Polygonum aviculare
(prostrate knotweed)

P. aviculare is native to Europe, but has been extensively introduced elsewhere. It is now a widespread weed in nearly all the temperate regions of the world and, more locally, in warmer climates. Its range may still be extending. Its extension towards equatorial regions could be restrained by chilling requirements.

P. aviculare has a very wide ecological amplitude with respect to soil texture, reaction and humidity. However, it is largely restricted to sites of pH >5.0 (Grime et al., 1988). It is favoured by soil compaction and heavy nitrogen fertilization, but tolerates drought and low nutrient fertility (Grime et al., 1988). In climates with a marked dry season, it is usually restricted to irrigated crops. P. aviculare (or at least some of its varieties) is extremely resistant to trampling and is a frequent component of the flora of cart-tracks; accordingly, it can spread in agricultural areas damaged by vehicles and implements (Kress, 1988).

P. plebejum prefers moist soils, being particularly frequent along irrigation ditches (Pandey et al., 1995) and in low lying areas such as temporary ponds and pools (Sant and Kamlesh, 1979).

P. aviculare is typically a weed of spring crops (Tottman and Wilson, 1990). It is one of the relatively few weeds that are equally abundant in both cereals and broad-leaved crops and is a principal or serious weed of a wide range of crops in temperate climates (Holm et al., 1997). It is also a principal weed of specific warm climate crops, including soyabean, sweet potato, cotton, sugarcane and rice. It is locally frequent in orchards, especially in Mediterranean climates, in pastures, and in nurseries. Finally, it is a frequent weed of amenity turf and is particularly troublesome on heavily-worn and compacted areas such as football pitches.

P. aviculare regenerates exclusively by seed (Grime et al., 1988) and is often one of the most abundant species in seed banks of cultivated soils in temperate Europe (Macchia et al., 1996). Densities as high as 200 to 5000 seeds/m² are common (Holm et al., 1997), with most seeds being buried in the upper 5 cm of soil. Optimum germination depth is 0-2 cm. Seed viability in soil decreases exponentially with time, with less than 10% remaining viable after 2 years (Holm et al., 1997). Deep, undisturbed seeds are the most likely to remain dormant. Viable seeds have been recovered after up to 60 years of burial (Campagna and Rapparini, 1997; Holm et al., 1997).

P. aviculare has a cyclic dormancy pattern in soil; fresh seeds show innate dormancy and low temperatures stimulate release from dormancy of imbibed seeds (chilling requirement is typically <5-10 °C for 10 weeks) while high temperatures induce secondary dormancy. Therefore, P. aviculare usually germinates in a single flush during a short period early in the spring (between late February and early May in England, between August and November in Argentina). Seeds that fail to germinate in the spring enter secondary dormancy and no subsequent germination usually occurs in the summer (Courtney, 1968). In Argentina, the temperature range in which germination could occur was estimated to fall between 8°C and 25°C and the estimated required thermal time for germination of 50% of the non-dormant fraction was 80 degree-days above a base temperature of 0°C (Kruk and Benech-Arnold, 1998).

The first seeds are usually shed two months after emergence, and can be produced over an extended period of time (up to 6 months or more unless the plant is killed by frost, for instance from June to November in England (Grime et al. 1988)). A single plant can produce up to 6000 seeds, but this number is highly variable, depending on competition and resource availability (Meerts, 1995; Holm et al., 1997).

The removal of shoot apices resulted in reduced apical dominance and increased branching intensity and this was accompanied by reduced fitness estimates (McPhee et al., 1997).

P. aviculare, even defined in the narrow sense as specified in the Notes on Taxonomy and Nomenclature, is a highly polymorphic species. There exists considerable genetic variation in growth habit and life history traits, including growth rate, flowering date, number of seeds and life span (Meerts, 1992, 1995; Meerts and Garnier, 1996). Chilling requirement also varies among populations (Holm et al., 1997). In addition, the species shows extensive phenotypic plasticity in response to soil fertility, soil moisture, and disturbance regime (trampling) (Meerts, 1995). Genotypes with contrasting life-histories might be adapted to different cultivation practices and disturbance regimes. For instance, short-lived, early flowering genotypes could be favoured in crops subjected to weeding activities in early summer; by contrast, longer-lived genotypes can resume growth and reproduction after harvest of the crop and are therefore well adapted to cereals.

The flowers seem to be mostly (if not always) self-pollinated (Grime et al., 1988; Meerts et al., 1998), although they can be visited by insects (Bugg et al., 1987).

