Wild buckwheat

Polygonum convolvulus L. = Fallopia convolvulus (L.) Á. Löve

Images above: Upper left: Wild buckwheat seedlings (Antonio DiTommaso, Cornell University). Upper right: Wild buckwheat mature fruits (Antonio DiTommaso, Cornell University). Bottom: Wild buckwheat plant (Antonio DiTommaso, Cornell University).

Identification

Other common names:  black bindweed, knot bindweed, bear-bind, ivy bindweed, climbing bindweed, corn-bind, climbing buckwheat, dullseed corn-bind, climbing knotweed, devil's-bindweed, blackbird bindweed

Family:  buckwheat family, Polygonaceae

Habit:  Twining summer annual herb.

Description:  Seedlings have two elongate, oval cotyledons that are 0.25-1.25” (0.64-3.18 cm) long by less than 0.25” (0.64 cm) wide, with round tips and a gritty, waxy surface.  Cotyledons are often at a 120° angle from one another.  The seedling stem is red-purple.  Young leaves are arrow shaped with a tapered, pointed tip and basal, backwards pointing lobes.  The first true leaves are blue-green on their upper surface and red on their lower surface.  All true leaves have a thin, papery covering extending up the stem from the base of the leaf stalk (ocrea).  Mature plants have smooth, weak, twining stems that branch near the base and reach up to 6.5 ft (2 m) in length.  Leaves are alternate, hairless, 0.75-2.5” (1.9-6.4 cm) long, arrow shaped with long pointed tips and backward pointing lobes, and attached by long leaf stalks.  Ocreae are two leaf-like appendages enclosing the stem (stipule-sheaths), each 0.13-0.2” (0.33-0.51 cm) long, and present at all leaf-stem junctions.  Leaves lower on the stem will be more triangular or heart-shaped than younger, narrower leaves near the tip.  The root is fibrous.  Greenish-pink or greenish-white flowers are grouped into clusters of 2-6 on a long stem arising from axils of upper leaves or on tips of small branches.  Individual flowers are 0.2” (0.5 cm) wide.  Seeds are dull, black, sharply three sided, and may be covered by the papery, brown remnants of flower structures.

Similar species:  Tartary buckwheat [Fagopyrum tartaricum (L.) Gaertn.] and domestic buckwheat (Fagopyrum esculentum Moench) do not twine onto other plants and have more compact inflorescences and larger seeds.    Hedge bindweed [Calystegia sepium (L.) R. Br.] and field bindweed (Convolvulus arvensis L.) resemble wild buckwheat, but both are perennials with thick spreading roots.  Leaves of wild buckwheat are more tapered and pointed at the tip than leaves of either of the bindweeds.  The lobes of hedge bindweed leaves are squarish in outline when laid flat whereas wild buckwheat and field bindweed leaf-lobes are pointed.  The flowers of both bindweeds are large and either white or pink, and they occur singly. This is unlike wild buckwheat flowers, which are small, green and clustered.  The bindweeds lack ocrea.

Management

Wild buckwheat is a severe weed in spring sown cereal grains and a problem in many other field and vegetable crops.  In cereal grains, management during preceding crops in the rotation is critical since options are limited in spring grains.  A vigorous winter grain crop will tend to suppress the seedlings.  Properly timed cultivation can control the weed in row crops like corn and soybean.  If this species is causing severe problems in your spring grains and you rotate with corn, cleaning up plants in the corn rows with a flame weeder may be worthwhile.  Including later planted crops like soybean, dry bean and summer vegetables in the rotation allows a period in the spring for a cultivated fallow which will reduce the seed bank going into a subsequent cereal grain the next year (Hume et al. 1983).  If time permits, work the soil to a depth of about 3” (7.6 cm) at about one-week intervals to insure that all seeds in the surface soil get exposed to enough warmth to prompt germination.

For this species, effectiveness of tine weeding (harrowing) and rotary hoeing are limited in both grains and row crops because many of the seedlings arise from depths below the seeding depth of the crop.  Some seedlings will be buried by a tine weeder, however, so weeding soon after the first flush of seedlings emerge can provide some control.  A dense planting of grains helps reduce losses in fields infested with wild buckwheat (Hume et al. 1983, Stevens 1947).  Although the species climbs, it does not begin climbing until about a month after emergence.  Rapid canopy closure and root proliferation from a densely sown crop can help suppress this species (Gruenhagan and Nalewaja 1969, Mertens and Jansen 2002).  Slightly lower than optimal nitrogen fertility suppresses wild buckwheat more than it does grain crops (Hume et al. 1983).  Because the majority of seeds remain on the plant at harvest, seed capture and destruction during harvesting operations is an option for reducing the seed population (Burton et al. 2017).

