by Leslie Cox; Thursday, July 25, 2013

Heracleum mantegazzianum, also known as giant cow parsley, giant cow parsnip, wild rhubarb and wild parsnip, is a member of the carrot family. An incredibly tall perennial, it can grow to eight feet (2.5 m)…15 feet (4.5m) when in flower…or even taller.

The flower itself is made up of many small white florets in a large, flat-topped umbel…much like Queen Anne’s Lace, only bigger.

Stout, hollow stems are either reddish-purple or green with purple spots in colour and two to four inches (5-10 cm) in diameter. Dark green, compound leaves are deeply incised and can measure up to five feet (1.5 m) across.

(FYI…the giant hogweed plants in the photo are on a steep downward slope behind my granddaughter, Shelby, so they look shorter than they really were. This photo was taken in Victoria along Dallas Rd.)

Native to southwest Asia, Heracleum mantegazzianum was introduced as a garden ornamental into North America in 1917. In BC, it can be found in the central Kootenay region, Lower Mainland, Fraser Valley, on the Gulf Islands and Vancouver Island from Victoria as far north as Campbell River.

While preferring damp soil conditions and full sun, giant hogweed will also tolerate shade and drier aspects. Many roadside ditches, right-of-ways, vacant lots, wetlands and agricultural areas have been infested with this weed.

It does not take long. Once the plants reach flowering maturity, each one is capable of producing up to 100,000 winged seeds. Distribution can be far-flung. The seeds are also capable of floating for three days before sinking should they land in water. And, they can remain viable in the soil for up to 15 years.

In the Pacific Northwest, Heracleum mantegazzianum sprouts in early spring, either from the roots or from seed. Seedlings are identified from mature plants by their more palmate leaf pattern. Flowering occurs in June with seed formation following in July. However, young plants can take anywhere from two to four years to flower which makes identification a little more difficult.

The real danger with giant hogweed lies in its extreme toxicity. The stalks and blister-like pustules on the stems exude a clear watery sap containing glucosides called furanocoumarins. These toxic organic compounds are photodynamic agents, meaning they operate through light energy by absorbing ultraviolet photons. Certain plants naturally produce furanocoumarins to combat fungus diseases and to ward off insects and animals.

It is this phototoxin ability in the sap that sensitizes the skin to ultraviolet radiation if you should brush against the plant. Symptoms of photosensitization are heat, redness, itchiness, swelling and blistering which can last for many days, weeks…even months.

It depends on the levels of furanocoumarins in these plants, as well as an individual’s DNA make-up that determines photosensitivity.

Phototoxic plants are generally found within four families: Umbellifereae (or Apiaceae), Rutaceae, Moraceae (fig) and Leguminoseae (or Fabaceae). Beware the sap in parsnips which is in the Apiaceae family, as a friend painfully found out.

If you should get any sap on your skin, immediately wash with soap and cold water. Keep exposed skin out of sun and treat affected area as for a surface burn.

If you get the sap in your eyes, it can cause temporary…sometimes permanent blindness. Immediately flush eyes out with copious amounts of water and see a doctor right away.

Properly outfitted, you can deal with removing giant hogweed yourself when the plant first appears in spring. Dress in waterproof clothing with long sleeves, use safety goggles and heavy rubber gloves. The sap will seep through non-waterproof clothes and regular gardening gloves.

Dig out the plant, going down a minimum of four inches to make sure you are getting all of the root. Dispose of the plants in heavy duty garbage bags…heavy enough that the sides will not be pierced by any sharp pieces of stalk. Do not compost them!