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Giant Hogweed - Heracleum mantegazzianum


This dangerous plant can be found along roadsides, stream banks and in scattered locality areas in southern Ontario.


It has a flowering stem that can stand up to 5 meters high and 10 centimeters in diameter.  The stem is hollow except at the nodes.  Both the stems and petioles have conspicuous red-purple flecks that can become solid purple near the base.  The leaves are rosette and can be up to 1 meter across.  It is a compound leaf with 3 large deep cut leaflets, each having deep irregular lobes with coarse, sharp teeth on all margins. 

It flowers between June to August producing anywhere from 30 to 50 branches, each ending in a simple umbel with approximately 30 to 40 flowers.  The compound umbel is flat bottom with a gently rounded top.

It's watery, clear sap contains photosensitizing compounds, which has serious health hazards in you come in contact with it.  When contacted with your skin and combined with UV rays, it can cause burning.  Depending on the part of the plant, burning effects can very.  Skin reactions can differ between peoples individual sensitivities.

Redness and swelling will appear within 24 hours followed by an inflammatory reaction after 3 days.

Contact with this plant should be avoided at all times.  Mild reactions can be treated with a cool wet wash cloth or a non-steroidal anti-inflammatory.  More severe cases should seek medical treatment.