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LivingGardeningGardening in Almonte: More on Invasive Plants

Gardening in Almonte: More on Invasive Plants


More on Japanese Knotweed

The cards and letters keep on coming! I received a further comment from a reader to the effect that Japanese Knotweed “doesn’t set seeds – it spreads only by rhizomes”. I agree that it spreads virtually completely by rhizomes, but it does on occasion produce seeds. The information sheet produced by the Ontario Ministry of Agriculture, Food and Rural Affairs (which I generally regard as an authoritative source) states as follows:

Flowers & Fruit: Flowers individually small but numerous and forming showy, greenish-white branching panicles from the axils of upper leaves; flowers unisexual, the sexes on separate plants; seeds only rarely produced, these hanging from the branches, triangular, about 7mm (¼in.) long, shiny, enclosed in a papery, 3-winged, teardrop-shaped, dry calyx. Flowers from July to September.

I know of at least two very healthy stands of this plant in Almonte – one behind the Post Office and the other along the rail trail. Why is it of concern? Japanese Knotweed can severely degrade the quality of wetland and riparian habitats where it becomes established. Dense thickets of Japanese Knotweed can reduce sunlight penetration by more than 90 per cent and its thick mats of dead and decaying vegetation in fall/spring prevent other plant species from growing, by shading them out. This plant can significantly damage infrastructure. It is able to grow through concrete/asphalt up to 8 cm thick and building foundations. It is of particular concern in new housing developments. In the United Kingdom, developers must dispose of soil containing knotweed fragments at hazardous waste facilities. (Information Source: Ontario Invasive Plant Council).

Spread of Invasive Plants

While we can discuss whether the spread of invasive plants is primarily by rhizomes or by seed, the reality is that the primary vehicle for the spread of many invasive plants is the automobile! Gardeners plant many invasive plants because they are very attractive. Japanese Knotweed, for example, was introduced to North America as a horticultural plant in the late 19th century and was widely planted as an ornamental, for the purposes of erosion control, and as forage for livestock. And it can be a very attractive structural plant, resembling bamboo, but this is one tough hombre to confine!

Goutweed – the worst garden thug!

Goutweed is often seen in perennial gardens – I have found it the absolute worst thug to eliminate in a garden. The irony is that people still continue to plant it and offer it to other gardeners. As the following photos show there are two forms – a variegated version and the all-green one. If tightly controlled, for example, between a house foundation and a sidewalk, it can be quite attractive, but it has to be watched carefully as it spreads underground.

Aegopodium podagraria, commonly called ground elderherb gerardbishop’s weedgoutweedgout wort, and snow-in-the-mountain, and sometimes called English masterwort and wild masterwort is a perennial plant in the carrot family (Apiaceae) that grows in shady places.  Goutweed was widely cultivated as a vigorous perennial ground cover because of its strong spreading rhizome system, but the same characteristics enable it to escape from cultivation and invade lawns, hedges, gardens, roadsides and waste places, the habitats in which it occurs throughout most of southern Ontario. Some cultivated forms have variegated leaves, the leaflets being green near the centre but whitish around their margins. These are usually less aggressive than forms with entirely green leaves.

Once established, goutweed is nearly impossible to eradicate. The smallest piece of rhizome left in the ground will quickly form a sturdy new plant. All-green goutweed may be more persistent and spread more rapidly than ornamental, variegated goutweed varieties, making the all-green type particularly difficult to control. It readily spreads over large areas of ground by underground rhizomes. Once established, the plants are highly competitive, also in shaded environments, and can reduce the diversity of ground cover, and prevent the establishment of tree and shrub seedlings. The primary vector for dispersal to new areas is human plantings as an ornamental, medicinal or vegetable plant, as well as by accidentally spreading rhizomes by dumping of garden waste. It spreads rapidly under favourable growing conditions.

Attempts to control goutweed in a garden with landscape cloth, bark mulch, and hand weeding are largely unsuccessful because sprouting occurs from either rhizomes or root fragments left in the soil. Hand pulling, raking, and digging followed by monitoring to control goutweed may be effective; however, caution must be taken to remove the entire rhizome and root system. Removing flowers before seed set may help control the spread of goutweed. (Information sources: Ontario Ministry of Agriculture, Food and Rural Affairs and Wikipedia).

When advising people on how to eliminate this thug I usually advise that the only 100 per cent certain approach is to sell your property and move! The best control is to not plant it in the first place! If anyone offers it to you – run!!





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