By Jennifer Holmes


It is well documented that Japanese knotweed has several hybrids, so this blog looks at what a hybrid is, how they form and what hybridisation means for the future of invasive species – factors the weed control industry must not ignore. The important take-out here is that Japanese knotweed cannot grow from seed (in the UK) whereas knotweed hybrids can! This can have real impact on what we as an industry will need to do in the future to control knotweed AND its hybrids.

Further on we show you what the Japanese knotweed hybrids look like, but first a brief explanation .

Japanese knotweed in its native range can propagate via seed, but in the UK the seed produced via a hybrid tends not to survive our wet winters (as yet) however in the Cardiff study by Sophie Hocking, “Connollys Hybrid” was discovered with a seedbank. Connollys is rare in the wild but is the most frequently produced by seed found on Japanese knotweed plants in the UK. The Cardiff study goes on to state If future climatic conditions become favourable for these seeds to germinate successfully, our Japanese knotweed problem might get worse.”

What is a plant hybrid

A hybrid plant is the resulting offspring of two different plant varieties that have been ‘cross pollinated’. We’ll describe this process later on. Some hybrids occur naturally but many are commercially bred to gain certain attributes from each of the parents such as larger or sweeter tasting fruits. Common examples include sweetcorn, strawberries and clementines. However, hybrids are not always intentionally grown for the benefit of mankind, and can be extremely detrimental to their surrounding environments. Results of recent studies point towards hybridisation as a precursor to the evolution of invasiveness in plants. We know how invasive plants cause havoc to ecology and human health, so here we’ll highlight the drivers and impacts of hybridisation, and give a few UK examples to look out for.

Natural cross pollination

Cross pollination works as simply as the pollen from one plant being transferred to the flower of another. This transaction can occur naturally between plants growing close to one another, with the aid of wind and wildlife action. Resulting seeds from cross pollinated plants are transported once again, and grow upon their landing. Due to natural promiscuity in plants, it is likely that none of the plant species we see today are ‘pure bred’. Hybridisation between closely related species is very common, and can produce rather unexpected characteristics

Manual cross pollination

Humans produce hybrids manually using different techniques. One method is hand pollination; the act of transferring pollen with a cotton swab or brush from one plant to the flower of another. Bisexual flowers can also be cross pollinated by brushing the petals of a male flower against female flower stigmas. However, rates of plant hybridisation have soared in the last couple of decades, due to human disturbances to the environment. This phenomenon has been historically tricky to regulate due to the method of hybridisation almost always being case-specific. As such, it is often impossible to tell whether a hybrid has occurred naturally or anthropogenically.

Natural drivers of hybridisation

Whether or not plants can hybridise fundamentally depends on biological, geographical, and ecological factors. Biologically, plant species sharing more DNA are more likely to hybridise and hybrids are often unlikely to produce offspring and continue a hybrid lineage.  Geographical distribution and separating features such as mountains influence whether plant species that are able to hybridise can actually do so. Ecologically speaking, some hybrid species likely occupied similar niches prior to cross-pollination, and cross-pollination may depend on assistance from other species within this niche.

Human disturbances

Humans influence these factors through exploitation of natural resources and the resulting disturbance this causes. For instance, activities disturbing the geographical distribution or ecology of a species can enable naturally impossible hybridisation events to occur. Hybridisation events may be induced by climate change, as previously separated species are brought into contact with one another. These disturbances can manifest as changes in animal migration, species range, or phenology, which can already be observed in species such as Japanese knotweed. Thus, drivers of hybridisation also include worldwide transport, global warming, deforestation and farming.

“A third of the wild hybrids found in the British Isles have one or more foreign parents brought there knowingly or unknowingly by humans.”

Invasion by hybrids

Hybrid plants can have enhanced resistance to desiccation, pathogens and grazing, larger plant size, and longer flowering stages. Studies conclude these traits could assist the evolution of invasiveness in previously harmless plant species, with four main routes for invasion through hybridisation:

  • Hybridisation between natives
  • Hybridisation between exotic and native congener
  • Hybridisation between two exotics
  • Introduction and subsequent spread of hybrids

Problems as an industry to control hybridisation

Hybridisaton is a potential threat to biodiversity due to its role in the evolution of invasiveness, and the impacts of genetic and demographic swamping. Gene swamping – the replacement of rare taxa by hybrids, and demographic swamping result in reduced population growth rates due to production of unviable hybrids.

Studies found a “remarkable number of cases in which hybridisation preceded the emergence of successful invasive populations” and given that invasives are important for so many reasons, considerable effort has been spent trying to develop generalisations to determine which species are likely to become successful. An example of this is when a native southern English grass mated with an introduced North American grass, resulting in English Cordgrass. This species has since spread on a global scale, establishing as a noxious invasive plant in some areas.

Hybrids not only evolve into invasive species, but also act as bridges to exchange genes between native and non-native species, diluting the identity of native species beyond recognition. This phenomenon has been coined “extinction by mating”.

Knotweed identifiers

Hybrids may have different traits to their origin plant species but can look very similar. Since one plant may be toxic and the other not, it is important we know how to tell the difference. Several of the plants managed by Japanese Knotweed and Environmental Controls Ltd. are hybrids. Since Japanese knotweed was our primary target for eradication and still runs rife across the UK, we’ve detailed a few identification features for other less known knotweed species below:

Giant knotweed (Reynoutria sachalinensis)

The main indicators of the Giant knotweed is the enormous size of the leaves; heart shaped leaf-bases; the wavy hairs on the leaf underside; and the long, fluffy, dense flowering clusters.

Key points:

  • Generally, plants are very similar to hybrid knotweed but
  • everything about them is much larger.
  • Larger leaves (20cm to 40cm long)
  • Taller plants (up to approximately 4m tall)
  • Leaves have large lobes at their base, making them heart shaped.
  • Long trichomes (hairs) on the undersides of the leaves.
  • Flowers have a light green hue.


Himalayan knotweed (Persicaria wallichii)

Key Points:

  • Plants are shorter (around 1m tall)
  • Leaves are long and slender.
  • Leaves have evenly spaced, parallel vein
  • The undersides of the leaves are paler than the topsides.
  • The midrib on the topside of the leaves is distinct and is a
  • pale green/pink.
  • Fused stipules wrap around the nodes, forming sheaths of
  • thin membranes that cover the rings on the stems where the
  • leaves branch off.


Dwarf knotweed (Reynoutria japonica var.‘Compacta’)

Key Points:

  • Shorter plant
  • Slightly curled edges to the leaves
  • Has both male and female flowers
  • Capable of forming hybrid varieties


Hybrid knotweed (Reynoutria x bohemica)

The main things to look out for are the crinkly leaves, short stout hairs on the leaf undersides, and heart-shaped leaf bases.

Key points:

  • Mature leaves are slightly larger.
  • Leaves are a darker shade of green.
  • Leaves have slightly crinkled edges.
  • Lobes at base of the leaves make them appear heart
  • On the undersides of the leaves there are trichomes (hairs)
  • which can be of variable length

What should we be doing?

A key to managing conservation concerns is knowing the factors influencing destructive versus constructive hybridisation. Management priorities should be minimising introduced hybridisation-prone exotic plants, and restoring mature and diverse habitats that are resistant to hybrid establishment. Japanese knotweed is one of the most well known invasive plants in the UK. Control and eradication is on-going and will curb the increase of hybridisation.

If you need help identifying whether you have Japanese knotweed or one of its hybrids you can send some pictures of the suspect plant to our experts who will quickly identify it for you.


This article was written by Eloise M.K. Holmes Msc, Bsc.


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