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Care of Your
in Eastern Ontario
Giant hogweed (Heracleum
mantegazzianum) is an invasive plant species that has gone viral in more
ways than one. It looks like an overgrown Queen Anne's Lace, reaching up to
15 feet tall, but it can cause phytophotodermatitis, a chemical reaction on
skin due to sun exposure.
Why is it a problem?
Giant Hogweed presents a serious public health threat, the stems and leaves
are covered with small hairs coated with poisonous sap. Even the slightest
touch can cause painful blisters and severe skin irritation. The symptoms
can take a couple of days to develop, the skin becomes highly photosensitive
producing large watery blisters however the effects can be long lasting,
with contact resulting in recurrent dermatitis. This is a particular concern
for small children who may use the stems as pea-shooters or pretend
telescopes resulting in horrific injuries.
It is a threat to biodiversity out competing native species and monopolising
local environments. On riverbanks once it has died back in the winter the
area is vulnerable to erosion.
Coming into contact with the sap of giant hogweed, followed by exposure to
sunlight, can produce painful, burning blisters, also known as
photo-dermatitis. Hogweed stems contain a large amount of juice that squirts
out when stems are broken or cut. Contact with the toxic sap usually happens
when people cut down hogweed plants without taking precautions.
Hogweed Burns on Arms
Hogweed Burns on Ankle
Children are attracted to the large, hollow stems for
pretend swords or telescopes. Also, children may run through hogweed patches
and brush up against broken stems. Both activities can cause children to be
burned by hogweed sap.
When removing hogweed or working near it, wear protective clothing, such as
gloves, long sleeves, pants, hats and protective eyewear to prevent skin
contact with the sap. If skin comes into contact with sap, wash immediately
with soap and water. Burned skin is very sensitive to sunlight, so keep any
exposed areas covered when outside.
Contact your healthcare provider as soon as possible after exposure and tell
him or her you suspect you have touched giant hogweed. If they are
unfamiliar with the plant, have them contact the county or state noxious
weed program for more information.
After the burns subside, darkened areas or scars can persist for several
years. The affected areas remain sensitive to sunlight so it is important to
keep the burned areas away from direct sunlight as much as possible.
Sadly, children are the most likely to get hurt by hogweed because they
don't know they should avoid it. It is very important to check parks and
greenspace areas where children are playing to make sure there is no hogweed
in the area.
What does it look like ?
Very tall plants,
8-14 feet in height
Stems that are 2-4
inches in diameter with rigid hairs, purple blotches and are hollow
Leaves that are
unevenly lobed and up to 5 feet long
up to 2 feet across occurring from mid June to late July
Dead stems that
remain upright throughout the winter
Bears a close
resemblance to native cow parsley and hogweed
Has a reddish
purple stem with fine spines that make it appear furry (like a stinging
Has spotted leaf
May grow to 3-5m
Leaves can be up
to 1.5m wide
Plants can take up
to four years to reach their full height and flower
Flowers in June
Flower heads are
commonly 500mm (0.5m) wide
Flowers can each
produce 50,000 seeds every year
Seeds can stay in
the soil for up to 15 years before they develop.
If you must handle this plant be careful not to come into
contact with sap from broken stems. You should be well covered, wearing
gloves, long sleeves and long pants.
Giant Hogweed is a short lived perennial weed that reproduces only by seed
and is predominantly found along rivers, streams and wet land areas, but can
also be found in pastureland. The key to long term management of Giant
Hogweed is to limit seed production.
Often people will attempt to cut the flowering head off in
hopes of limiting seed production. However at this time of year, most
flowering heads of giant hogweed have already set mature seed. Therefore
cutting off seed heads now will only put you at risk of coming into contact
with the clear watery sap while helping the plant spread its seed around.
What You Should Do if You See Giant
Certainly Giant Hogweed is an invasive species which could
reproduce in an uncontrolled manner if left on its own
Oliver Reichl, a professional naturalist who lives in the Canadian 1000
Islands region say he can identify this invasive species and stop its
To contact Oliver
Contact the municipality where you live and they should
remove it or kill the weed
Hogweed Control .pdf
Coquitlam looking to coral giant hogweed
Giant hogweed creeps around Guelph