Smartweeds

Ladysthumb, Polygonum persicaria L. = Persicaria maculosa Gray

Pennsylvania smartweed, Polygonum pensylvanicum L. = Persicaria pensylvanica (L.) Small

Images above: Left: Ladysthumb seedling (Scott Morris, Cornell University). Right: Ladysthumb flowers (Scott Morris, Cornell University).

Images above: Upper left: Pennsylvania smartweed ocrea (Scott Morris, Cornell University). Upper right: Pennsylvania smartweed inflorescence (Joseph DiTomaso, University of California, Davis). Bottom: Pennsylvania smartweed plant (Randall Prostak, University of Massachusetts).

Identification

Other common names: 

  • Ladysthumb:  lady's thumb, smartweed, persicary, spotted smartweed, heartweed, spotted knotweed, red shanks, willow-weed, lovers-pride, lady's-thumb, ladythumb, heart's ease
  • Pennsylvania smartweed:  pinkweed, purple-head, glandular persicary, hearts-ease, swamp persicary, smartweed

Family:  buckwheat family, Polygonaceae

Habit:  Upright or sprawling, branched summer annual herbs.

Description:  Seedling have hairy edged cotyledons.  First true leaves are lance to ellipse shaped, hairy on upper surfaces and edges.  Seedling stems can be erect or prostrate.  Leaf-stem junctions are swollen and covered with transparent, papery membranes (ocreas) that extend up and around the stem. 

  • Ladysthumb:  Cotyledons are round-tipped, lance shaped, and 0.13-0.5” (0.3-1.3 cm) long by 0.13-0.3” (0.3-0.8 cm) wide.  Seedling stems are brown to bright red-pink.  Young leaves are lightly hairy on their upper surfaces and pale green with a black spot sometimes present on the leaf surface.  Stems are sometimes lightly hairy, otherwise they are similar to Pennsylvania smartweed stems.
  • Pennsylvania smartweed:  Cotyledons are ellipse to lance shaped and 0.13-1” (0.3-2.5 cm) long by 0.13-0.5” (0.3-1.3 cm) wide.  Seedling stems are pink at the base.  Young leaves are hairless, with purple-hued undersides.  The hairless, red-purple stems change angle at lower nodes, giving the young plant a zig-zag shape. 

Mature plants have green to red, leaning or upright, freely branching stems.  Leaf-stem junctions are swollen and covered by an ocrea.  Leaves are alternate, smooth to lightly hairy, and lance shaped to elliptical, and often have a black or purple spot in the center of the blade surface.  Roots are fibrous from a small taproot. 

  • Ladysthumb:  Stems are 1-3 ft (0.3-0.9 m) tall. Leaves are 1.25-6” (3-15 cm) long by 0.2-0.7” (0.5-1.8 cm) wide and sometimes covered in short, flat hairs; they lack the long stalks of Pennsylvania smartweed.  The darkened spot is often present on the blade surface.  Ocreas have a row of 0.06” (0.15cm) long bristly hairs at the top and extend further up the stem than they do up the leaf.
  • Pennsylvania smartweed:  Flat, stiff hairs are present on the 1-4 ft (0.3-1.2 m) tall stem.  The stalked leaves are 2-6” (5-15 cm) long by 1.25” (3 cm) wide and hairless to sparsely hairy.  The darkened spot is inconsistently present, and is often only a subtly different shade of green.  Ocreas are hairless, and wrap the stem and leaf to the same height.

Flowers are small, white to red, and clustered at stalk ends. 

  • Ladysthumb:  Flowers are pink to red-purple and rarely white. Clusters are 1” (2.5 cm) long.
  • Pennsylvania smartweed:  Flower stalks are hairy and occasionally sticky.  Flowers are pale to bright pink or white.  Clusters are 1.5” (3.8 cm) long.

Fruits and Seeds:  Seeds are flat, shiny, black, pointy tipped, and round to oval shaped.

  • Pennsylvania smartweed:  Seeds are 0.13” (0.33 cm) wide.
  • Ladysthumb:  Seeds are 0.1” (0.25 cm) wide, and occasionally 3 sided.

Similar species:  Tufted knotweed [Polygonum cespitosum Blume var. longisetum (Bruijn) A.N. Steward] ocreas also have bristles, but the bristles are longer, over 0.25” (0.6 cm) and the plant is shorter, less than 1 ft (30 cm).  Pale smartweed (Polygonum lapathifolium L.) has hairless ocreas, spotless, purplish-green, 2-6” (5-15 cm) long leaves, 3” (8 cm) long nodding flower clusters, and brown seeds. 

