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Ambrosia trifida

giant ragweed

Native to North America

Overview

Ambrosia trifida is a robust annual in the aster family, commonly reaching 3 to 12 feet (1-3.7 m) tall and occasionally taller, on stout, rough-hairy stems. The large leaves are opposite, 4-12 inches (10-30 cm) long, and usually divided into three or five coarsely toothed lobes, rough to the touch on both surfaces. It is wind-pollinated and monoecious, bearing long, upright spikes of small greenish-yellow male flower heads above inconspicuous female heads clustered in the upper leaf axils. The male spikes shed large amounts of airborne pollen from late summer into autumn, a leading cause of seasonal hay fever in its range. Each plant produces woody, beaked seeds that persist in the soil for years. It germinates early and grows fast, shading out shorter plants and competing strongly in crops and disturbed ground. The plant dies with the first hard frost, leaving bare stems.

Native Range

Native to North America, widespread across southern Canada and most of the United States, and naturalized in parts of Europe and Asia. It grows in moist, fertile, disturbed ground including floodplains, field margins, roadsides, and croplands.

Suggested Uses

Rarely cultivated, it occurs mainly as a tall weed of fertile, disturbed ground rather than a planted species. Its seeds are eaten by birds and small mammals in autumn and winter. In gardens and farmland it is managed as a weed because of its size, competitiveness, and allergenic pollen.

How to Identify

Recognized by its great height, stout rough stems, and large opposite leaves divided into three or five coarsely toothed lobes. In late summer it raises long, upright spikes of small greenish male flower heads at the stem tips. The woody, several-pointed seeds and heavy airborne pollen distinguish it from the more finely divided common ragweed A. artemisiifolia.

Appearance

Size & Dimensions

Height3' - 12'

Width/Spread1'6" - 3'

Colors

Flower Colors

green

yellow

Foliage Colors

green

Bloom Information

Bloom Period

~10 weeks

Summer Fall

Greenish male flower spikes elongate and shed pollen from August through October, peaking in late summer. The wind-borne pollen travels long distances and is a major hay fever trigger. Woody seeds ripen through autumn and drop around the parent plant.

Detailed Descriptions

Foliage Description

Green

Growing Conditions

Sun Requirements

SunFull Sun

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 - 7.5(Neutral)

357912

Soil Types

clay loam silt

Drainagemoist

Water & Climate

Water Needs

Medium

Care & Maintenance

Care Guide

Grows in full sun on moist, fertile soil and tolerates clay, periodic flooding, and a wide pH range. It germinates in early spring, grows several feet per month, and needs no care to reach full size. Dry, infertile soil limits its height but rarely stops it. The seeds remain viable in the soil for several years, so cleared ground can produce new stands. Cutting or pulling before seed set is the main way to reduce its spread. Plants die completely at the first hard frost.

Pruning

No pruning is needed for this annual. Cutting or mowing plants before the flower spikes shed pollen reduces both allergen load and seed set. Cut stems may resprout if severed high, so cutting low to the ground gives better control.