Fallopia x bohemica

It is generally an under-recorded component of the knotweed populations. It occupies the same sorts of habitat as Japanese knotweed (F. japonica) does in its adventive range. There is some evidence that it is more difficult to control than its parents (Bimova et al., 2003) and that it can exploit a wider range of habitat than its parents (Bailey and Wisskirchen, 2006). Some clones cover very large areas.

Bailey et al. (1996) give detailed information about the distribution in the British Isles with six figure grid references; more up to date data on a 10 km square basis is to be found in Preston et al. (2002).

The main method of reproduction is by vegetative propagation so there is a danger of river-borne fragments of rhizome crossing country borders. Also there are some reports of short, sea-borne transfer of viable rhizome fragments of Fallopia japonica from island to island in the west of Scotland (Beerling et al., 1994; Hayward, 2002) and there is nothing to suggest that F. x bohemica is any less adept in this ability so this possibility cannot be excluded. Richards et al. (2008) describe the presence of F. japonica and F. x bohemica in salt marsh habitats in the USA and performed experiments that indicated that both taxa exhibited considerable plasticity in their response to exposure to salt. There is little risk of inadvertent transport to continents not already occupied.

It occurs in habitats similar to those occupied by Fallopia japonica; principally in riparian and ruderal habitats, but also in salt marshes (Richards et al., 2008).

Not usually a problem in cultivated ground.

Genetics

No specific research has been carried out on F. x bohemica because it is rarely found in Japan, but much of what holds for Fallopia japonica might be expected to apply to the hybrid. F. japonica in Japan is attacked by a suite of natural enemies, both arthropod and fungal, not present in its adventive range. To date, 186 species of arthropod and around 40 species of fungus have been recorded from the plant in its native range of Japan (R Shaw, CABI, UK, personal communication, 2008). As a result of this attack, it is not able to compete with local flora as effectively as it does in the introduced range and does not normally reach the same massive size. Of these natural enemies, some exert significant damage such as the chrysomelid beetle Gallerucida nigromaculata, which is described as having potential as a biocontrol agent by Zwoelfer (1973). This is now thought to be Gallerucida bifasciata and not adequately specific. Recent observations by CABI Bioscience show fungal pathogens, as well as arthropods, are significant controlling factors in Japan, across the climatic range of the plant. This includes a number of obligate biotrophic fungi, as well as an ubiquitous leafspot pathogen. In its introduced range, F. japonica is attacked by the green dock beetle, Gastrophysa viridula, but this is only when its normal Rumex host has been consumed and beetle populations are elevated.

Animal feed, fodder, forage

• Forage

Environmental

• Erosion control or dune stabilization

Fuels

• Biofuels

Human food and beverage

• Emergency (famine) food

Materials

• Chemicals
• Pesticide

It is most likely to be mistaken for either of its parents Fallopia japonica and Fallopia sachalinensis. Identification is on the basis of leaf shape and the characteristic intermediate leaf hairs found on the lower epidermis of the leaves. For both of these characters, it is important that only large fully-expanded stem leaves are examined. The male-fertile clones are particularly easy to distinguish from F. japonica var. japonica. Clear characters for distinguishing the three taxa may be found in Bailey et al. (1996, 2008), Zika and Jacobson (2003) and Botta-Dukát and Balogh (2008).

Rapid Response

It is possible to eradicate knotweed if a new infestation of rhizome is spotted quickly and the resultant plants pulled or treated before the roots have become well established.

Eradication

There has been little success with eradication policies for F. japonica, but given the smaller population size, it is perhaps a realistic and worthwhile aspiration; although there is some evidence that it is more difficult to eradicate (Bimova et al., 2003), and contains greater reserves of genetic diversity than the clone of F. japonica var. japonica. Also see Child (1999), and Child and Wade (2000).

The UK Environment Agency have produced a Code of Practice, and the Cornwall and Devon Knotweed Forum have produced an excellent guide, which has advice on identifying the plant in the field at various stages of the season.

As a hybrid between Fallopia japonica and Fallopia sachalinensis it obviously shares many similarities with these plants. It should be noted that the tetraploid F. x bohemica is a cross between the dwarf montane F. japonica var. compacta and F. sachalinensis. The standard 6x and the rarer 8x F. x bohemica are both crosses between the F. japonica var. japonica clone and F. sachalinensis. The tetraploid and octoploid contain half japonica and half sachalinensis chromosomes, whereas the 6x is 66% japonica and 33% sachalinensis. This is reflected in their morphology; although they cannot currently be distinguished morphologically. It is also possible that the montane var. compacta component of the 4x may pre-adapt it to different habitats, because in the wild it is found up to 2500 m.a.s.l. (Bailey, 2003).

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.

Prevention

F. japonica appears on the UK Wildlife and Countryside Act (1981) and as such it is illegal to cause the plant to grow in the wild. It is listed as a noxious weed in many states and provinces of North America and appears on many weed lists around the world. F. x bohemica is not such a well-recognised taxon, but the same conditions apply to its treatment.

SPS measures

Vehicles should be inspected when moving from infected sites to new ones.

