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Reproductive Biology |
Department of Biology, University of Massachusetts–Boston, 100 Morrissey Boulevard, Boston, Massachusetts 02125 USA
Received for publication July 2, 2002. Accepted for publication November 8, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Fallopia japonica • germination • Japanese knotweed • Polygonaceae • Polygonum cuspidatum • Reynoutria japonica • sexual reproduction
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) and was being grown in the USA by 1877 (herbarium record, University of Massachusetts Amherst, Amherst, Massachusetts, USA). Fallopia japonica is a fast-growing and durable species and a prolific flower producer, making it a popular choice for planting as an ornamental, as forage, and for erosion control (Child and Wade, 2000
). With hollow, bamboo-like stems that can grow in excess of 3 m high, this species thrives in riparian ecosystems, where water is readily available, but is also found in disturbed areas such as roadsides and waste places. Once a ramet of F. japonica is established, it spreads clonally via rhizomes, forming virtual monocultures that displace native plant species and reduce available habitat for birds, mammals, and other organisms (Wade and Child, 2001
). The underground stem system makes established knotweed populations extremely difficult, if not impossible, to eradicate. Within the past several decades, managers of recreational areas are increasingly being forced to deal with tall, dense stands of knotweed that block access to parks and rivers. Limited techniques are currently available for eradicating F. japonica. Extensive removal of vegetation and soil is expensive, causes major disturbance, and creates problems with disposal. Herbicide application can be effective, but only if treatments are repeated over several seasons, and use of these chemicals is often restricted along waterways or near parks. Due to these issues, the focus remains on management of existing stands rather than eradication, for all but the smallest infestations.
The problem of F. japonica invasion has recently received attention on a global scale, as researchers have identified the species as a major problem in Europe, the United Kingdom, the United States, and Canada. In the UK, where F. japonica is so widespread that legislation has been passed making it illegal to plant or spread the species (Child et al., 1992
), it has been shown that F. japonica can hybridize with congeners F. sachalinensis (F. Schmidt) Ronse Decr. (Polygonum sachalinense F. Schmidt; giant knotweed) and F. baldschuanica (Regel) Holub (Polygonum baldschuanicum Regel, Russian vine, silver lace vine). However, the establishment of either true breeding or hybrid seed is considered a rare event (Bailey, 1994
; Bailey et al., 1995
).
The breeding system of F. japonica has been characterized as both dioecious and gynodioecious (Beerling et al., 1994
), though in New England it has been observed to be subdioecious, with male and female flowers on separate plants and males that sometimes set seed (J. Forman and R. V. Kesseli, unpublished manuscript). There is also a common misconception that the species does not spread by seed in the wild, though female plants typically bear thousands of fruits each fall. Few studies have been done on sexual reproduction of F. japonica species growing in North America. Locandro (1973)
reported that female plants often produced empty achenes and that although there were sites (in New Jersey) with high numbers of seedlings, they rarely developed beyond the three-leaf stage and never survived beyond mid-summer, as observed over 5 yr. The Element Stewardship Abstract for Japanese Knotweed, published by The Nature Conservancy, mentions that the species can reproduce by seed, but that it is not "a significant mode of reproduction" (Seiger, 1995
, section III). Seeds collected by Seiger from F. japonica in Washington, D.C., USA and germinated on filter paper following 2 yr of dormancy had germinability ranging from 50% to 63%, while seeds with no dormancy had germinability of less than 10%. These seedlings appeared to be hybrids with F. baldschuanica. No seedling establishment was observed in the wild (Seiger, 1993
).
In Europe, seedling establishment in the wild has been noted, but several documented cases have turned out to be hybrids between F. japonica and F. sachalinensis (Alberternst, 1995 as cited in Bailey, 1999
; Bailey et al., 1995
). Researchers have concluded that virtually all true F. japonica (referred to as var. japonica) in Britain is female (Hollingsworth and Bailey, 2000
). While Fallopia japonica, F. sachalinensis, and F. baldschuanica have been introduced to the USA, the extent of hybridization among these species is unknown.
In its native range of Asia, F. japonica reproduces both asexually and by seed, with seed dispersal in the fall and germination in spring (Maruta, 1976
). Plants in Asia are typically found in early successional habitats and are frequently among the first to colonize volcanic rock (Schnitzler and Muller, 1998
). First- and second-year seedlings in Japan grow alongside adults (Maruta, 1981
).