The high colonizing success of P. aviculare in various kinds of man-disturbed habitats has been ascribed to a suite of complementary attributes (Grime et al., 1988; Meerts, 1995): long lasting seed bank; extensive phenotypic plasticity; selfing mating system; and high genetic diversity compared to other, self-pollinating weeds. It has been shown that populations from arable fields, either tetra- or hexaploid, consist of fixed heterozygotes at several enzymatic loci (Meerts et al., 1998).

P. aviculare contains water-soluble allelochemicals, including long-chain fatty acids and phenolic glycosides, that inhibit germination and seedling growth of crops (lettuce, alfalfa, medic, rice) and several species of weeds, most notably Cynodon dactylon and Chenopodium album (Alsaadawi and Rice, 1982a, 1982b; Kloot and Boyce, 1982; Alsaadawi et al., 1983; Lovett et al., 1986; Chung et al., 1994)

The species has effective mechanisms of seed dispersal by human or other agencies. Seeds occur both as an impurity in the harvested crop and as a contaminant of sown seed. They may be dispersed in mud on footwear or tyre treads and can survive ingestion by stock or by birds (Grime et al., 1988). They can also be transported by irrigation water (Holm et al., 1997).

Local decreases in abundance of P. aviculare due to attacks by natural enemies have been reported in Belgium (Bulcke et al., 1994), Italy (Marocchi, 1994) and China (Hu et al., 1989). In the latter two cases, it was concluded that the chrysomelids Gastrophysa polygoni and Entomoscelis orientalis, respectively, deserved further investigation as promising agents for potential biological control programmes.

Environmental

• Erosion control or dune stabilization

Human food and beverage

• Emergency (famine) food

Materials

• Poisonous to mammals

Medicinal, pharmaceutical

• Traditional/folklore

See also Notes on Taxonomy and Nomenclature.

P. arenastrum is a closely related taxon of disputed rank. Flowers and fruits offer the most reliable discriminant characters: P. arenastrum typically has the tepals fused for one third to one half of their length, achenes 1.4-2.5 mm long, with two convex and one much narrower, concave, side (P. aviculare sensu stricto: tepals fused for less than one third of length, achenes 2-3.5 mm long, with three concave sides). In addition, P. arenastrum usually has a more prostrate growth habit and smaller leaves, but these and other vegetative traits show extensive phenotypic plasticity in the whole aggregate. P. arenastrum most often occurs in trampled sites and is apparently much less often found as a weed of cultivated fields than P. aviculare sensu stricto.

P. plebejum is a common weed in a restricted region of central Asia, where it occurs in wheat (Singh, 1997), onion (Iqbal et al., 1990), lentil (Dangol, 1990), paddy rice, maize and potato (Neogi and Rao, 1980). P. plebejum replaces, or co-exists with, P. aviculare as a widespread weed in a restricted region of central Asia, including India (Madhya Pradesh: Singh, 1997; Bihar: Pandey et al., 1995; Meghalaya: Neogi and Rao, 1980), Pakistan (Iqbal et al. 1980) and Nepal (Dangol, 1990). It is also present in Australia, Egypt, Taiwan and Vietnam (Holm et al., 1979), but its status as a weed in those countries is less clear. P. plebejum is smaller, with a short compact stem, 6-15 (-30) cm long, and short internodes. Leaves narrower, 6-25 mm long, 1-2 mm wide, with parallel sides. Perianth-segments ca 1.5 mm long, bright pink. Nut ca 1-1.5 mm long, black and shiny (Lousley and Kent, 1981).

P. patulum is a related species occasionally found as a weed from southern Europe and the Mediterranean area eastwards to Central Asia. It can be distinguished by its inflorescences, consisting of very lax, elongate slender terminal spikes with the flowers in the axils of much reduced, lanceolate-subulate bracts.

Other, taxonomically unrelated, species share a superficial resemblance to P. aviculare due to a prostrate growth habit, and small, greenish flowers in the axils of entire leaves, for instance: Euphorbia maculata, E. prostrata, E. nutans and Herniaria spp.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Careful control is necessary to prevent the build up of large seed reserves in persistent seed banks.
 

Cultural Control

The following methods have been successfully applied, but they are rarely sufficient on their own; they should preferably be combined with chemical control.

- hoeing with or without chain harrowing, in winter wheat in Iran (Rafii, 1993);

- two harrowings in winter reduced the need for pre-emergence treatment in beet in Italy (Re et al., 1996);

- solarization was successful in Spain (Dalmau et al., 1993) and China (Zhang et al., 1992);

- in an organic farming system the negative consequences of late weed development in wheat were diminished by undersowing with Medicago lupulina and Trifolium repens (Hartl, 1989);

- drilling (Fernandez Quintanilla et al., 1984) and shallow soil disturbance (tining) (Pollard and Cussans, 1981);

- use of long straw wheat varieties, sown at high densities with high nitrogen (Grundy et al., 1997).

