giant ragweed (Ambrosia trifida) — identification, habitat & look-alikes

Giant ragweed is a tall plant native to North America and a troublesome weed in crop fields. Its large, rough leaves usually have three to five lobes, and its late-season pollen can cause allergies.

Appearance

Appearance: The upright stems and leaves feel rough and hairy. Most leaves grow in opposite pairs and are deeply divided into three to five lobes.
Growth form: This summer annual is commonly 3–12 ft (0.9–3.7 m) tall and can occasionally reach about 20 ft (6.1 m).
Flowers: The small green male flower heads form narrow spikes at branch tips. Female flower heads sit below them and in the upper .
Flowering time: In the northeastern United States, plants flower from midsummer into late summer.
Fruit: Each female flower makes a hard, one-seeded fruit with blunt points arranged like a tiny crown.

Look-alikes

Jerusalem artichokeHelianthus tuberosus

Young plants can look similar. Jerusalem artichoke develops unlobed, oval-to-lance-shaped leaves and showy yellow sunflower heads, while giant ragweed usually has three- to five-lobed leaves and narrow green flower spikes.

Why they get mixed upBoth are tall, rough plants in the sunflower family, and their lower leaves may be opposite.

common ragweedAmbrosia artemisiifolia

Common ragweed has much smaller, finely divided leaves. Giant ragweed has broad leaves usually split into three to five large lobes.

Why they get mixed upBoth have inconspicuous green flowers and share the ragweed name.

Where it lives

Typical habitats: crop fields
floodplains
roadsides
field edges
Range: It is native across much of North America and is especially common in central and eastern parts of the United States.
Preferred conditions: It grows especially well in moist, fertile, disturbed ground.

Life cycle

Reproduction: Male and female flower heads occur separately on the same plant, and the flowers are .

Conservation

IUCN Red List: NENot EvaluatedNot yet assessed

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