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Have you seen Siam Weed (Chromolaena odorata)?
Hawaii Early Detection Network Priority Pest for the island of Oahu

Siam Weed leaf
Siam Weed leaves have a pitchfork shaped veins
Image: Tony Rodd
Siam Weed flowers
Siam Weed flowers
Image: Satish Nikam
Siam Weed smothering habit
Siam Weed has a smothering habit
Image: Phuong Tran

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 external link

Siam Weed look-alikes:

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]).


Maui Pamakani

Maui Pamakani (Ageratina adenophora)
Images, unless noted: Forest & Kim Starr

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.




Download this Siam weed priority pest identification flyer PDF icon and share with your neighbors, friends, and family. Help keep Hawaii free from new pests!

Last Updated: Tuesday October 30 2012. If you have any questions about the Hawaii Early Detection Network contact
Funding and support for this project was made possible by the Hawai'i Invasive Species Council, the USDA Forest Service State and Private Forestry assistance, and University of Hawai'i-Pacific Cooperative Studies Unit.