Siam Weed Plant Care

Siam Weed (Chromolaena odorata) belongs to the Asteraceae family within the Chromolaena genus. Siam Weed is native to the Americas, from Florida and Texas in the United States south through Mexico and the Caribbean to South America. Siam Weed is also known as Jack in the bush, Triffid Weed, Devil Weed.

Siam Weed displays an upright, spreading shrub growth habit, reaching up to 3 ft tall with a spread of approximately 2 ft. Siam Weed produces small white or lavender flowers in clusters.

Leaf size: approximately 6 inches.

Water a young or newly planted Siam Weed every 7–10 days, soaking the whole root zone, until it is established — usually one to two growing seasons. Established shrubs mainly need water during long dry spells. In a container, water whenever the top few centimetres of soil are dry.

Siam Weed thrives in full sun to partial shade.

Chromolaena odorata tolerates temperatures of 65–85°F. Siam Weed prefers humidity levels of 40–70%.

Well-draining sandy loam with added organic matter. Good drainage is essential for Siam Weed to prevent waterlogging, which leads to root rot and fungal issues.

Feed Siam Weed every 4 months during the growing season with a balanced, slow-release fertilizer at the rate on the label. Established shrubs in good soil often need little extra feeding, so let weak growth or pale foliage guide you.

Stop feeding by late summer so new growth can harden before winter.

The PlantoScan app can remind you when to water and when to fertilize each plant.

Choose a spot with full sun to partial shade and well-draining sandy loam with added organic matter. Avoid low spots where water collects after rain.

Siam Weed is suited to USDA Zone 9b – 12b.

Plant container-grown or balled-and-burlapped shrubs in spring or early autumn, setting the root flare at soil level, and water well after planting.

Siam Weed can spread to about 2 ft (0.6 m) and reach about 3 ft (1 m) tall. Space plants about 2 ft (0.6 m) apart so mature crowns don't crowd each other, and plant at least half that distance from buildings, fences and paths.

Planning a row or hedge? Try the plant spacing calculator.

Prune Siam Weed while it is dormant in late winter: first remove dead, damaged and crossing branches, then thin lightly for shape.

If it flowers in spring, prune right after flowering instead so you don't remove next year's buds. Never take off more than about a quarter of the shrub in one year.

Siam Weed is frost-tender (hardy to USDA zone 9). In colder areas grow it in a container and move it to a bright, frost-free place before the first frost.

Siam Weed can be propagated through seeds, stem cuttings. Similar propagation techniques also apply to bigelow's thoroughwort plant care, which shares the same genus.

See all plants toxic to cats and plants toxic to dogs.

Siam Weed is susceptible to aphids, spider mites, whiteflies. Inspect the foliage regularly, especially along leaf undersides and stems. Treat infestations promptly with insecticidal soap or neem oil to prevent spread.

Yellow leaves, brown tips or drooping? See common plant problems and how to fix them.

Siam Weed grows outdoors in USDA Zone 9b – 12b. It survives winters down to planting zone 9b. Siam Weed is native to the Americas, from Florida and Texas in the United States south through Mexico and the Caribbean to South America. In cooler regions, grow Siam Weed as a houseplant or bring it indoors before the first frost.