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Identification: Siam weed is a shrub that forms dense, tangled thickets 4-6' high (although some branches can grow up trees to 65' high), its leaves are triangular in shape, have velvety hairs, and leaf edges have large teeth or are smooth. Leaves also have a distinctive 3-vein "pitchfork" pattern and a distinct odor when crushed. Flowers are very small, white to mauve in color, and bunched together in round clusters. A long, whispy structure extends beyond the flower petals. Seeds are dark, .12-.16" long, with a .2" long fluffy structure (pappus). All parts of the plant have a distinctive odor when crushed.
Impacts: Siam weed grows quickly to form dense thickets that smother nearby plants, can reach tall heights, and promotes fires. In addition to being an allergen and toxic to humans, Siam weed is also toxic to livestock and contains chemicals that prevent the growth of other plants around it. The spread of this species would threaten watershed forests and agricultural lands.
Dispersal Mechanism: Each plant produces up to 800,000 seeds per year that are mostly spread by wind, but they are also easily spread by sticking to animals, hikers, vehicles and equipment.
More information about this pest 
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Maui Pamakani (Ageratina adenophora):
Maui pamakani is a widespread weed that has leaves and flowers that are similar to Siam Weed. Maui Pamakani is not hairy, has stems that are dark red or purple, dark green leaves, and seeds that are smaller (.06" long, with a .16 long fluffy structure [pappus]).
THIS LOOK-ALIKE IS ALSO A PEST!
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Sourbush (Pluchea carolinensis):
Sourbush can be differentiated from Siam weed by its dull, grey-green leaves that are oblong to elliptic shaped and its smaller seeds (.04" long, with a .08-.12" long fluffy structure [pappus]). Sourbush does not grow into a tangled shrub.
THIS LOOK-ALIKE IS ALSO A PEST!
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Sourbush
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and share with your neighbors, friends, and family. Help keep Hawaii free from new pests!)
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