Safety warning — extreme hazard
DANGER: the watery sap plus sunlight can cause severe burns and blistering
Giant Hogweed
Heracleum mantegazzianum
A very large, hazardous plant with thick blotchy stems and huge white umbrella-shaped flower heads. Its watery sap can cause severe skin burns and blisters when skin is exposed to sunlight afterward.
Giant hogweed in flower (~3.5 m tall) — observe from a distance only; do not touch.
Image source: Ontario weed-identification reference (OMAFRA / Ontario CropIPM)
At a glance
- Key clue
- Giant white umbrella flowers on a blotchy, hairy stem (2–5 m)
- Best months
- June to August, when the giant white umbrella flower heads are visible — observe from a distance only
- Often found
- Ravine edge, Wetland / pond edge, Vacant lot
- Easiest to remove
- Professional removal recommended
Why it's prohibited
Giant hogweed is a serious health hazard: contact with its sap, followed by sunlight, can cause painful burns, blistering, and long-lasting scars, and sap in the eye can damage sight. It also spreads readily and crowds out other plants.
Main identification features
- Very tall — often 2–5 m when in flower
- Thick green stems (4–10 cm) with purple-red blotches and stiff white hairs
- Enormous, deeply divided leaves up to 1 m or more across
- Large white flower heads shaped like umbrellas (umbels), up to 0.5–0.8 m wide
- Sturdy, ridged hollow stems
Similar plants & look-alikes
- Cow parsnip — Cow parsnip is smaller (1–2 m), with fuzzy stems lacking the strong purple blotches and a smaller flower head. Its sap can also irritate — handle any look-alike with care.
- Angelica — Angelica has smooth, often purplish rounded stems and dome-shaped (not flat umbrella) flower clusters.
- Queen Anne's lace — Queen Anne's lace is small (under 1.2 m) with a lacy flat flower and a single tiny dark flower in the centre.
Month-by-month identification timeline
June to August, when the giant white umbrella flower heads are visible — observe from a distance only.
April–May (Early spring)
· 0.5–1.5 mHuge, deeply lobed leaves expand near the ground; the plant is already large but not yet flowering.
Enormous jagged leaves — do not touch.
May–June (Late spring)
· 1.5–3 mA thick, blotchy, hairy stem rises rapidly, carrying the developing flower head.
Thick stem with purple blotches and white hairs.
June–August (Summer)
· 2–5 mTowering plant topped with a massive white umbrella-shaped flower head.
Giant white umbrella flower head — unmistakable, view from afar.
August–September (Late summer)
Flowers turn to large flat, dry seeds; the plant begins to decline after seeding.
Big flat seed heads forming.
Fall–winter (Dormant)
Top growth dies back to tall dry stalks; sap on dead stems can still irritate.
Dead hollow stalks — still avoid contact.
Growth-stage photo gallery
Photos of giant hogweed at different growth stages — note the visual features described under each one to identify it confidently.
A newly emerged seedling beside a mature stalk — even young plants should not be touched.
Large rosette leaves from a plant that emerged in spring.
Huge, deeply divided leaves on elongated stems.
The green stem with reddish-purple speckles and stiff, whisker-like hairs — a key warning sign.
A flowering plant about 3.5 m tall with a huge white umbrella flower head. View from a distance only.
Click any photo to enlarge. Image source: Ontario weed-identification reference (OMAFRA / Ontario CropIPM).
How to remove it safely
Safety first. Always recommended. Giant hogweed should be assessed and removed by trained professionals using full protective equipment.
Safest resident-friendly approach
For giant hogweed, safety comes first — do not attempt casual removal. Because the sap is hazardous, the City and health authorities advise reporting it and, in most cases, having it removed by trained professionals with full protective equipment. If you believe you've found it, keep people and pets away and seek guidance rather than cutting it yourself.
Tools & protection
- Do not handle without full protective equipment
- Professional removal strongly recommended
Best time of year
Leave timing and method to professionals; the priority is to avoid any skin contact with the sap.
Monitoring
Professionals monitor the site for several years; report any new plants to the City.
Prevent regrowth
Professionals typically manage the root and monitor the site over multiple seasons; report any regrowth.
What not to do
- Do NOT touch, cut, mow, or weed-whip it — this splatters hazardous sap
- Do NOT let the sap contact skin or eyes; do not burn it
- Do not let children or pets near the plant
Disposal
Disposal should be handled by professionals. Do not compost or chip giant hogweed, and do not move material around with the sap still present.
Safety cautions
- DANGER: the watery sap plus sunlight can cause severe burns and blistering
- Sap in the eyes can cause serious, possibly permanent eye damage
- If sap contacts your skin, wash immediately with soap and water, keep the area out of sunlight, and seek medical advice
- Keep children and pets well away; do not attempt DIY removal
When to get professional help
Always recommended. Giant hogweed should be assessed and removed by trained professionals using full protective equipment.
Do not handle or compost. Arrange professional removal and disposal; report the location to the City and avoid all skin contact.
Sources
Report a prohibited plant
Found this plant growing on a property? Report it to the City through official Toronto 311.
Report through Toronto 311This page is provided for general educational and reference purposes only. Plant identification results are not a legal determination. Always confirm prohibited plant information using official City of Toronto resources. For hazardous species or large infestations, consider seeking professional advice.
Image credits & usage
Plant photographs are reproduced for non-commercial public education and identification reference only. Sources include Ontario weed-identification (OMAFRA / Ontario CropIPM) and invasive-species fact-sheet materials; all image rights remain with their respective owners. If you are a rights holder with a concern, please contact us through the Feedback page. Photos on this page: Ontario weed-identification reference (OMAFRA / Ontario CropIPM). Contact us.