Polygonum aviculare L. (1753)

More info for the term: density

Prostrate knotweed seeds form a persistent seed bank [32]. Some prostrate knotweed seeds (<1%) were viable after 19.7 years of burial in subarctic conditions near Fairbanks, Alaska. Seeds buried at shallower depths lost viability faster than those buried at greater depths; over the course of the study, the annual rate of viability decline was 40% for seeds buried at 1 inch (2 cm) and 29% for seeds buried at 6 inches (15 cm) [30]. From mine sites in the United Kingdom, prostrate knotweed seeds germinated from soil samples stored for 4 years, and germinated from samples taken from as deep as 7 feet (2 m) in the soil [43].

The density of prostrate knotweed seeds in the soil seed bank is variable, and may be high even in areas where prostrate knotweed does not occur in the extant vegetation. At saline sites in Ohio, the mean number of seeds found in 100 cm² of soil ranged from approximately 50 to 225 [58]. Seeds of prostrate knotweed were found at a low density (4.3 seeds/m²) in the seed bank of a forested woodlot in southern Ontario [21]. In northeastern Ohio, prostrate knotweed was not present in the extant vegetation but occurred in the soil seed bank (2,631.6 seeds/m²) of a highly saline saltpan [51]. In Argentina, prostrate knotweed was present in the soil seed bank of 2- to 4-year-old successional fields but was not present in the extant vegetation. It was a dominant species in nearby croplands [39].

Plants green or bluish green, green after drying, sometimes whitish from powdery mildew, homophyl-lous or heterophyllous. Stems prostrate to erect, branched, flex-uous, 5-200 cm. Leaves: ocrea 3-15 mm, proximal part cylindric or ± funnelform, distal part silvery, hyaline, soon disintegrating into persistent fibers or nearly completely deciduous; petiole 0.3-9 mm; blade green to gray-green, narrowly elliptic, lanceolate, elliptic, obovate, or spatulate, 6-50(-60) × 0.5-22 mm, margins flat, apex acute, obtuse, or rounded; stem leaves 1-4 times as long as adjacent branch leaves; distal leaves overtopping flowers. Inflorescences axillary; cymes uniformly distributed or aggregated at tips of stems and branches, 1-6(-8)-flowered. Pedicels enclosed in or exserted from ocreae, 1.5-5 mm. Flowers closed or semi-open; perianth 1.8-5.5 mm; tube 20-57% of perianth length; tepals overlapping or not, green or reddish brown with white, pink, or red margins, petaloid, not keeled, oblong to obovate, often cucullate in fruit; midveins branched or unbranched, thickened or not; stamens 5-8. Achenes enclosed in or exserted from perianth, light to dark brown, ovate, (2-)3-gonous, 1.2-4.2 mm, faces subequal or unequal, apex not beaked, edges slightly concave, dull, usually coarsely striate-tubercled, sometimes obscurely tubercled; late-season achenes common or not, 2-5 mm.

More info for the terms: cover, density, fire management, invasive species, natural, prescribed fire, shrubs

Impacts: Most reported impacts of prostrate knotweed are related to its establishment in crop fields [123]. Prostrate knotweed is problematic in >60 crop species worldwide. Its density in agricultural fields was as high as 28.3 plants/m², as was recorded in a barley field in Alberta. Prostrate knotweed establishment reduces yield for some crops. Its stems may inhibit the mechanical harvest of other crops (e.g., onions, carrots) (review by [32]) and may act as an alternate host for crop pathogens [123]. Prostrate knotweed is also considered a nuisance in lawns, sidewalks, and paved areas (review by [32]).

Prostrate knotweed's impact on native plant communities is not well documented. A weed information guide suggests that dense mats of prostrate knotweed may smother herbaceous species and small shrubs [157]. Prostrate knotweed also has allelopathic qualities (review by [32]). In laboratory tests, soil collected from under prostrate knotweed inhibited the growth of several plant species, including Bermuda grass, Madagascar dropseed (Sporobolus pyramidatus), lambsquarters, sorghum (Sorghum bicolor), and Creole cotton (Gossypium barbadense). The soil used in this study was collected 4 months after prostrate knotweed plants died in the fall, suggesting that toxins may persist in the soil. Prostrate knotweed aboveground parts, roots, and root exudates also inhibited germination and growth of several crop and nonnative plant species [1].

