Have you spotted giant hogweed? Try to remove it in spring, advises OMAFRA specialist

Thursday, May 14, 2015

by SUSAN MANN

Spring is the best time to get rid of giant hogweed with followup extermination in the fall, says provincial agriculture ministry weed management lead Mike Cowbrough.

The fall followup removal is needed “because sometimes you’d have new seedlings that germinate later in the season,” he notes. “If you can tackle this plant in the spring and in the fall, then that’s very successful.”

For spring removal, “you’re largely dealing with plants that are at knee level or below” in height. Otherwise the plant can hit eight to 10 feet  (2.5 to four metres) in height, have leaves up to three feet (one metre) in breadth and thick two- to four-inch (five to 10 cm) hollow stems. Its stems and the undersides of its leaves have coarse hairs, while its large, umbrella-shaped flowers are white and can be more than one foot (30 cm) in diameter.

The risk of exposure to the plant’s noxious sap “is a lot higher” with the bigger plants, he says, noting exposure happens when plant material is broken open.

In a May 12 press release, Conservation Halton says the plant produces sap that sensitizes skin to ultraviolet light, which can cause severe and painful burns and blisters. Conservation Halton is Halton Region’s conservation authority. The region is located about 40 kilometres west of Toronto.

The invasive giant hogweed species is easiest to identify when the plant is huge and flowering later in the summer. “Every year for the past 10 years we get media calls in July” because people are tackling the removal of it and someone gets burned from the noxious sap, he says.

The release says people should wear protective clothing, including waterproof fabric with long sleeves, high shoes, gloves, face and eye protection when trying to remove it. Small patches can be handled by digging up plants in the spring. For larger patches, herbicides can be used according to label directions.

Cowbrough says spot spraying with glyphosate or Roundup, the trade name of glyphosate herbicide, works very well.

Digging up giant hogweed plants is tricky because the roots can extend beyond three feet (one metre) into the ground. Similar to dandelions, the entire root of giant hogweed has to be removed for extermination to be successful. If any roots are left in the ground, the plant will grow back.

The plant is rarely a problem for farmers, Cowbrough says. It mainly grows along streams and creeks but it can occasionally spread inland to pasture and forage fields. BF