This study seeks to determine the capability for sexual reproduction in F. japonica and the possibility that it contributes to the dispersal of the species in North America, with a focus on populations in Massachusetts. Viability of seed was tested over a period of 3 yr in the laboratory. In addition, field sites were monitored for two seasons for evidence of seedling germination and establishment. Seedling viability was also examined both in the laboratory and in the field. The implications for management of this invasive plant are great if sexual reproduction is indeed a factor in the survival of the species.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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In February 2000, seeds from 24 parent plants were randomly assigned to soil-filled plugs in trays (14 x 7 plugs) and germinated at room temperature in full light for 6 wk. For each parent, 33 seeds were planted unless fewer seeds were available. To fill all seven trays, two additional seeds were used from one parent plant (Table 1). Surviving seedlings were transplanted to larger pots in May 2000, while non-germinating seeds were removed for examination. For each seedling, the parent plant, germination date, survival to adulthood, and flowering date were recorded.
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Fruits, each of which contained a single seed, were collected in April 2001 from the dead stems of plants at Nep 2 and 3. These seeds were sowed outdoors in soil flats alongside seed from the mesh bags. Beginning with the first observed germinations, seedlings from all flats were recorded and removed until no new germinations occurred.
Seedling survival in the wild
During the spring, summer, and fall of 2001 (April–November), Br 1 and Nep 2 and 3 were monitored for germination and seedling establishment. Br 1, located along the Monatiquot River in Braintree, Massachusetts, USA, is marked by a female genet established on a slope 2.4–3.0 m above the riverbank. Nep 2, located along the Neponset River in Milton, Massachusetts, USA, has a large dense stand of knotweed (approximately 25 m2) between an abandoned railway bed and a cemetery. The Nep 3 site, located behind a construction area in Dorchester, is lined with knotweed plants on both sides of an abandoned railway bed and is characterized by dry, sandy soil. As is typical for F. japonica in Massachusetts, populations at each site consisted of both males and females.
All three sites were monitored beginning in April 2001. Several patches of seedlings still in the cotyledon stage were marked at each site for observations during the growing season. Seedlings were collected from Nep 2 and taken back to the greenhouse to confirm their identification as F. japonica. Sites were checked on a biweekly basis for signs of new germinations, seedling development, or seedling death. In November 2001, wire markers were placed around surviving seedlings at Br 1. One seedling was removed in order to record rhizome and root size, and was then replanted.
At the start of January 2002, seedlings collected from Nep 2 in April 2001 and seedlings grown from seed collected in spring 2001 from Nep 3 were placed in a greenhouse storage room to acclimate them to winter temperatures. The room maintained daytime temperatures no greater than 10.5°C. After 6 wk, these seedlings were placed outside. The outdoor greenhouse seedlings and second year seedlings at Br 1 were monitored throughout August 2002.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Overall, seeds stored indoors for 4–16 mo retained up to 100% viability. Germination began 7 d after planting, and generally occurred within a few days (Fig. 1), with two exceptions. First, seeds collected in 1998 germinated at low frequency and at various periods during the experiment. Only seeds from NBP-1B, NBP-5, and NBP-16 germinated, and none of the offspring survived to adulthood. Second, the main periods of seed germination for the cultivated F. japonica plants occurred over longer durations, with weighted averages ranging from 9 to 21 d, compared to a range of 7–9 d for seed collected from wild parents in 1999. This was due to the appearance of several new seedlings after the initial 7–10 d period during which the majority of germinations occurred. When looking at seeds collected in 1999, those from cultivated parents had much lower germinability (47%) than seed from wild parents (83%).
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Seed from the cultivars had a range of germinability from 27% to 61%, with all offspring that attained adulthood eventually flowering. Offspring from the CB cultivars varied in degree of pink coloration in flowers and foliage, while all offspring from the Var'd cultivars developed variegation beginning with the appearance of the first true leaf. Offspring from both cultivars had leaf sizes and stature that ranged from the cultivars to wild F. japonica, evidence that the progeny were crosses with wild F. japonica.
2000–2001: overwintering effects on germination
Flats with seeds sown directly on top of the soil began germinating several days earlier than the seeds covered with soil. Seeds exposed to typical New England winter conditions still retained high viability, in most cases higher than in the preliminary tests done in November (Table 2). There was no significant difference among treatments (one-way ANOVA, P < 0.8, Fig. 2), though seeds from CB-A, which had only 18% germinability in December, had germination levels following overwintering that were closer to that of the other parents (55–59%). Germinability for overwintered seeds ranged from 43% for NBP-30 to 100% for NBP-35.
