Identifying Giant Ragweed (Ambrosia trifida) in Ontario

A 2.5 m (8 ft) plant with fuzzy stem, single 5-lobed lower leaf and multiple 3-lobed upper leaves in Windsor, Ontario is almost certainly giant ragweed (Ambrosia trifida), a native annual weed common across North America.

Key ID Traits

• Coarse, hairy stem.
• Leaves: lower = large, 3–5 deep lobes; upper = smaller, 3 lobes.
• Inconspicuous green flowers in spikes; massive pollen producer (hay fever).
• Appears in disturbed soil—dug perennial beds are perfect.

Management

1. Pull before flowering (July–August) to prevent seed set.
2. Wear gloves/mask if pollen-sensitive.
3. Do not compost flowering plants.
4. Mulch beds to suppress future germination.

While a weed, it supports native insects. If not in high-traffic area, consider leaving a few for biodiversity.