Spring is when many homeowners are outside doing work in their gardens, and part of that work is pulling weeds or invasive species which may be present. It is a great time to do this work in your garden before these plants get fully established, and is why in many cases now is the best time to remove them!
One invasive species which is best to be removed now, if you have it, is Giant Hogweed (Heracleum mantegazzianum). This plant has become well known due to its tremendous height and the dangers from its sap which has created plenty of public attention.
Giant Hogweed has two major negative impacts. Firstly, like all invasive plant species, it poses a threat to native plants and biodiversity. Secondly, Giant Hogweed is a public health hazard. It produces a noxious sap which sensitizes the skin to ultraviolet light. This is known as photosensitivity, which can result in severe and painful burning and blistering. It is important to avoid any skin contact with this plant.
If you have seen Giant Hogweed on your property, then now is the time to investigate, to see if any small hogweed plants are coming up, and take steps to control or eradicate them. Late in the summer, when Giant Hogweed is a giant plant with big white flowers, it is easy to spot, but that is not necessarily the ideal time to try and control it. In fact, you could be putting yourself at significant risk, from the sap in the plant’s stem and leaves. Right now in the spring, when the plants are just starting to grow, is the best time to try and control it, and prevent it from producing seeds.
If you are considering doing the removal yourself, it is very important to wear proper protective clothing (waterproof fabric with long sleeves, rubber boots, gloves, face and eye protection), so as to avoid skin contact with the sap. If you have a small patch of Giant Hogweed, then you can dig the plants up, but be sure the entire root is removed, and check the area again in two weeks for any re-growth.
Alternatively, if you have a large infestation, other control methods will need to be considered. In some cases a herbicide, used according to the label, and by a certified professional, is the best method of controlling this invasive plant.
Even if you undertook removal, or control, late last year, there likely will still be seedlings sprouting this year. To control an invasive species like hogweed you must be vigilant and return to the site to ensure control has been effective.
Giant Hogweed is known for its enormous size, reaching heights of 2.5 to 4m (8 to 14ft), with leaves up to 1m (3 ft) in breadth. It has a thick, 5 to 10cm (2 to 4in) hollow stem. Its stem and the undersides of its leaves are covered in coarse hairs. Its large, umbrella-shaped flowers are white in colour and can be more than 30cm (1ft) in diameter. You can find more information about Giant Hogweed on the Conservation Halton website.
Giant Hogweed has several lookalikes, including a smaller, harmless plant which has a similar (but much smaller) white flower called Queen Anne’s Lace. If you are unsure whether you have Giant Hogweed, feel free to seek confirmation from an expert. At Conservation Halton, you can contact Brenda Van Ryswyk, by phone 905-336-1158, ext. 2282 or send an email to Brenda or use the invasive species reporting form which can be found by clicking here.
For help identifying the young plants see the video produced by the Ontario Ministry of Agriculture, Food and Rural Affairs.
For general inquiries, questions on invasive species or to report a sighting anywhere in the province, you can use the Invasive Species Hotline: 1-800-563-7711. You can also report or find more information on invasive species through the Early Detection and Distribution Mapping System, or EDDMaps for short.