1 / 10

Parthenium hysterophorus

santa maria feverfew

Asteraceae Subtropical Americas

Deer Resistant Drought Tolerant

Native to North America

SunFull Sun – Part Shade

WaterLow · drought-tolerant

Overview

Parthenium hysterophorus, known as Santa Maria feverfew, famine weed, or congress grass, is a fast-growing annual in the daisy family, native to the subtropical Americas and now a widespread invasive weed across the warmer parts of the world. It forms an erect, much-branched plant 1-5 feet (30-150 cm) tall on ridged, roughly hairy stems. The lower leaves are pale green, soft-hairy, and deeply divided into many narrow lobes, forming a basal rosette before the stems elongate. The flowering branches carry many tiny cream-white heads about 0.15-0.2 inch (4-5 mm) across, each producing a few seeds; a single plant can set tens of thousands of seeds in a season and self-pollinates freely. It germinates and flowers across the warm months and colonizes roadsides, pastures, waste ground, and crop fields, where its allelopathic chemicals suppress neighbouring plants. The foliage and pollen contain parthenin, which causes severe allergic dermatitis, hay fever, and asthma in people and poisons grazing livestock, tainting milk and causing mouth ulcers. These traits make it a regulated noxious weed in many countries and hard to control once established.

Native Range

Native to the subtropical Americas, including Mexico, Central and South America, and the southern United States. It has spread as an invasive weed across Africa, Asia, and Australia.

Suggested Uses

This species has no ornamental or garden use and is managed only as a weed to be controlled. It is listed as a noxious or quarantine weed in many countries. It is the target of biological and chemical control programmes across its invasive range.

How to Identify

Identify it by deeply cut, pale green, soft-hairy lower leaves in a basal rosette and tall, ridged, branching stems carrying many tiny cream-white flower heads. Each small head is five-sided with a few short white rays. The whole plant is roughly hairy and gives off a strong smell when crushed.

Appearance

Size & Dimensions

Height1' - 5'

Width/Spread1' - 2'

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

~20 weeks

Spring Summer Fall

Flowering occurs through the warm months, often spring to autumn, and nearly year-round in frost-free climates. Plants begin flowering within weeks of germinating. Seed is produced continuously and remains viable in the soil for years.

Detailed Descriptions

Flower Description

white to cream

Foliage Description

pale green

Growing Conditions

Sun Requirements

SunFull Sun SunPartial Shade

Requires 6-12 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 Range5.5 - 8.5(Neutral)

357912

Soil Types

clay loam sand rocky

Drainagewell drained

Water & Climate

Water Needs

Low

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Parthenium hysterophorus grows in full sun on almost any disturbed soil, from sand to clay, and tolerates drought, heat, and low fertility. It needs no cultivation and germinates wherever bare ground and warmth allow. Control is difficult because of its huge, long-lived seed bank, so management combines preventing seed set with removing plants before they flower. Contact with the plant causes skin and respiratory reactions, so it is cleared with protective gear rather than bare-handed. Mowing alone tends to provoke regrowth and more seed. It is a regulated weed in many regions and is eradicated rather than grown.

Pruning

Plants are pulled or cut before flowering to stop seed production, with repeated passes through the season. Cut material is bagged and destroyed rather than composted, because seed ripens on cut stems. No ornamental pruning applies.

⚠️ Toxicity Warning

Toxic to pets and humans