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Field
Seedling germination was observed during April 2001 at all three field sites (Br 1, Nep 2, and Nep 3). At Nep 2, seedlings in marked plots were soon lost in the dense cover of the surrounding vegetation that resprouted from established rhizomes. No seedlings could be located after a period of 1 mo, and it is assumed that they did not develop beyond the cotyledon stage. At Nep 3, seedlings grew in an area that was less densely populated with adult knotweed, but the site remained extremely dry throughout the summer. These seedlings developed very slowly compared to those germinated in the greenhouse, remaining at the cotyledon stage for over 4 wk. A few seedlings eventually developed a single true leaf, but no leaf exceeded 5–6 mm in length. Later in the summer marked plots were buried under 10–15 cm of dirt during construction of a bike path, presumably killing the seedlings.
At Br 1, dead stems from a female plant were observed to have collapsed downslope during the winter. Over 100 seedlings germinated at the bottom of that slope, likely all offspring of that female, since they were in a clearing at least 2.5 m from any adult knotweed. By June 2001, vegetation of nearby woody and herbaceous species limited the amount of sunlight available to the seedlings, but development continued throughout the growing season (Fig. 3), with several individuals losing their cotyledons and developing five or more true leaves. After November frost, leaves of the seedlings were mostly yellowed, as were leaves on adults. As indicated in Fig. 4, surviving seedlings developed a small, thick rhizome by the end of the fall. This development reflects the accumulation of resources in preparation for overwintering and resprouting the following spring.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. The offspring viability tests indicate that germinated seeds grow up to be healthy adult plants. The majority of offspring were large, healthy, and grew aggressively. Under optimal conditions, seedlings grew quickly and flowered within a single growing season. Many seedlings showed resilience in attaining sexual maturity and following a period of dormancy produced adult vegetation and flowered at the end of the growing season.
Sexual reproduction and seed set in F. japonica is not an anomaly in the USA, as has been previously reported. While Locandro (1973)
reported that female knotweed plants produced empty achenes when males were not available to pollinate plants, our observations in the greenhouse and in the wild indicated that female plants isolated from pollen sources had ovaries that aborted with no remnant seed detected. In addition, each of our study sites, along with dozens of other field sites throughout New England, was observed to have at least one male-fertile plant within pollination distance of each female, suggesting that isolation of females is not an issue in this region of the country. Whether the population biology of F. japonica in New England differs from that in other parts of North America or a change in population biology has occurred since studies were last done in the 1970s and 1980s requires further investigation.
Seeds from the two horticultural varieties of F. japonica showed lower viability than their wild counterparts. The two variegated plants in the UMB greenhouse are functionally male, and have a very low fruit set compared to female plants, bearing only one or two fruits on an inflorescence with over 100 flowers. Of the few seedlings from variegated parents that did germinate, only two survived to adulthood. Low seed production and viability is typical of hermaphrodites in subdioecious species. In contrast, CB plants, which are female, have a seed set comparable to that of wild F. japonica, but their seeds have lower germinability and a wider range of days to germination. The increase in germination percentages when seeds were exposed to winter conditions indicates that this variety benefits from a period of dormancy during which seeds are exposed to cold temperatures and suggests the presence of genetic variation for this evolutionarily important trait.
Variegated Japanese knotweed is available for sale through nurseries and catalogs that advertise it as non-invasive and has been spotted in gardens in Massachusetts. In addition, an educational website (http://fisher.bio.umb.edu/knotweed) set up to warn the public about problems caused by F. japonica, related species, and cultivars regularly receives requests for information about where to purchase variegated knotweed. While a variegated plant has fewer chloroplasts for photosynthesis and would be expected to exhibit less aggressive growth patterns than wild F. japonica unless exposed to constant bright sunlight, all observed plants of this variety have been male fertile and can thus be a pollen source for wild F. japonica. Further studies are being done to determine the genetic basis of variegation.
The CB cultivar of F. japonica is also sold by nurseries and catalogs as non-invasive. The plants used in this study had a consistently high fruit set, outcrossed with wild male plants in an open-pollinated greenhouse, and produced offspring with characteristics of both CB and wild F. japonica. The seed produced by these offspring showed high germinability in preliminary testing. This type of contribution to the problem of wild F. japonica invasion is likely not to be anticipated by those opting to buy a knotweed touted as non-invasive. It is also unlikely that gardeners, purchasing a plant marketed to them in part for its pink flowers, would be willing to remove those flowers to avoid seed production, just as they would not wish to remove the attractive inflorescences of variegated plants to eliminate a pollen source. We therefore recommend that both cultivars be removed from nursery lists.
