Mature Giant Hogweed

Mike Cowbrough, Weed Management Field Crops Program Lead

When we were here in May we talked about May - early May, late April's the best time of year to control this species - and now while we're in the end of July here's why: the risk of coming into contact with the sap of Giant Hogweed is at its most when you physically break up vegetation.

Take a look at that seed head, the seed head is now produced seed, it's almost fully mature, you can even see at the edge here the seeds actually fallen off, so if I come in here and try and cut these plants down, all that's going to happen is this seed's going to shatter and you're gonna expose yourself to the vegetation and leaf tissue and actually increase your risk of coming into contact with Giant Hogweed.

The one thing you have to appreciate about Giant Hogweed is it only flowers once in its lifetime. It's a perennial weed, it only reproduces by seed.

Within a population of Giant Hogweed you'll have a huge diversity of age of plants.

The age of plant, on something like this the literature is varied, but it can be anywhere from six to nine years old where it starts to produce a flower. They can range anywhere from maybe eight or nine feet tall, all the way up to fifteen feet tall.

This is one of the species that a lot of people confuse with Giant Hogweed, it's called Wild Carrot or Queen Anne's Lace. Here's the major difference: one, if it's flowering in less than three metres, it's not Giant Hogweed. The second thing is the flowering head of Giant Hogweed can span about a metre wide, so a huge width. Lastly, you look at the leaf of Wild Carrot or Queen Anne's Lace, keep in mind this the same genus and species as cultivated carrot, the leaf is very small, its very lacy it's not like Giant Hogweed which is big and more like a maple leaf shape.

So here's a couple of great learnings that we had in terms of management with the University of Guelph this year.

This was a fairly expansive research site. We had over fifteen different treatments here looking at mechanical and chemical control. A couple of the key things we tried is things that typically homeowners might try to do.

The first thing was spot applications of a round-up product following up to remove any vegetation that you missed the first time around and then throwing a bark mulch or some sort of mulch to stop new seeds from germinating. The second equally effective method was in that late-April, early May to go in with a shovel to dig as much of that root as possible, to lay some sort of black sheet of plastic overtop and even throw bark mulch on top of that just to smother those plants out.

So you have non-chemical options, you have chemical options, they both work.

Giant Hogweed's a fairly minor concern from an agricultural perspective.

The concern usually in hay or pasture fields is will it be toxic to my livestock? And livestock won't die from eating Giant Hogweed. That said, Giant Hogweed, because of the chemical compounds in it, they can affect the rates of production of livestock, right, so they won't gain weight as fast as they normally do. And secondly the chemical compounds have been known to interrupt hormone balances and can affect fertility in female livestock, so when we see it in those environments it is best just to manage it and get it out of there.

At this point, make note of it for next year, and do the work next year.

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