange to almost white poppy flowers from November to February. The leaves are deeply lobed, spiny and often have white blotches.
First leaves: First leaf often 3 lobed.
Leaves: Crowded rosette of shortly stalked leaves. Alternate.
Petiole - none on stem leaves. Short on the rosette leaves.
Blade - Prickly, 60-250 x 15-100 mm. Variable shape. Deeply lobed into 7-11 lobes. Lobes have sharp yellow spines. Wavy indentations on lobes and between lobes. Blue-green, blotched with white along veins on the upper surface. Covered with a powdery bloom. Edges wavy. Conspicuous veins. Prickles scattered on the underside.
Stem leaves - Shallower lobes than basal leaves. Stem clasping. Variable shape.
Stems: Blue-green. Up to 900 mm tall. Leafy. Often branched near base. Copious yellow to orange sap. Pithy. Smooth or slightly furry between scattered stiff yellow prickles.
Flower head:Single at the ends of stems with 2 or 3 bracts underneath. Stem leaves close underneath.
Flowers: 30-60 mm diameter. No stalk or very short stalk.
Ovary - spiny surface. Purple to dark red stigma with 3-6 lobes pressed against each other. Short style.
Sepals - 3. 50-100 mm long. Hood-like. Tip has long conical, 7- 10 mm point or spine. Free with horn-like appendage. Bristly. Fall off before the flower opens. Sparsely prickled with a large spine just below the top. Sepals fall off as the flower opens.
Petals - 4-6. Egg shaped, delicate. 20-30 mm long, 14-40 mm wide. Yellow- orange to almost white.
Stamens - Many. Orange.
Fruit:Capsule, 1 celled. Egg shaped to oblong, spiny, thin walled. 15-50 mm long x 20 mm diameter. Ribbed, usually with 5 lengthwise pale grooves separated by blue green. 3-6 sections. Persistent style on top. Open from the top downwards leaving a structure like the ribs of an umbrella formed from where the style ribs are attached to the stigma.
Seeds:Dark brown to black. Rounded, globular. Lengthwise rows of warts, 1-2 mm diameter. Oily. Covered with a network of veins.
Roots:Taproot with a few branches and many fine laterals.
Key Characters:Yellow-orang to white poppy flowers with 4-6 petals.
Annual. Flowers spring to summer. Seeds germinate at any time of the year. Rosettes of leaves are formed in winter and the flowering stem emerges in spring. Flowering and seed set normally starts in November and continues through the summer. Dormant seed is produced and is expected to survive in the soil for several years.
By seed only.
Flowering times:November to February.
Seed Biology and Germination:Seed has an after ripening period (dormancy) of a few weeks to several months.
It produces chemicals that affect the growth of other plants such as wheat, sorghum and finger millet. At high concentrations it inhibits plants whilst at lower levels it increases growth. In India a Mexican poppy mulch is spread to improve rice and Sesbania growth.
Population Dynamics and Dispersal:Large numbers of seeds (up to 40,000 per plant) are produced, most of which germinate close to the parent. Movement in water, mud attached to animals and machinery or in produce and grain are the main methods of spread.
Seedlings don't establish readily in the presence of perennial pastures.
Origin and History:Central America and Mexico.
Probably imported as a contaminant of wheat seed and first reported near Sydney in 1845.
Distribution:ACT, NSW, NT, QLD, SA, VIC, WA.
Occurs as a serious weed in many countries.
Courtesy Australia's Virtual Herbarium.
Sub-tropical, sub humid and semi arid scrub lands.
At altitudes from sea level to 2900 metres.
Soil:Sand banks, silty areas in flood plains and the beds of intermittent creeks. Tolerates a wide range of soils but is often more common on sandy soils. Well adapted to growing in soils with a low nutrient status. Common on heavy self mulching soils.
The ochroleuca subspecies dominates in nitrogen poor soils and the mexicana subspecies in soils with low phosphorous levels.
Listed as one of the world's worst weeds.
Beneficial:Honey and pollen plant.
It is one of the few plants that can reduce soil pH.
Detrimental:Contaminates wool with its prickly fruit. Weed of flood plains, disturbed areas, roadsides, mine sites, rabbit warrens, cultivated areas, fallows, vegetables, cereal and sugar cane.
Reduces growth and egg production in poultry when fed contaminated feed.
Toxicity:Toxic due to nitrate levels and alkaloids but it is usually not eaten by stock. Seed contaminating grain may be toxic to poultry and man. The seed is the most toxic part of the plant.
Symptoms:Violent colic in horses.
Deaths are rare.
In poultry - darkening of the comb, oedema of wattles and sub-mandibular space occur 9-12 days after ingestion. Growth rates are depressed at low levels (0.5%) of contamination.
In man - dropsy and glaucoma has been attributed to the use of cooking oil made from contaminated oilseeds.
Treatment:Avoid feeding produce contaminated with seed to animals. Don't expose hungry, unaccustomed animals to infestations.
Legislation:Noxious weed of WA.
Management and Control:Prevent seed set. Mowing and slashing often fail because of regrowth. Cultivation of young plants is effective. Hormone herbicides provide good control of young plants. Glyphosate provides good control in non-selective situations. A number of selective herbicides can be used in crops. In pasture, the establishment of perennial pasture species usually results in the slow disappearance of the weed.
Prevent seed set. Establish competitive perennial pastures.
Herbicide resistance:None reported.
Biological Control:Related plants:
American Poppy (Argemone subfusiformis). American poppy has cream to pale yellow flowers (or deep yellow in central and northern NSW), the capsule is spindle shaped and the seed is longer (about 2 mm).
Plants of similar appearance:Thistles are similar before flowering but can be distinguished by their flower type later in the season.
References:Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P196. Photo.
Black, J.M. (1965). Flora of South Australia. (Government Printer, Adelaide, South Australia). P366.
Burbidge, N.T. and Gray, M. (1970). Flora of the Australian Capital Territory. (Australian National University Press, Canberra). P178. Diagram.
Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P312-313. Photo.
Everist, S.L. (1974). Poisonous Plants of Australia. (Angus and Robertson, Sydney). P563. Photo.
Gardner, G.A. and Bennetts, H.W. (1956). The toxic plants of Western Australia. (West Australian Newspapers Ltd, Perth). P30. Diagram.
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). #123.2.
Marchant, N.G., Wheeler, J.R., Rye, B.L., Bennett, E.M., Lander, N.S. and Macfarlane, T.D. (1987). Flora of the Perth Region. (Western Australian Herbarium, Department of Agriculture, Western Australia). p67.
Meadly, G.R.W. (1965). Weeds of Western Australia. (Department of Agriculture - Western Australia). P74-76. Diagram.
Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P534-537. Photos.
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