General Description: Biennial or perennial,
flowering only once in its lifetime and reproducing only by seed. Plants
forming rosettes to 1m (40in.) high the first year; in the second year,
either sending up a flowering stem, or remaining vegetative and producing
a very large rosette of huge leaves, these including their petioles, up
to 2m (80in.) high, and flowering in the third year.
Giant hogweed (A - stand of mature 2-yr-old plants; leafy
canopy is about 1.5m (5ft) tall and seed-bearing stems about 3m (10ft)
tall; B - a small plant beginning to flower; C - lower part of a stem
and leaf petiole with a hunting knife for scale).
Giant hogweed. Second-year plant with rosette leaves
up to 125cm (50in.) high plus a central stem 2 leaves emerging through
the top of the rosette. The central stem will later produce a very large
umbel of flowers.
Stems & Roots: flowering stems up to
5m (16ft) high and up to 10cm (4in.) in diameter, hollow except at the
nodes; both stems and petioles having conspicuous reddish-purple flecks
throughout and sometimes nearly solid purple near the base; lower stem
often very rough with sharp-pointed, irregularly-spaced bumps.
Leaves: Leaf blades of rosette leaves very
large, up to 1m (40in.) across, compound with 3 large deeply cut leaflets,
each leaflet with deep irregular lobes and coarse, sharp teeth on all
margins; their petioles often with sharp-pointed bumps similar to those
on the flowering stems; leaves on the flowering stem of similar shape
but smaller, the upper ones often not divided but just deeply 3-lobed.
Flowers & Fruit: Inflorescence a compound
umbel up to 120cm (4ft) across, consisting of many (approximately 30 to
50) branches, each branch ending in a simple umbel with approximately
30 to 40 flowers; the whole compound umbel with a flat bottom and gently
rounded top; petals white or rarely pinkish and up to 12mm (½in.)
long; fruit oval in outline, 7- 3mm (¼-½in.) long, 6-10mm
(¼-2/5in.) wide and 1-3mm (1/25-1/8in.) thick, smooth to softly
hairy, with usually 4 prominent dark-coloured oil tubes. Flowers from
June to August.
Habitat: Giant hogweed was introduced from
Europe, presumably as an ornamental, and has escaped along roadsides,
streambanks and waste areas in scattered localities in southern Ontario.
Similar Species: It is distinguished by its
huge size, its very large, compound leaf blades, its tall, thick, hollow,
often sharply roughened stems, and its large flat-topped compound inflorescence
with white flowers and large, flat fruits with prominent dark-coloured
oil tubes. Flowering inflorescences are often heavily infested by aphids.
Caution: It has been implicated as a cause
of severe dermatitis in a few susceptible individuals.
