Nature Notes – From the Backyard to the Biosphere. Some Thoughts on Noxious Weeds

The lovely late summer wildflowers are out in full bloom at this time of year — real charmers for those who enjoy this last show of colours, textures and smells. However, given what this display represents — the flowering of a group of plants loosely referred to as weeds — it can be nothing short of an alarming example of disarray and a threat to the well-being of other systems.

There is no doubt that the yellows, whites and blues of these flowering plants brighten up roadsides and ditches across the continent, adding visual interest to the landscape for those passing by. However, these plants, many of which are for one reason or another considered noxious weeds, serve as a prime example of the importance of perspective in determining how vegetation is viewed.

Revered or reviled, scraggly or elegant, fragrant or lacking in scent, of high wildlife value or of ecological concern due to invasive tendencies — weeds have a very interesting background. The business of classifying plants as species of real concern has in most cases been related to their impact on agriculture. Species considered detrimental to crop productivity are listed provincially and federally as noxious. These plants are considered to have such negative potential impact on farmland that people occupying or owning property containing listed species are obliged by law to control and/or remove them. Non-compliance means fines and, potentially, further legal ramifications.

Ecologically speaking, weed control regulations can be of as much benefit to nature as to the farm field. Some of the very same plants that pose a threat to agriculture also have strong negative impacts on natural systems and controlling their spread would be a boon to all. This is particularly so in cases where the plant in question is an introduced species with a tendency to take over.

However, noxious weed classification is not always an environmental bonus. One of the most unfortunate instances of such classification is that of milkweed. Considered noxious almost right across the board, the regulation of this plant is such that landholders are duty-bound to eradicate it, whether by chemical or physical means.

Several species of this plant (e.g. common, swamp, and butterfly milkweed) exist in Canada, and each is essential for the well-being of one of our most familiar and well-loved insects, the Monarch butterfly. While adult Monarchs are able to feed on the nectar of a variety of plants, the only larval food source for this insect is the milkweed. Hence, Monarch populations can only exist in areas in which milkweed is present. Suffering population losses of up to 90% in recent years, this insect’s future is in real question, in part due to the destruction of its larval host plant, the “?noxious’ milkweed.

Not all cases of weed classification place agriculturalists and environmentalists at such odds, however. Thistles (Canada thistle and nodding thistle, for example) and burdock are two examples of plants introduced from afar that can take over both agricultural and non-agricultural areas. The rapid and significant growth of these Eurasian species, the fact that they effectively shade out, and in some cases even chemically suppress other plants, mean that thistles and burdock can cause significant impact in both farming and natural systems.

Interestingly, though, both thistles and burdocks do best in disturbed conditions, even in their homeland where native insect predators and plant pathogens exist to exert some means of control. Disturbed conditions are those in which, for one reason or another (e.g. overgrazing), the natural balance has been thrown off. What we view to be a problem (the presence of this plant) can therefore be seen to be directly related to a type of landscape alteration that allows it to become a problem in the first place. Controlling these plants sustainably over the long term may thus require examining landscape management techniques as much as investigating removal strategies. It is an interesting lesson in weed science.

Another dimension to the weed story is the fact that a plant may be classified as noxious because of its effect on human health, rather than its impact on crop productivity. Ragweed is the prime example of a plant thus categorized. Seen by many allergy sufferers to be the bane of their existence, the effects of this wind-pollinated plant can be felt hundreds of kilometres away from the source. Causing misery amongst those at the receiving end, who could argue with the listing of this introduced species as noxious?

One odd problem is, however, associated with the listing of this species in that it is often confused with another plant, one that is both native to Canada and of high wildlife value – that being the goldenrod. People unclear on plant identification and highly motivated by their understandably strong dislike of Ragweed, often mistakenly remove goldenrod when trying to eradicate their true nemesis. Such confusion is not so good on a number of fronts. Providing a supply of seeds to birds and other animals throughout the winter, and acting as the larval host plant for a particular species of insect, goldenrod is a plant of ecological importance, making its removal a significant, but common, mistake.

So, as in many aspects of life, it seems that things are never as simple as they seem. A weed is not a weed is not a weed. With an ever-increasing number of invasive introduced species raising their heads these days, the issue of classification, of understanding the ecological and economic impacts of these plants, and figuring out how to deal with them responsibly will only continue to grow in importance. With so many angles to every noxious weed story, it seems that this will be a difficult web to untangle.

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