Management

Crop rotation is an important component in managing Pennsylvania smartweed and ladysthumb.  Since nearly all the emergence of these species occurs by late spring, tillage before a summer planted crop like tomatoes or a late planting of soybean will largely eliminate these species for that year.  A winter grain crop will be competitive against these weeds when they emerge in spring.  High crop sowing density and close row spacing (4” or 10 cm) can suppress ladysthumb in spring wheat, and these tactics are probably helpful in any cereal grain (Mertens and Jansen 2002).  These species die out of the seed bank at an intermediate rate, and consequently grains following alfalfa were found to have 27-29% as many smartweeds as grains following grains (Ominski et al. 1999).  These species can, however, establish in later years of a forage crop and the hay must therefore be mowed before seed set. 

Tine weed spring grains and row crops pre- and post-emergence to kill seedlings.  Since seedlings commonly emerge from up to 2” (5 cm), tine weeding will be more effective than rotary hoeing, though a well-timed rotary hoeing will kill part of the population.  If weed density is high, tine weed winter grains in the spring as seedlings emerge.  Since the seedlings do not become tall quickly, throwing 2” (5 cm) or more of soil into the crop row can effectively bury most seedlings that escape tine weeding in row crops.

Organic and synthetic mulches can effectively suppress these weeds.  These species are likely to respond to fertility amendments at least as much as the crop, so avoid over fertilization.

Ecology

Origin and distribution:  Ladysthumb is a native of Eurasia where it occurs from western Europe eastward through central Russia and the Middle East to northern India and southward across North Africa.  It also occurs in Japan and has been introduced into Australia, New Zealand and North and South America (Simmonds 1945).  In North America, it occurs throughout the conterminous U.S.A and southern Canada and northward into the Yukon and Alaska (USDA Plants).  Pennsylvania smartweed is native to eastern North America.  It occurs in most areas from the Atlantic to the prairie states and provinces but is more sporadic in the intermountain and far western states (USDA Plants).

Seed weight:  ladysthumb, 1.4-4.0 mg, mean 2.0 mg (EFBI), 1.4-2.7 mg (Simmonds 1945), 1.8 (Stevens 1932); Pennsylvania smartweed, 3.6 mg (Stevens 1932), 4.5 mg (Stoller and Wax 1973), 6.8 mg (Jordan et al. 1982).

Dormancy and germination:  The seeds of both species are dormant when shed from the parent plant and require several months of after ripening before they will germinate (Boumeester and Karssen 1992, Jordan et al. 1982).  A few weeks of cold, wet conditions break dormancy (Baskin and Baskin 1987, Boumeester and Karssen 1992, Jordan et al. 1982, Timson 1965).  Once dormancy has been broken, ladysthumb germinates better at 86 °F (30 °C) than at 50 or 68 °F (10 or 20 °C) (Boumeester and Karssen 1992), and day-to-night temperature fluctuations of at least 14 °F (8 °C) greatly increase germination (Thompson and Grime 1983).  Similarly, non-dormant seeds of Pennsylvania smartweed germinate best in warm, fluctuating temperatures, for example, 86/59 or 95/68 °F (30/15 or 35/20 °C).  However, exposure to temperatures over 59 °F (15 °C) under conditions unfavorable for germination re-induces dormancy of ladysthumb (Boumeester and Karssen 1992), and drying also has been reported to induce secondary dormancy (Simmonds 1945).  As both species show greatly reduced emergence during summer, induction of secondary dormancy by warm, dry soil conditions probably occurs in Pennsylvania smartweed also.  Nitrate stimulates germination of ladysthumb, but the effect is weak and only acts on relatively new seeds (Bouwmeester and Karssen, 1992).  Germination of Pennsylvania smartweed was greater than that of several other common annual weeds under wet conditions, but lower under dry conditions (Raynal and Bazzaz, 1973). 

Seed longevity:  In experiments in annually tilled soil, ladysthumb seeds declined at a rate of 20-28% per year (Lutman et al. 2002), but, in a similar experiment, seeds declined at 37% per year (computed from Barralis et al. 1988).  In several five-year experiments in which the top 3” (7.5 cm) of soil was stirred three times per year, seeds of ladysthumb declined by an average of 38-43% per year (computed from Roberts and Neilson, 1980).  In another experiment, the number of seedlings emerging in an annually tilled soil declined 38% per year (Popay et al. 1994).  However, some seeds of ladysthumb can survive in the soil for 20 to 30 years if left undisturbed (Timson 1965, Toole and Brown 1946).