Cultivation in row crops should be aimed at uprooting plants while they are small, and burying newly emerged seedlings in the row.  Re-rooting becomes an increasing problem as the wild buckwheat grows.  Consequently, tools that cut are more effective against wild buckwheat than are those that dig.  If this species is a problem, consider investing in an implement that can work close to the crop when it is still small.

If you do not already have wild buckwheat, inspect uncertified grain and cover crop seed for the characteristic three-sided seeds of wild buckwheat before you sow it. 

Ecology

Origin and distribution:  Wild buckwheat originated in Europe, but is now found throughout the world’s temperate regions, and as far north as Greenland and Alaska (Hume et al. 1983).  The species occurs throughout the U.S.A. and Canada (USDA Plants).

Seed weight:  4.65-7.0 mg (Stevens 1932), 6.5 mg (Gardarin et al. 2010), 6.9 mg (Gaba et al. 2019).

Dormancy and germination:  Seeds are dormant when shed from the parent plant, and very few will germinate until subjected to a prolonged period of cold, wet conditions (Timson 1966).  Dormancy results from a hard seed coat which, when scarified, allows germination (Hsiao 1979, Metzger 1992).  A cold, moist period of at least two months with daily alternating temperatures between 36 and 50 °F (2 and 10 °C) will break dormancy and induce maximum germination similar to mechanical seed coat removal (Metzger 1992).  Non-dormant seeds germinate well at 68-77 °F (20-25 °C) (Metzger 1992, Stevens 1947, Timson 1966).  Most seeds enter secondary dormancy during hot, dry summer weather (Hume et al. 1983) and then require a second period of cold, wet conditions before germination is again possible.  Light (Hsiao 1979) and nitrate (Timson 1966) do not affect germination.  In the field, germination and emergence is slow, requiring from 9 to 31 days (Forsberg and Best 1964).

Seed longevity:  Seed longevity may vary between populations (Thompson et al. 1997).  Most viable seeds germinate the year after production, but some may survive for several decades deep in cool moist soil.  In soil stirred four times per year, the number of wild buckwheat seeds declined by 32-50% per year, whereas in undisturbed soil they declined at 20-25% per year (Roberts and Feast 1972, 1973).  In Alaska, annual mortality of buried seeds was 52% (Conn et al. 2006), and, in France, mortality was 46% (Barralis et al. 1988).  Based on these studies, the seed bank is moderately persistent but susceptible to management.

Season of emergence:  Most emergence occurs in early spring with some emergence continuing into summer (Doll 2002, Chepil 1946, Forsberg and Best 1964, Matthews 1971, Roberts and Feast 1973, Stevens 1947, Witts 1960).  Maximum emergence occurs at soil temperatures less than 60 °F (16 °C) (Matthews 1971).

Emergence depth:  Most seedlings emerge from anywhere in the top 2” (5 cm) of soil, but occasional seedlings emerge from as deep as 5” (13 cm) or more (Barralis et al. 1988, Chancellor 1964, Forsberg and Best 1964).  Emergence was often best from 0.4-1.6” (1-4 cm) depths rather than from the soil surface or from deeper (Chancellor 1964, Forsberg and Best 1964).

Photosynthetic pathway:  C3

Sensitivity to frost:  Wild buckwheat is killed by hard frost (Stevens 1924).

Drought tolerance:  This species absorbs soil moisture efficiently and competes well with crops under dry soil conditions (Hume et al. 1983). 

Mycorrhiza:  Wild buckwheat is not mycorhizzal (Harley and Harley 1987, Pendleton and Smith 1983).

Response to fertility:  The growth response of wild buckwheat to N is similar to wheat – both species continue to increase in size up to application rates beyond 214 lb N/A (240 kg N/ha) but the incremental increase with greater N is moderate (Blackshaw et al. 2003).  The species is highly responsive to P application rates up to 164 lb P2O5/A (183 kg P2O5/ha) (Blackshaw et al. 2004).  Increasing fertility tends to favor wild buckwheat more than field crops like wheat and flax (Hume et al. 1983). 

Soil physical requirements:  Wild buckwheat occurs on a wide range of soil types (Hume et al. 1983).  It is best adapted to heavier soil with good moisture holding capacity (Matthews 1971).  Soil compaction does not affect seed germination.

Response to shade:  The vining habit of wild buckwheat potentially allows it to avoid shade by climbing up competing crop plants (Hume et al. 1983).  However, the species does not begin twining onto crops until about a month after emergence, and it can be suppressed by shade if the crop canopy closes quickly.

Sensitivity to disturbance:  Since many individuals typically emerge from relatively deep in the soil, shallow disturbance with a tine weeder or rotary hoe leaves many individuals unaffected.  In contrast, tools that cut seedlings and young plants at or just below the soil surface kill the weed.  The fibrous root system holds soil and helps larger plants re-root after cultivation.

