Rapid spread of Wild Parsnip (Pastinaca sativa) and Giant Hogweed or Cow Parsnip (Heracleum mantegazzianum) along our roadsides has set off spray programs using the Dow herbicide Clearview in eastern Ontario municipalities.
Alarmist media reports warned us to not go near the parsnips without eye and full body protection. Many were alarmed. But many others, particularly farmers, had been living with the parsnips for decades. They managed because they learned that the severe skin burning and eye damage from the parsnips was caused by plant sap that was activated by exposure to sunlight. Burns could be avoided by washing off the oily sap with soap and water before going out in the sun. So there was an alternative to widespread spraying with Clearview.
Health Canada's testing of Clearview's effects is reported as incomplete but here are the basics that are known.
The active chemical in Clearview mimics the growth hormones (auxins) in plants, disrupts the natural chemistry of the plant's cells and prevents plant growth. The active chemical is aminopyralid which is highly water-soluble and can move in the soil and surface waters for up to 533 days. Predictions from computer models were that it posed negligible risk to humans and to wildlife.
Clearview also contains 9.5% of another chemical (metsulfuron-methyl) that stops cell division in plants at very low doses and also is highly water-soluble. It lasts for up to180 days in soil and about three weeks in water. As a spreading agent Clearview also contains Gateway which includes petroleum distillates that are toxic to aquatic organisms.
Clearview lasts about two years in soil so its effects on parsnips will stop and respraying will be needed after two years.
Clearview is not selective in the sense that it kills only wild parsnips; it damages or kills many shrubs and trees for up to two years if sprayed near or on them.