Pennsylvania smartweed seeds also can survive up to 30 years of burial (Toole and Brown 1946).  Pennsylvania smartweed seeds buried in late October had 40-63% viability 10 months later (Stoller and Wax 1973).  In an Alaskan study, undisturbed seeds declined by an average of 25% per year and, after burial for 19.7 years, 3.3% of seeds were still viable (Conn et al., 2006).

Season of emergence:  Both species emerge in early to mid-spring, with some emergence continuing into late spring for Pennsylvania smartweed (Doll 2002, Stoller and Wax 1973, Werle et al. 2014) and early summer for ladysthumb (Boumeester and Karssen 1992, Simmonds 1945, Roberts and Neilson 1980). 

Emergence depth:  Ladysthumb seedlings emerge best from the top 1.6” (4 cm) of soil but a few seedlings can emerge from as deep as 2.4” (6 cm) (Chancellor 1964, Barralis et al. 1988).  Pennsylvania smartweed emerges readily from anywhere in the top 2” (5 cm) of soil and a few seedlings can emerge from 4” (10 cm) (Stoller and Wax, 1973). 

Photosynthetic pathway:  C3

Sensitivity to frost:  Neither species tolerates temperatures below 32˚ F (0˚ C) (USDA Plants).   A 27 °F (-3 °C) frost killed about half the leaves on Pennsylvania smartweed (Stevens, 1924).  

Drought tolerance:  Ladysthumb tolerates a range of soil moisture from drought (daily wilting) to continuous flooding (Sultan and Bazzaz, 1993b).  Flexibility in allocation of resources to roots and optimization of carbon assimilation in relation to transpiration explain the adaptive responses of this species to moisture conditions (Heschel et al. 2004).  Pennsylvania smartweed does best in moist, but well drained habitats and is more tolerant of overly wet than dry conditions (Pickett and Bazzaz, 1976, Raynal and Bazzaz 1973).  It maximizes biomass production under saturated soil conditions, but maximizes seed production under field capacity conditions (Haukos and Smith 2006).  Photosynthesis of Pennsylvania smartweed declines more rapidly than velvetleaf or giant foxtail with increasing drought stress, probably because it closes stomates and maintains a higher plant water potential than the other species (Wieland and Bazzaz 1975). 

Mycorrhiza:  Mycorrhiza have been reported in some samples of ladysthumb (Harley and Harley 1987) and absent in others (Dhillion and Friese 1994, Harley and Harley 1987).

Response to fertility:  Ladysthumb is moderately to highly responsive to fertility, with substantial differences among populations. Plant size increased 3-10 fold when balanced N:P:K fertility increased from very low to high rates.  Excessively high fertility neither helped nor harmed the plants (Sultan and Bazzaz 1993c). 

Moderate fertilization of an agricultural field with 15:30:15 fertilizer increased growth of Pennsylvania smartweed by 43% (Mabry et al. 1997).  The equivalent of 50 lb/A (56 kg/ha) each of N, P, and K added to a low fertility potting mix increased growth about 3-fold, but higher fertility rates did not cause additional growth (Lee et al.1986, Zangerl and Bazzaz 1983a).  Thus, Pennsylvania smartweed appears to be less responsive to fertility than many crops. 

Soil physical requirements:  Pennsylvania smartweed and ladysthumb tolerate a wide range of pH from 4.0-8.5 (USDA Plants).  Pennsylvania smartweed occurs on fine to course textured soils (USDA Plants), and ladysthumb is found on a wide variety of soils including muddy river banks, silty alluvium, heavy clay, sand, black muck, peat, cinders, and manure heaps (Simmonds 1945). Both species are moderately well adapted to anaerobic conditions and commonly occur in wetlands (USDA Plants).

Response to shade:  Pennsylvania smartweed and ladysthumb are generally considered shade-intolerant (USDA Plants).  Ladysthumb growth and seed production was decreased by shade (Sultan and Bazzaz 1993a), however, this species adapts to shade by reducing leaf thickness and allocating more resources to leaves (Griffith and Sultan 2005, Sultan and Bazzaz 1993a).  Pennsylvania smartweed growth is significantly reduced by shade (Lee et al. 1986, Zangerl and Bazzaz 1983a) and maximum photosynthesis is only reached in full sunlight (Zangerl and Bazzaz 1983b). 