Field studies elucidated what is not yet widely accepted in the general community: germination of F. japonica seeds does occur in the wild and is not rare. Studies done in the USA, Europe, and Japan concluded that seeds are viable but that seedling survival in the USA and Europe is negligible (Locandro, 1973
; Hru
ková and Hofbauer, 1999
). Our study found that seedlings that germinate underneath well-established stands of knotweed are not likely to survive, because the rapid growth of vegetation from the rhizomes creates a canopy in early spring that blocks most sunlight. However, seeds dispersed into open areas can survive through the next growing season, and do not necessarily die at an early stage as previously reported (Locandro, 1973
; Seiger, 1993
). A study by Maruta (1976)
estimated that seedlings on Mt. Fuji, Japan needed roots of approximately 6 cm in length to survive low water availability. Maruta (1983)
also determined that rhizomes needed to attain a threshold size for seedlings to survive winter conditions. As there were few seedlings remaining at the Braintree field site by the end of the growing season, and it was unclear whether any would survive through the winter, we chose not to sacrifice any individuals to measure rhizome dry mass. However, it is clear that a typical seedling growing in Massachusetts at the end of the growing season has a long root and a developing energy reserve in the rhizome (Fig. 4).
Our research indicates that if seedlings survive the growing season, they are likely to resprout the following spring if they have achieved sufficient growth (five-plus true leaves) in the previous growing season, as was observed with seedlings at Br 1. Fallopia japonica has been recorded in 40 of 50 states in the USA and six of 13 Canadian provinces (Thompson, 1997
; USDA and NRCS, 2001
). Given that the seedlings survived a winter in Massachusetts, USA, that was slightly milder than normal, regions of North America with milder winters should have the right conditions for extensive seedling survival. The advent of global warming (Dukes and Mooney, 1999
) could also lead to milder winters in more northerly regions, resulting in greater rates of seedling survival. Results of this study suggest that seedling survival is likely to be more dependent on the availability of adequate resources (light and water) than on temperature. Previous studies may have found no seedling establishment because they focused on knotweed-infested areas where heavy competition prevented any plant from growing, especially seedlings.
Vegetative reproduction is undoubtedly the major method of dispersal for F. japonica in America. However, ignoring the evolutionary and ecological importance of sexual reproduction could produce flawed management plans. The recombination that occurs during sexual reproduction generates new phenotypes that have the potential to be better competitors and more successful invaders (Jain and Martins, 1979
). Hybridization between related species also introduces new diversity that can lead to stronger, more aggressive plants (Arnold and Hodges, 1995
). While hybrids may have problems with sexual reproduction, hybrid vigor could lead to more successful vegetative reproduction (Vila and D'Antonio, 1998
). In Europe and the UK, F. japonica has been shown to hybridize with F. sachalinensis. Ellstrand and Schierenbeck (2000)
identified this plant (F. xbohemica) as part of a group of hybrids that have been found to be more aggressive than either parent plant. It is likely that the spread of F. xbohemica, which is now more common in the UK, is enhanced by clonal reproduction, but molecular studies have shown that several genotypes exist in the wild in the UK (Hollingsworth et al., 1999
). This is surprising since genotypic variation in F. japonica in the same region is extremely low (Hollingsworth and Bailey, 2000
) and suggests that hybridization is a recurring event. Further investigations are needed to determine what role, if any, that hybridization plays in the breeding system and evolution of F. japonica in America.
Recognizing that F. japonica is reproducing sexually may lead to improved management techniques. While the focus should remain on removal of vegetative growth, care should be taken to avoid allowing plants to reach flowering stage. Any removal or herbicide application should take place before August, when flowering normally commences, and sites should be monitored from late summer until a killing frost to ensure that resprouting vegetation is not producing new inflorescences. In addition, since horticultural varieties of knotweed produce viable seed and can have viable pollen, alternatives should be found in the nursery and landscaping industries so that these plants are no longer introduced in areas where they could outcross with wild populations.
