tcm (r&d) : Hybrid knotweed

Hybrid knotweed

Fallopia x bohemica

• Species
• Family
• Description
• General Information 

Species

Fallopia x bohemica.  It is possible that an earlier name attributed to this hybrid was Polygonum "Cookii," though plants remained largely unrecognized until their formal description by Czech scientists in 1983. Until then, they were taken to be Fallopia japonica.

Family

Polygonaceae

Description

Hybrid Knotweed is a hardy, herbaceous, rhizomatous perennial. Plants are dioecious, with some male-fertile and some functionally female (male-sterile) individuals. Leaves measure up to 23cm by 19cm, ovate-cordate, sometimes somewhat oblong; glabrous except for numerous short, stout trichomes on the undersides. Leaf tips are somewhat acuminate. The stems are stout, hollow, and measure up to 4.5 metres; bases erect, branching with height. The flowers are in short panicles, similar to Japanese Knotweed. The fruits, where produced, are likewise similar.

General Information

Fallopia x bohemica refers to the progeny of Fallopia sachalinensis and Fallopia Japonica var. japonica or Fallopia japonica var. compacta. The highly invasive forms widely encountered in the UK are likely to be of sachalinensis x japonica var. Japonica origin. The male and female plants of this hybrid are also capable of producing fertile seeds in their own right. That Fallopia x bohemica results from hybridisations on many different occasions is borne out by the fact that at least 5 genotypes have been noted in the UK. Each of these bears between 4, 6 and 8 single sets of chromosomes.

Fallopia x bohemica is usually dispersed in the same way as Fallopia japonica, with equally small portions of rhizome being able to develop into new plants. In all other respects it also behaves in the same way as Japanese Knotweed, though often growing larger.  Many large stands of Knotweed, in the Pacific northwest of the USA, are now being shown to be of hybrid origin. The UK incidence is likewise increasing. Though controls applicable to Japanese Knotweed are equally applicable to the hybrid forms, this may not always be the case because of the possibilities of genetic mutations in seed originated plants. Developments in chemical or biological control could likewise be highly effective for the single-clone Japanese Knotweed, whilst having limited or short lived effect on hybrid forms.

Comparison with our native Elm trees makes a good illustration of the point. The Common Elm, Ulmus procera, once widespread throughout the southern English counties, and Elms of the East Anglian group, Ulmus carpinifolia, rarely set any fertile seed, but have a great propensity to sucker from their bases and to throw up suckers some distance from the main tree. Throughout these counties of England, the big landowners of the eighteenth century planted tens of thousands of elms from these very suckers. This created the basis for a huge population from a very small genetic base. When Dutch Elm disease came along, these clonal elms had equally high susceptibility, and we lost nearly all of them. In the West Midlands, Wales, and the North, our other main native elm, the Wych Elm, Ulmus glabra, has remained far less affected by elm disease, as many individuals are totally resistant because they reproduce exclusively by seed. No two Wych Elms will be exactly genetically alike, and disease resistance will vary from tree to tree.

We may find a biological control or chemical spray that wipes out Fallopia japonica var. Japonica, which is all one huge female clone of genetically identical plants. It may also kill most, but not all, Fallopia x bohemica. The resistant individuals will cross, spread and generate further resistance.