- 99% decline of seed bank in 5 years was observed in autumn sown crops (wheat and rape) that were ploughed annually, during which period no return of weed seed was permitted (Lawson et al., 1993).

By contrast, P. aviculare was resistant to flooding in Japan (Tsuruuchi, 1986) and was favoured by bark mulching in apple orchards in Poland (Scibisz et al., 1995). P. aviculare was unpalatable to domestic white China geese (Wurtz, 1995), and grazing was therefore ineffective.

Chemical Control

P. aviculare is often reported as being difficult to control, even though true resistance to herbicides has only rarely been demonstrated; to triazine (van Oorschot and Straathof, 1988) and amitrole (Bulcke et al., 1988). As a rule, it is much more resistant when mature than at the seedling stage.

Pre-emergence treatments are recommended, with simazine, terbuthylazine, metribuzin, ethofumesate, cyanazine, metamitron, lenacil, or combinations of these, such as metamitron + ethofumesate + lenacil (May and Hilton, 1991; Strijckers, 1992). Seedlings and very young plants (less than six leaves) can be controlled with ioxynil, bromoxynil + bromofenoxim or fluoroxypyr (Strijckers, 1992).

In beets, successful chemical treatments include: phenmedipham or phenmedipham + desmediphame (Strijckers, 1992); triflusulfuron with phenmedipham (Toth and Peter, 1997); pre-emergence treatment with glyphosate, followed by metamitron + lenacil at reduced dose (Campagna and Rapparini, 1997), with two harrowings in winter reducing the need for pre-emergence treatment in Italy (Re et al., 1996). There is some indication of antagonism between chloridazon and triflusulfuron which can result in poor control of P. aviculare (Fisher et al., 1995).

In spring barley, thiameturon-methyl + metsulfuron can be used early or late post-emergence (Espir, 1987). The activity of sulfonylurea herbicides (metsulfuron-methyl and, or tribenuron-methyl and, or triasulfuron) was greatly improved by two adjuvants (an alkoxylated fatty acid polymer and an organosilicone non-ionic surfactant) (Davies et al., 1997). Thifensulfuron-methyl with metsulfuron-methyl is used as a post-emergence herbicide for control of broad-leaved weeds in wheat, barley, oats and triticale in Australia (Arends and Pegg, 1990).

Clomazone is a post-emergence treatment in poppies (Papaver somniferum) in Tasmania (Macleod, 1997).

More than 90% control was achieved with isoxaben, pronamide [propyzamide] and terbutryn in a common vetch-oat intercrop (Caballero et al., 1995).

In peas, pendimethalin + prometryn are used as pre-emergence herbicides in the UK (Brown et al., 1991); bentazone + pendimethlin are also successful (Birkler, 1988). In chickpea, pre-emergence application of cyanazine, metribuzin and terbuthylazine, or their mixtures, provided an effective level of control of broadleaf weeds (Dastgheib et al., 1995).

In Jordan, the critical period of weed interference in bean (Phaseolus vulgaris) was between 14 and 21 days after emergence (Qasem, 1995).

Aclonifen (+ linuron) are successfully used in maize, sunflowers, tobacco, tomatoes, potatoes and peas in Italy (Anon., 1993).

Aziprotryne + clethodim are in use in cabbage (Dastgheib and Popay, 1995).

In Pimpinella anisum, trifluralin, linuron and prometryn, applied before sowing, pre-emergence and post-emergence, respectively, gave good control (Tepe et al., 1994).

In flax and linseed, bentazone with bromoxynil applied post-emergence are effective (Brochard and Gosselin, 1995).

In apple orchards, pendimethalin or terbacil can be added to simazine (Clay et al., 1990).

In amenity turf, if P. aviculare seedlings are evident before reseeding during April-May in the UK, 2,4-D can be applied 2-3 weeks before seeding (Shildrick, 1990); pendimethalin is also efficient (Johnson and Murphy, 1989). Imazethapyr is used for control in grass stands in the USA (Ferrell et al., 1992).

Mamarot and Rodriguez (1997) provide suggestions for use of herbicides and herbicide mixtures in a wide range of crops in France. These include atrazine in maize and sorghum, bromoxynil plus bentazon inn linseed, thifensulfuron plus tribenuron, or metsulfuron in wheat and barley, oxadiazon in soyabean and sunflower, propyzamide in peas.

Biological Control

Two natural enemies have been suggested as being of potential interest for biological control: the chrysomelids Entomoscelis orientalis and Gastrophysa polygoni, both of which have been circumstantially reported to cause local decrease in abundance of P. aviculare during particular years (Hu et al., 1989; Marocchi, 1994). However, extensive field tests have apparently never been performed.