Control

Due to a large and persistent rhizome system, F. japonica is highly resistant to control efforts (Ainsworth et al., 2002), and the same also applies to F. x bohemica. The effectiveness of control and eradication interventions has recently been reviewed thoroughly by Kabat et al. (2006), who included 65 articles in their meta-analysis. Six categories of intervention were included, none of which could eradicate Japanese knotweed or its hybrid in the short term. Cutting treatments alone were not found to result in significant decreases in knotweed abundance. However, statistically-significant reductions in abundance can be achieved by short-term application of: glyphosate; imazapyr; imazapyr and glyphosate; cutting followed by filling stems with glyphosate; and cutting followed by spraying with glyphosate (Kabat et al., 2006). However, these authors were still unable to conclude long-term efficacy for any control measure.

Physical/mechanical control

Mechanical control of F. japonica is difficult, but continual mowing will reduce the resources of the extensive rhizome system if carried out throughout the growing season. Glasshouse trials have shown that repeated cutting at least every 4 weeks and at least 7 weeks prior to senescence, can be effective (Seiger and Merchant, 1997). Pulling up plants complete with root systems can eliminate small stands and is appropriate for local eradication in sensitive areas, but only if carried out continually over a number of years (Baker, 1988). However, digging up roots is even more challenging because they can extend to a depth of 2 m, and 7 m away from the crown, and despite the best efforts, this normally results in an increased stem density. This may be useful for integrated control. With its Fallopia sachalinensis component, F. x bohemica may be slightly more palatable for stock. Certainly F. sachalinensis was promoted as fodder for livestock in the early 1900s (Bailey and Conolly, 2000).

Biological control

Much work has been done on F. japonica at CABI Biosciences (Shaw et al., 2009), but this was mainly based on the European clone of F. japonica, and the aim was to obtain agents specific to that clone. The F. japonica programme has been underway, on behalf of UK and North American sponsors, since May 2003, with two candidate agents, namely a Mycosphaerella leafspot and the psyllid Aphalara itadori. Both of these agents have undergone extensive host range testing and have good potential as biological control agents. Given the difficulty faced by property developers, there would appear to be a market for a mycoherbicide, although registration costs are hindering this approach. As a newly arisen hybrid it is doubtful that any specialist predators actually exist for F. x bohemica and it will be a matter of selecting agents that are restricted to F. japonica and F. sachalinensis.

Chemical control

For F. japonica, chemicals that are permitted on or near water are normally restricted as will be the potential for full control. Child and Wade (2000) recommended five herbicides for F. japonica control, to be applied as foliar sprays. Triclopyr and imazapyr can be applied to young, actively-growing shoots when grasslands need to be protected; glyphosate is suitable during active growth periods when leaves are fully expanded, although larger plants may need to be sprayed using a telescopic/long lance sprayer; picloram can also be used as a soil drench due to its persistence, but not where planting is required within 2 years; and 2,4-D amine is used during the active growing period and as a selective translocated herbicide to be used in grassland, amenity areas and forest situations, although this may depend on which formulation is used in which country. Of the five herbicides, only glyphosate and 2,4-D amine can be used near water. In general, cutting and removing dead stems at the end of the season, prior to a spraying regime the following season, is advisable to aid access. F. japonica is a very resilient plant and unless extremely toxic chemicals are appropriate, repeated well-timed applications should be anticipated, and follow-up spot treatments of any regrowth will often be required.

Stem injection of various herbicides is a relatively modern phenomenon and can produce very good results in some conditions, but concerns remain over the amount of chemical that is actually applied per hectare exceeding statutory maxima. Hagen and Dunwiddie (2008) discovered that using glyphosate, through the injection method, results in the short-term dieback of injected stems. However, drawbacks to its use in certain scenarios should be considered when developing an integrated management plan for knotweed control.

Much of the above will also apply to F. x bohemica, although the larger leaf area and the fact that chemical control is more difficult with F. x bohemica (Bimova et al., 2003), may point to the need for some experimentation with the dosage used and the times and frequency of application.

IPM

For F. japonica the use of a combination of mechanical and chemical techniques has proved effective, i.e. cutting and a follow-up spray of new growth, but it is necessary to apply the chemical more than once a season (de Waal, 1995). There are two basic methods: to cut plants to 5 cm height and immediately apply a 25% solution of glyphosate or triclopyr to the cut stems; or cut or mow infestations when the plants reach the early bud stage in the late spring or summer and treat the regrowth in the autumn with glyphosate or triclopyr. If deep digging is used to effectively increase the above ground:below ground biomass ratio, then subsequent chemical application can reduce the time required to achieve effective control (Child et al., 1998). Another herbicide strategy is an integrated strategy with mowing or cutting.

Similar approaches for F. x bohemica should be attempted in order to assess their effectiveness.

Monitoring and Surveillance

There are various GIS surveys on-going in the UK, the first being in Swansea, followed by Cornwall and Devon. Professor Denis Murphy and Daniel Jones at the University of Glamorgan are currently working in this area. Whilst it is highly unlikely that the system would ever be able to discriminate between the different knotweed taxa, it is nevertheless a powerful tool for charting the distribution and extent of this group.

Mitigation

Rapid eradication of F. x bohemica is only possible if rhizome growth has not been too extensive.

Ecosystem Restoration

F. japonica is able to hyper-accumulate heavy metals, including copper, zinc and cadmium, more effectively than other angiosperm species as has been demonstrated in Japan (Nishizono et al., 1989) and Croatia (Hulina and Dumija, 1999). However, the abilities of F. x bohemica in this direction remain untested.
 

Do the Wildlife and Countryside Act and subsequent Duty of Care regulations actually apply to the hybrid F. x bohemica and if not the appropriate legislative changes should be made?

07/07/09 Original text by:

John Bailey, University of Leicester, UK