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FOOTNOTES |
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2 Author for reprint requests (Jennifer.Forman{at}umb.edu
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Bailey J. P. 1994 Reproductive biology and fertility of Fallopia japonica (Japanese Knotweed) and its hybrids in the British Isles. In L. C. de Waal, L. E. Child, P. M. Wade, and J. H. Brock [eds.], Ecology and management of invasive riverside plants, 27–37. John Wiley & Sons, Chichester, UK
Bailey J. P. 1999 The Japanese Knotweed invasion of Europe; the potential for further evolution in non-native regions. In E. Yano, K. Matsuo, M. Syiyomi, and D. A. Andow [eds.], Biological invasions of ecosystem by pests and beneficial organisms, 141–158. National Institute of Agro-Environmental Sciences, Tsukuba, Japan
Bailey J. P. L. E. Child M. Wade 1995 Assessment of the genetic variation and spread of British populations of Fallopia japonica and its hybrid Fallopia x bohemica. In P. Pysek, K. Prach, M. Rejmanek, and M. Wade [eds.], Plant invasions: general aspects and special problems, 141–150. SPB Academic Publishing, Amsterdam, Netherlands
Beerling D. J. J. P. Bailey A. P. Conolly 1994 Fallopia japonica (Houtt.) Ronse Decraene (Reynoutria japonica Houtt.; Polygonum cuspidatum Sieb. & Zucc). Journal of Ecology 82: 959-979[CrossRef]
Child L. E. L. C. de Waal P. M. Wade 1992 Control and management of Reynoutria species (Knotweed). Aspects of Applied Biology 29: 295-307
Child L. E. P. M. Wade 2000 The Japanese knotweed manual: the management and control of an invasive alien weed. Packard, Chichester, UK
Dukes J. S. H. A. Mooney 1999 Does global change increase the success of biological invaders?. Trends in Ecology and Evolution 14: 135-139
Ellstrand N. C. K. A. Schierenbeck 2000 Hybridization as a stimulus for the evolution of invasiveness in plants?. Proceedings of the National Academy of Sciences, USA 97: 7043-7050
Hollingsworth M. L. J. P. Bailey 2000 Evidence for massive clonal growth in the invasive weed Fallopia japonica (Japanese Knotweed). Botanical Journal of the Linnean Society 133: 463-472[CrossRef]
Hollingsworth M. L. J. P. Bailey P. M. Hollingsworth C. Ferris 1999 Chloroplast DNA variation and hybridization between invasive populations of Japanese knotweed and giant knotweed (Fallopia, Polygonaceae). Botanical Journal of the Linnean Society 129: 139-154[CrossRef]
Hruková H. J. Hofbauer 1999 Germination capacity of Japanese knotweed (Reynoutria japonica Houtt). Rostlinná v
roba 4: 189-191
Jain S. K. P. S. Martins 1979 Ecological genetics of the colonizing ability of Rose Clover (Trifolium hirsutum All). American Journal of Botany 66: 361-366[CrossRef][ISI]
Locandro R. R. 1973 Reproduction ecology of Polygonum cuspidatum. Ph.D. dissertation, Department of Botany, Rutgers University, New Brunswick, New Jersey, USA
Maruta E. 1976 Seedling establishment of Polygonum cuspidatum on Mt. Fuji. Japanese Journal of Ecology 26: 101-105
Maruta E. 1981 Size structure in Polygonum cuspidatum on Mt. Fuji. Japanese Journal of Ecology 31: 441-445
Maruta E. 1983 Growth and survival of current-year seedlings of Polygonum cuspidatum at the upper distribution limit on Mt. Fuji. Oecologia 60: 316-320[CrossRef][ISI]
Schnitzler A. S. Muller 1998 Ecologie et biogeographie de plantes hautement invasives en Europe: les renouees geantes du Japon (Fallopia japonica et F. sachalinensis). Revue d'aecologie: la terre et la vie 53: 3-38
Seiger L. 1993 The ecology and control of Reynoutria japonica (Polygonum cuspidatum). Ph.D. dissertation, Columbian College and Graduate School of Arts and Sciences, George Washington University, Washington, D.C., USA
Seiger L. 1995 Element stewardship abstract: Polygonum cuspidatum. Nature Conservancy, Arlington, Virginia, USA
Seiger L. 1997 The status of Fallopia japonica (Reynoutria japonica; Polygonum cuspidatum) in North America. In J. H. Brock, M. Wade, P. Pysek, and D. Green [eds.], Plant invasions: studies from North America and Europe, 95–102. Backhuys Publishers, Leiden, Netherlands
Thompson M. 1997 Invasive plants of Canada: an introduction. Canadian Botanical Conservation Network (http://www.rbg.ca/cbcn/en/invasives/invade1.html), Ontario, Canada
Townsend A. 1997 Japanese knotweed: a reputation lost. Arnoldia 57: 13-19
USDA and NRCS. 2001 The PLANTS database, version 3.1 (http://plants.usda.gov). National Plant Data Center, Baton Rouge, Louisiana, USA
Vila M. C. M. D'Antonio 1998 Fitness of invasive Carpobrotus (Aizoaceae) hybrids in coastal California. Ecoscience 5: 191-199
Wade M. L. Child 2001 Getting to grips with Japanese knotweed. Enact 9: 4-7
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