The annual smartweeds Polygonum lapathifolium L., P. pensylvanicum L., and P. persicaria L. grow on alluvial river bars during the summer and autumn when their substrate is free of flood waters and physical disturbance. They are dependent upon dispersal for the annual recolonization of this habitat. Achenes are the only diaspores (dispersal units), and hydrochory (water dispersal) appears to be a major dispersal method.An experimental study of water dispersal of these species revealed several adaptive features. The coordination of fruit fall with the availability of the agent (flood water) ensures that the achenes are picked up. The duration of buoyancy is adequate for successful transport downstream. The presence and condition of the perianth, degree of water turbulence, and achene size are factors which determine the duration of buoyancy. Alteration of the degree of dormancy but no loss of viability occurs as a result of water transport. After deposition the achenes may remain dormant or germinate rapidly depending on environmental conditions. The deposition stage (and therefore, hydrochory) is unsuccessful for most achenes of P. persicaria, which rapidly lose viability in sites where deposition is likely to occur. The presence of this species on alluvial bars must be due to other means of dispersal.

Royle (Himalayan balsam) is an invasive alien annual up to 3 m in height with showy flowers that are generally pink or purplish. Native to the Himalayan region, I. glandulifera was first recorded in Canada in 1901 in Ottawa, and is now found in eight Canadian provinces: British Columbia, Manitoba, Ontario, Quebec, Nova Scotia, New Brunswick, Prince Edward Island and Newfoundland. Impatiens glandulifera is typically found in riparian habitats and may spread rapidly because its seeds are readily transported via waterways. Up to 2500 seeds are produced per plant and dispersed explosively up to 5 m from the parent plant. This can result in dense monotypic stands which prevent establishment of native plants and make stream banks vulnerable to erosion when the shallow-rooted plants die back. Impatiens glandulifera is susceptible to glyphosate but because herbicide use in riparian areas is not advised, other control methods such as hand weeding, mowing or flaming have been used. Methods for eradication are most successful when upstream populations are controlled first, as the plants spread downstream. Removal of I. glandulifera should be managed synchronously with non-native control measures and ideally be accompanied by planting native species to ensure the restoration of native species composition. The prognosis for curbing its spread in Canada seems poor as it has quite rapidly become established along waterways in many regions, following a pattern seen over the past two centuries in Europe.Key words: Impatiens glandulifera, Impatiens roylei, Himalayan balsam, impatiente glanduleuse, policeman's helmet riparian, invasive plant, weed biology Clements, D. R., Feenstra, K. R., Jones, K. et Staniforth, R. 2008. La biologie des plantes exotiques envahissantes au Canada. 9. Impatiens glandulifera Royle. Can. J. Plant Sci. 88: 403Á417. Impatiens glandulifera Royle (impatiente glanduleuse) est une annuelle envahissante pouvant mesurer jusqu'a`trois me`tres de hauteur. Ses fleurs tre`s voyantes sont ge´ne´ralement roses ou mauves. Cette plante originaire de l'Himalaya a e´te´signale´e pour la premie`re fois a`Ottawa en 1901, mais on la retrouve ap re´sent dans huit provinces (Colombie-Britannique, Manitoba, Ontario, Que´bec, Nouvelle-É cosse, Nouveau-Brunswick, Iˆle-du-Prince-É douard et Terre-Neuve). Impatiens glandulifera affectionne habituellement les milieux riverains et se multiplie rapidement, car les cours d'eau en transportent aise´ment les graines. Chaque plant donnera jusqu'a`2500 semences qui seront disperse´es de manie`re explosive jusqu'a`5 me`tres de distance de la plante me`re. Il en de´coule parfois de denses peuplements monotypiques qui empeˆchent les plantes indige`nes de s'e´tablir et peuvent rendre les berges vulne´rables al 'e´rosion lorsque meurent ces plantes peu profonde´ment enracine´es. Impatiens glandulifera est sensible au glyphosate, mais l'usage d'herbicides e´tant de´conseille´dans les zones riveraines, on recourt a`d'autres me´thodes de lutte, notamment le de´sherbage manuel, la tonte ou le br...

Opuntia fragilis (Nutt.) Haw. is restricted to south-facing rock outcrops on the shores of certain rivers and lakes in the boreal forest of southeastern Manitoba. Limitations in propagule dispersal, establishment, and maintenance were studied at Bird River (50°25′ N, 95°41′ W) between October 1979 and October 1982. Propagules studied were stem units (pads) because this cactus propagates entirely by vegetative means in this region. Mature terminal pads broke off most easily in the late spring while turgid and rolled downhill (geochory), thus establishing new plants locally. Some pads fell into the river and could float long distances (hydrochory) before being washed ashore. Perhaps less frequently, pads were dispersed over long distances on animals (epizoochory). Marked pads placed in potential habitats showed significantly greater establishment (α = 0.05) on south-facing outcrops than on north-facing or forest sites. Censuses (1979–1982) of an established colony indicated that pad number was increasing by 51% per year. Approximately 6% of the pads were dispersed from the outcrop. The present distribution of O. fragilis appears to be both habitat and dispersal limited.

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The annual smartweeds Polygonum lapathifolium L., P. pensylvanicum L., and P. persicaria L. are weeds of cereal crops, waste places, and wet ground in Canada. Their ‘weediness’ is attributed, in part, to the production of large numbers of achenes which are capable of lying dormant in the soil for several years or until suitable conditions arise for germination.Dormancy–germination characteristics of achenes from Ontario plants were examined in the laboratory and in the field. Three types of dormancy have been identified, each of which prevents germination under a different set of unfavourable conditions. An innate dormancy prevents germination in fresh achenes immediately prior to winter. An enforced dormancy prevents germination during the winter and in achenes which are deep in the soil. An induced dormancy is caused by hot or dry periods (at least in P. persicaria). Germination occurs as a series of flushes whenever achenes are brought to the soil surface and (or) whenever rainy spells occur.