Sensitivity to disturbance:  Ladysthumb tolerates wheel traffic and trampling by livestock.  If clipped repeatedly in the vegetative state it becomes dwarfed.  If clipped when flowering, it may overwinter in areas with a mild winter (Simmonds 1945).  Stem cuttings are capable of regenerating into plants (Sultan and Bazzaz 1993a,b,c).

Time from emergence to reproduction:  Pennsylvania smartweed and ladysthumb begin flowering at 6-9 weeks after emergence and plants can have mature seeds by July (Doll 2002, Pickett and Bazzaz 1976, Simmonds, 1945).  Both species continue producing seeds until fall (USDA Plants).

Pollination:  Ladysthumb is primarily self-pollinated but some cross pollination by insects probably also occurs (Simmonds 1945).  Pennsylvania smartweed is self-compatible and pollinated by insects, including ants (Kubetin and Schaal 1979). 

Reproduction:  When growing with relatively little competition, ladysthumb typically produces 200-800 seeds per plant, but large plants may produce 1,200 (Simmonds 1945) to 4,500 seeds (Salisbury 1978).  In a spring wheat crop, however, plants produced 40-150 seeds per plant, depending on row spacing and seeding rate of the crop (Mertens and Jansen 2002).

When grown as a monoculture in North Carolina, Pennsylvania smartweed produced 93,000 to 119,000 seeds per plant, whereas when grown with cotton, it produced 19,000 to 22,000 seeds per plant (Askew and Wilcut 2002).  A dense population of Pennsylvania smartweed growing with giant foxtail and common ragweed produced an average of 60 seeds per plant (Raynal and Bazzaz 1975).

Dispersal:  Seeds of both species pass through grazing animals intact (Harmon and Keim 1934, Simmonds 1945) and thus disperse when animals are moved and when manure is spread (Mt. Pleasant and Schlather 1994).  Cottontail rabbits eat both species, but only the smaller seeds of ladysthumb pass through the rabbits unharmed (Staniforth and Cavers 1977).  Seeds of ladysthumb have reportedly been moved in mud on the feet of gulls (Simmonds, 1945).  In the 1940's, ladysthumb was a common contaminant of forage legume and grass seed, with red clover being particularly prone to contamination (Simmonds 1945).  Poorly cleaned forage and cover crop seed probably continues to be a mode of dispersal today.  Pennsylvania smartweed is found in irrigation water (Wilson 1980).  Ladysthumb seeds float (Simmonds 1945) and are dispersed in irrigation water (Kelley and Bruns 1975).  As both species are commonly found along the edges of streams and rivers (Kelley and Bruns 1975, Staniforth and Cavers 1977), seeds are likely deposited in lowland fields during flood events.

Common natural enemies:  Birds and cottontail rabbits eat seeds of both species (Simmonds 1945, Perry and Uhler 1981, Staniforth and Cavers 1977).  The widespread specialist aphid Capitophorus hippophaes infested an experimental population of Pennsylvania smartweed in Illinois but had little influence on growth and reproduction (Mabry et al. 1997).  Pennsylvania smartweed seeds frequently carry pathogenic fungi that damage or kill young plants (Kirkpatrick and Bazzaz 1979).

Palatability:  Ladysthumb and Pennsylvania smartweed are considered to have low forage quality and palatability for grazing animals (USDA Plants, Simmonds 1945, Temme et al. 1979). However, Pennsylvania smartweed was as palatable to sheep as oats (Marten and Anderson 1975) and had crude protein content similar to and digestibility early in the season higher than that of a fescue/legume mix (Bunton et al. 2020).  Pennsylvania smartweed is a valued species in wetlands because its seeds are abundant and have superior nutritional quality for supporting migratory birds and other wildlife (Haukos and Smith 2006).

