Lee Valley & Veritas Gardening
Newsletter
Lee Valley 35 Years  
  Volume 8, Issue 8 - August 2013    
 
What Is It?
What Is It?
 
Identifying this weed can be difficult, since a number of species look very similar, including cow parsnip, purplestem angelica, woodland angelica, valerian, lovage and Queen Anne's lace. This plant grows much larger than those mentioned, however. In fact, in ideal conditions a mature specimen can grow to 5.5m (18'). While its white flower clusters do resemble those of Queen Anne's lace, they tend to be more widely spaced and can form a flower head almost 1m (3.2') wide. When identifying this toxic plant, known as giant hogweed (Heracleum mantegazzianum), its huge size is a key indicator.
 
 
Giant hogweed flowers
  The flowers look similar to those of Queen Anne's lace.
Native to central and southwest Asia, giant hogweed was likely brought over as an ornamental plant and is now naturalizing across North America. This perennial, a member of the carrot family, can thrive in many habitats and grows particularly well in areas where the soil has been disturbed (wastelands, riverbanks, roadsides, along railroads, etc.). Depending on the conditions in which it grows, it can quickly dominate an area because of its size and ability to spread rapidly.
 
The plant is becoming renowned for its toxicity. If you encounter it, use extreme caution. Cutting the plant or even simply brushing against it can cause the sap to get on your skin. After exposure to sunlight, this will cause chemical burns. The painful blisters can appear within 48 hours after exposure and can recur for several years. Even after the blisters subside, purplish-colored scars can form. If you happen to get the sap in your eye, it can cause severe irritation and possibly blindness. If you do come into contact with the sap of this plant, the United States Department of Agriculture (USDA) recommends seeking medical attention immediately.
 
How will you know if the plant is giant hogweed and not one of its look-alikes? Aside from its massive size, the Ontario Ministry of Agriculture and Food suggests looking for two additional attributes particular to this weed — it has a thick, hollow, purple-blotched stem and very jagged, lobed leaves, both of which are covered with bristles similar to those found on the stinging nettle.
 
Thick purple-blotched stem and jagged leaves   Thick purple-blotched stem and jagged leaves
To identify giant hogweed, look for its thick purple-blotched stem and extremely jagged leaves.
 
If you do spot the plant, the USDA advises not to touch it, move it, cut it or weed whack it, and to seek advice from a professional plant control specialist.
 
Photos provided by Peter Smith, University of Guelph
 
Further Reading
www.invadingspecies.com
 
 
 
 
     
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