Time from emergence to reproduction:  On average, wild buckwheat begins flowering at 6 to 8 weeks after emergence and the first seeds mature about 3 weeks later in Saskatchewan (Forsberg and Best 1964).  In Wisconsin, 12 weeks were required between emergence and flower initiation (Doll 2002).  Maturation is faster in warm weather, but the plants are less vigorous (Hume et al. 1983).  Because of the indeterminant flowering habit of wild buckwheat, flowers, immature seeds, and mature seeds may be found on the same plant.  Up to half of the seeds of late emerging plants may not reach maturity by the first killing frost (Forsberg and Best 1964).

Pollination:  Wild buckwheat primarily self-pollinates, and seed set often occurs even though the flowers remain closed.  Even open flowers do not attract pollinating insects.  (Hume et al. 1983, Matthews 1971)

Reproduction:  Wild buckwheat reaches maturity midseason and continues flowering and releasing seeds for most of the growing season.  Large, old plants often lose most of their leaves but still continue maturing seeds.  Large plants can produce as many as 12,000-30,000 seeds (Forsberg and Best 1964, Stevens 1932).  Less than 3% of seed were shattered at harvest of field pea and 5-31% of seeds were shattered at harvest of spring wheat (Burton et al. 2017).

Dispersal:  Wild buckwheat spread throughout the world primarily in contaminated seed grain (Hume et al. 1983).  The seeds float and can spread in surface irrigation water (Wilson 1980).  They also move about on tires, tillage machinery and combines (Hume et al. 1983).  Wild buckwheat seeds can survive for several months in silage and survive rumen digestion for 24 hours (Blackshaw and Rode 1991), thus, the species probably disperses in manure.

Common natural enemies:  The beetle Gastrophysa polygoni eats the foliage and larvae of the fly Pegomyia setaria mine the leaves.

Palatability:  The seeds are edible but low in oil and protein.  The stems and leaves have low forage value for livestock.  (Hume et al. 1983)

References:

  • Barralis, G., R. Chadoeuf, et J. P. Lonchamp.  1988.  Longevité des semences de mauvaises herbes annuelles dans un sol cultivé.  Weed Research 28:407-418.
  • Blackshaw, R. E., and L. M. Rode.  1991.  Effect of ensiling and rumen digestion by cattle on weed seed viability.  Weed Science 39:104-108.
  • Blackshaw, R. E., R. N. Brandt, H. H. Janzen, T. Entz, C. A. Grant, and D. A. Derksen.  2003.  Differential response of weed species to added nitrogen.  Weed Science 51:532-539.
  • Blackshaw, R. E., R. N. Brandt, H. H. Janzen, and T. Entz.  2004.  Weed species response to phosphorus fertilization.  Weed Science 52:406-412.
  • Burton, N. R., H. J. Beckie, C. J. Willenborg, S. J. Shirtliffe, J. J. Schoenau, and E. N. Johnson.  2017.  Seed shatter of six economically important weed species in producer fields in Saskatchewan.  Canadian Journal of Plant Science 97:266-276.
  • Chancellor, B. J.  1964.  Depth of weed seed germination in the field.  Proceedings of the Seventh British Weed Control Conference, pp. 607-613.
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  • Doll, J.  2002.  Knowing when to look for what: Weed emergence and flowering sequences in Wisconsin.  University of Wisconsin.  https://extension.soils.wisc.edu/wp-content/uploads/sites/68/2016/07/Doll-2.pdf
  • Forsberg, D. E., and K. F. Best.  1964.  The emergence and plant development of wild buckwheat (Polygonum convolvulus).  Canadian Journal of Plant Science 44:100-103.
  • Gaba, S., P. Deroulers, F. Bretagnolle, and V. Bretagnolle.  2019.  Lipid content drives weed seed consumption by ground beetles (Coleoptera, Carabidae) within the smallest seeds.  Weed Research 59:170-179.
  • Gardarin, A., C. Dürr, M. R. Mannino, H. Busset, and N. Colbach.  2010.  Seed mortality in the soil is related to seed coat thickness.  Seed Science Research 20:243-256.
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  • Hsiao, A. I.  1979.  The effect of hypochlorite, gibberellic acid, and light on seed dormancy and germination of wild buckwheat (Polygonum convolvulus) and cow cockle (Saponaria vaccaria).  Canadian Journal of Botany 57:1735-1739.
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  • Metzger, J. D.  1992.  Physiological basis of achene dormancy in Polygonum convolvulus (Polygonaceae).  American Journal of Botany 79:882-886
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  • USDA Plants.  USDA Natural Resources Conservation Service Plants Database.  http://plants.usda.gov/java/
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