References:

  • Askew, S. D., and J. W. Wilcut.  2002.  Pennsylvania smartweed interference and achene production in cotton.  Weed Science 50:350-356.
  • Barralis, G., R. Chadoeuf, and J. P. Lonchamp.  1988.  Longevité des semences de mauvaises herbes annuelles dans un sol cultivé.  Weed Research 28: 407-418.
  • Baskin, J. M., and C. C. Baskin.  1987.  Temperature requirements for after-ripening in buried seeds of four summer annual weeds.  Weed Research 27:385-389.
  • Bouwmeester, H. J., and C. M. Karssen.  1992.  The dual role of temperature in the regulation of the seasonal changes in dormancy and germination of seeds of Polygongum persicaria L.  Oecologia 90:88-94.
  • Bunton, G., Z. Trower, C. Roberts, and K. W. Bradley.  2020.  Seasonal changes in forage nutritive value of common weeds encountered in Missouri pastures.  Weed Technology 34:164–171.
  • Chancellor, B. J.  1964.  Depth of weed seed germination in the field.  Proceedings of the Seventh British Weed Control Conference, pp. 607-613.
  • Conn, J. S., K. L. Beattie, and A. Blanchard.  2006.  Seed viability and dormancy of 17 weed species after 19.7 years of burial in Alaska.  Weed Science 54:464-470.
  • Dhillion, S. S., and C. F. Friese.  1994.  The occurrence of mycorrhizas in prairies: Application to ecological restoration.  Thirteenth North American Prairie Conference pp. 103-114.
  • Doll, J.  2002.  Knowing when to look for what: Weed emergence and flowering sequences in Wisconsin.  https://extension.soils.wisc.edu/wp-content/uploads/sites/68/2016/07/Doll-2.pdf
  • EFBI.  Ecological Flora of the British Isles.  http://www.ecoflora.co.uk/
  • Griffith, T. M., and S. E. Sultan.  2005.  Shade tolerance plasticity in response to neutral vs green shade cues in Polygonum species of contrasting ecological breadth.  New Phytologist 166:141-147.
  • Harley, J. L., and E. L. Harley.  1987.  A check-list of mycorrhiza in the British flora.  New Phytologist 105:1-102.
  • Haukos, D. A., and L. M. Smith.  2006.  Effects of soil water on seed production and photosynthesis of pink smartweed (Polygonum pensylvanicum L.) in playa wetlands.  Wetlands 26:265-270.
  • Harmon, G. W., and F. D. Keim.  1934.  The percentage and viability of weed seeds recovered in the feces of farm animals and their longevity when buried in manure.  Journal of the American Society of Agronomy 26:762-767.
  • Heschel, M. S., S. E. Sultan, S. Glover, and D. Sloan.  2004.  Population differentiation and plastic responses to drought stress in the generalist annual Polygonum persicaria.  International Journal of Plant Science 165:817-824.
  • Jordan, J. L., D. W. Staniforth, and C. M. Jordan.  1982.  Parental stress and prechilling effects on Pennsylvania smartweed (Polygonum pensylvanicum) achenes.  Weed Science 30:243-248.
  • Kelley, A. D., and V. F. Bruns.  1975.  Dissemination of weed seeds by irrigation water.  Weed Science 23:486-493.
  • Kirkpatrick, B. L., and F. A. Bazzaz.  1979.  Influence of certain fungi on seed germination and seedling survival of four colonizing annuals.  Journal of Applied Ecology 16:515-527.
  • Kubetin, W. R., and B. A. Schaal.  1979.  Apportionment of isozyme variability in Polygonum pensylvanicum (Polygonaceae).  Systematic Botany 4:148-156.
  • Lee, H. S., A. R. Zangerl, K. Garbutt, and F. A. Bazzaz.  1986.  Within and between species variation in response to environmental gradients in Polygonum pensylvanicum and Polygonum virginianum.  Oecologia (Berlin) 68:606-610.
  • Lutman, P. J. W., G. W. Cussans, K. J. Wright, B. J. Wilson, G. M. Wright, and H. M. Lawson.  2002.  The persistence of seeds of 16 weed species over six years in two arable fields.  Weed Research 42:231-241.
  • Mabry, C. M., M. Jasieński, J. S. Coleman, and F. A. Bazzaz.  1997.  Genotypic variation in Polygonum pensylvanicum: nutrient effects on plant growth and aphid infestation.  Canadian Journal of Botany 75:546-551.
  • Marten, G. C., and R. N. Anderson.  1975.  Forage nutritive value and palatability of 12 common annual weeds.  Crop Science 15:821-827.
  • Mertens, S. K., and J. H. Jansen.  2002.  Weed seed production, crop planting pattern, and mechanical weeding in wheat.  Weed Science 50:748-756.
