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Chromolaena odorata

siam weed

Asteraceae Tropical Americas

Attracts Pollinators Attracts Butterflies Deer Resistant Drought Tolerant

Native to North America

SunFull Sun – Part Shade

WaterMedium · drought-tolerant

HardinessZones 10–11

Overview

Chromolaena odorata is a fast-growing perennial subshrub reaching 3-7 feet (1-2 m) tall as a freestanding plant and scrambling to 20 feet (6 m) where it can lean on other vegetation. Stems are soft and brittle when young, becoming woody at the base. The opposite, triangular leaves are 2-5 inches (5-13 cm) long, with three main veins and toothed margins, and release a pungent odor when crushed. Pale blue, lilac, or white flower heads, each rayless and 0.4 inch (1 cm) long, are borne in dense clusters at the branch tips, mainly in the dry season. Each plant produces large numbers of small, bristle-tipped seeds that spread on wind, clothing, and animals. Growth is rapid, and dense thickets form within a year or two, shading out other plants. The species is listed as an invasive weed in many tropical countries and is toxic to grazing livestock.

Native Range

Native to the Americas, from the southern United States through Mexico, Central America, and the Caribbean to tropical South America. Introduced and now widespread in tropical Africa, South and Southeast Asia, and parts of Australia and the Pacific, where it invades farmland, plantations, roadsides, and disturbed forest.

Suggested Uses

This species is not planted ornamentally and is regulated or banned as an invasive weed in many countries. In its native range it appears in fallow fields and disturbed ground and has been used in traditional medicine and as a fallow cover in some farming systems. Control programs target it to protect pasture, crops, and native vegetation.

How to Identify

Recognized by soft, sprawling stems, opposite triangular three-veined leaves with a pungent smell when crushed, and clusters of small pale blue to white rayless flower heads. The triangular leaf shape and strong odor separate it from many other weedy Asteraceae spp.. Mature plants form dense, tangled thickets 3-7 feet (1-2 m) high.

Appearance

Size & Dimensions

Height3' - 7'

Width/Spread3' - 7'

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

~8 weeks

Winter

Flowers mainly in the cool, dry season, often November through March in the tropics, though timing shifts with local climate. Flowering is brief and synchronized within a region, followed quickly by heavy seed set. A single flowering plant can produce tens of thousands of wind-dispersed seeds.

Detailed Descriptions

Flower Description

pale blue to white

Foliage Description

green

Growing Conditions

Sun Requirements

SunFull Sun SunPartial Shade

Requires 4-12 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range4.5 - 7.5(Neutral)

357912

Soil Types

loam clay sand

Drainageaverage

Water & Climate

Water Needs

Medium

Frost Tolerance

tender

Time to Maturity

1-2 years

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Grows in full sun to light shade across a wide range of soils, spreading fastest on disturbed, fertile ground in warm climates. It establishes quickly from seed and resprouts vigorously after cutting or burning, which makes it difficult to remove. The foliage suppresses nearby plants through shading and chemical effects on the soil. The plant is toxic to cattle and other grazing animals if eaten. It is frost-sensitive and dies back where winters are cold. In dry conditions the thickets become a fire hazard.

Pruning

Cutting alone does not kill the plant, as it resprouts from the base and roots. Repeated cutting before flowering reduces seed production over time. Removing whole plants with their roots before they set seed limits spread more than cutting.