Control: In all cases where invasive species are targeted for control, no matter what method is employed, the potential for other invasive species to fill their void must be considered [20]. Control of biotic invasions is most effective when it employs a long-term, ecosystem-wide strategy rather than a tactical approach focused on battling individual invaders [97].

Fire: For information on the use of prescribed fire to control this species, see Fire Management Considerations.

Prevention: It is commonly argued that the most cost-efficient and effective method of managing invasive species is to prevent their establishment and spread by maintaining "healthy" natural communities [97,127] (e.g., avoid road building in wildlands [145]) and by monitoring several times each year [76]. Managing to maintain the integrity of the native plant community and mitigate the factors enhancing ecosystem invasibility is likely to be more effective than managing solely to control the invader [74].

Weed prevention and control can be incorporated into many types of management plans, including those for logging and site preparation, grazing allotments, recreation management, research projects, road building and maintenance, and fire management [146]. See the Guide to noxious weed prevention practices [146] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.

Cultural control: Laboratory studies report that extracts from some cover crops, including rye (Secale cereale) and brown mustard (Brassica juncea), reduced germination of prostrate knotweed seeds and rootlet and shoot length of prostrate knotweed seedlings [52].

Physical or mechanical control: Mechanical control methods alone are usually not effective at controlling prostrate knotweed, but integration with other control methods (e.g., chemical) may improve treatment effectiveness. Soil solarization controlled prostrate knotweed in some areas (review by [32]). In interior Alaska, roadside prostrate knotweed seedlings establishing 2 years after fire were manually pulled in approximately 15 minutes. The following year, no prostrate knotweed seedlings were observed [31]. To prevent seed dispersal, a weed information guide suggests cutting plants prior to seed set [157] (e.g., late May in Pennsylvania [71]).

Prostrate knotweed's low stature makes mowing treatments largely ineffective [154]. Bark mulching favored prostrate knotweed in apple orchards (review by [32]). Flaming and hot-steaming did not control prostrate knotweed in Nova Scotia and Slovakia (Rifai and others 2001 as cited in [32])

Biological control: As of this writing (2010) no biological control agent has been identified to control prostrate knotweed. In North America, prostrate knotweed hosts several insects, nematodes, fungi, and viruses (review by [32]). In garden experiments in Pennsylvania, a fungal rust killed all prostrate knotweed seedlings. Seedlings emerging the following year also died, and the entire prostrate knotweed population was killed [71].

Biological control of invasive species has a long history that indicates many factors must be considered before using biological controls. Refer to these sources: [151,162] and the Weed control methods handbook [144] for background information and important considerations for developing and implementing biological control programs.

Chemical control: Both pre- and postemergent herbicides are effective at controlling prostrate knotweed (review by [32]), though a flora reports that prostrate knotweed resists herbicides [44]. The effectiveness of chemical control decreased with time in one cropping system experiment [175]. In commercial agricultural fields in California, exposure to several soil fumigants reduced the percentage of viable prostrate knotweed seeds. In areas exposed to the fumigant, 2.7% of seeds were viable, compared to 36.4% viability in areas not exposed to the fumigant [67].

Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but they are rarely a complete or long-term solution to weed management [23]. See the Weed control methods handbook [144] for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.

Integrated management: No information is available on this topic.

More info for the term: presence

Prostrate knotweed flowers are hermaphroditic. Chasmogamous and cleistogamous flowers may occur on the same plant. Most sources suggest that prostrate knotweed self-pollinates, though the presence of chasmogamous flowers suggests that cross-pollination is possible. There are numerous reports of flower visitation by insects [32]. In the Sacramento Valley of California, representatives of more than 36 insect taxa were observed feeding on the nectar of prostrate knotweed. Because flowers are often at or near ground level, they attract both aerial and terrestrial insects [22].