  • Mt. Pleasant, J. M., and K. J. Schlather.  1994.  Incidence of weed seed in cow (Bos. sp.) manure and its importance as a weed source for cropland.  Weed Technology 8:304-310.
  • Ominski, P. D., M. H. Entz, and N. Kenkel.  1999.  Weed Suppression by Medicago sativa in subsequent cereal crops: a comparative study.  Weed Science 47:282-290.
  • Perry, M. C., and F. M. Uhler.  1981.  Asiatic clam (Corbicula manilensis) and other foods used by waterfowl in the James River, Virginia.  Estuaries 4:229-233.
  • Pickett, S. T. A., and F. A. Bazzaz.  1976.  Divergence of two co-occurring successional annuals on a soil moisture gradient.  Ecology 57:169-176.
  • Popay, A. I., T. I. Cox, A. Ingle, and R. Kerr.  1994.  Effects of soil disturbance on weed seedling emergence and its long-term decline.  Weed Research 34:403-412.
  • Raynal, D. J., and F. A. Bazzaz.  1973.  Establishment of early successional plant populations on forest and prairie soil.  Ecology 54:1335-1341.
  • Raynal, D. J., and F. A. Bazzaz.  1975.  The contrasting life-cycle strategies of three summer annuals found in abandoned fields in Illinois.  Journal of Ecology 63:587-596.
  • Roberts, H. A., and J. E. Neilson.  1980.  Seed survival and periodicity of seedling emergence in some species of Atriplex, Chenopodium, Polygonum and Rumex.  Annals of Applied Biology 94:111-120.
  • Salisbury, E.  1978.  A note on seed production and frequency.  Proceedings of the Royal Society of London. Series B. Biological Sciences 200:485-487.
  • Simmonds, N. W.  1945.  Polygonum persicaria L.  Journal of Ecology 33:121-131.
  • Staniforth, R. J., and P. B. Cavers.  1977.  The importance of cottontail rabbits in the dispersal of Polygonum spp.  Journal of Applied Ecology 14:261-268.
  • Stevens, O. A.  1924.  Some effects of the first fall freeze.  The American Midland Naturalist 9:14-17.
  • Stevens, O. A.  1932.  The number and weight of seeds produced by weeds.  American Journal of Botany 19:784-794.
  • Stoller, E. W., and L. M. Wax.  1973.  Periodicity of germination and emergence of some annual weeds.  Weed Science 21:574-580.
  • Sultan, S. E., and F. A. Bazzaz.  1993a.  Phenotypic plasticity in Polygonum persicaria. I. Diversity and uniformity in genotypic norms of reaction to light.  Evolution 47:1009-1031.
  • Sultan, S. E., and F. A. Bazzaz.  1993b.  Phenotypic plasticity in Polygonum persicaria. II. Norms of reaction to soil moisture and the maintenance of genetic diversity.  Evolution 47:1032-1049.
  • Sultan, S. E., and F. A. Bazzaz.  1993c.  Phenotypic plasticity in Polygonum persicaria. III. The evolution of ecological breadth for nutrient environment.  Evolution 47:1050-1071.
  • Temme, D. G., R. G. Harvey, R. S. Fawcett, and A. W. Young.  1979.  Effects of annual weed control on alfalfa forage quality.  Agronomy Journal 71:51-54.
  • Thompson, K., and J. P. Grime.  1983.  A comparative study of germination responses to diurnally-fluctuating temperatures.  Journal of Applied Ecology 20:141-156.
  • Timson, J.  1965.  Germination in Polygonum.  New Phytologist 64:179-186.
  • Toole, E. H., and E. Brown.  1946.  Final results of the Duvel buried seed experiment.  Journal of Agricultural Research 72:201-210.
  • USDA Plants.  Natural Resources Conservation Service.  Plants Database.  http://plants.usda.gov
  • Werle, R., L. D. Sandell, D. D. Buhler, R. G. Hartzler, and J. L. Lindquist.  2014.  Predicting emergence of 23 summer annual weed species.  Weed Science 62:267-279.
  • Wieland, N. K., and F. A. Bazzaz.  1975.  Physiological ecology of three codominant successional annuals.  Ecology 56:681-688.
  • Wilson, R. G. Jr.  1980.  Dissemination of weed seeds by surface irrigation water in western Nebraska.  Weed Science 28:87-92.
  • Zangerl, A. R., and F. A. Bazzaz.  1983a.  Response of an early and late successional species of Polygonum to variations in resource availability.  Oecologia 56:397-404.
  • Zangerl, A. R., and F. A. Bazzaz.  1983b.  Plasticity and genotypic variation in photosynthetic behavior of an early and a late successional species of Polygonum.  Oecologia 57:270-273.