“Why isn’t Japanese knotweed such a problem in Japan?”
This is a well considered question we hear over and over at workshops and conversations about invasive plant species in the UK. The short answer is that Japanese knotweed lacks natural predation and competition outside its native environment.
Natural checks and balances
Philipp Franz von Siebold was the industrious German botanist and physician who first introduced Japanese knotweed into Europe in the mid 19th century. His mistake is understandable: when von Siebold first came across knotweed on the slopes of a volcano in Nagasaki Prefecture he would not have seen the prodigious density and fearsome growth of knotweed stands so familiar along riverbanks and urban development sites in Europe.
In Japan knotweed exhibits characteristics of a ruderal species, spreading by seed at high altitude habitats, and also spreading by natural dispersal of vegetative material from landslides at lower altitudes.
A hint of Japanese knotweed’s resilience and invasive potential is that it grows successfully on the scree and lava fields lining the slopes of Japan’s many active volcanoes. However, in this environment the plants are typically much smaller in size than in Europe due to poor soils, 'central die back' of the plant (similar to fairy rings, common in the UK), and the repeated coverings of volcanic ash and landslides that serve to limit knotweed growth.
In its Japanese habitat knotweed is further kept in check by a large native ecosystem of similarly vigorous giant herbs such as the grasses Miscanthus and Bamboo, and natural invertebrate pests such as the psyllid Aphalara itadori. A range of Japanese soil fungi and plant diseases also attack all parts of the knotweed plant.
As a whole, this more hostile native environment helps to suppress Japanese knotweed in Japan. In urban areas knotweed is still however a problem warranting chemical control and physical management - only not to the degree that it is Europe.
Strengthened non-native growth
Outside of it’s native habitat, lacking predation, competition, and suppression by volcanic ash, a strong aboveground Japanese knotweed plant is able to sink much more energy into a hardy underground rhizome. In turn, at different stages of the year, the rhizome is then able to pour more energy back into aboveground growth.
Repeated cycles of strengthened above and belowground growth result in a larger, stronger and more troublesome plant in the UK compared to Japanese knotweed in it's native habitat.
Von Siebold returned to Europe with over 12,000 foreign plant specimens and quickly established his Royal Society for Encouragement of Horticulture. In 1850 he sent a speculative batch of his plants to Kew Gardens in London. Unwittingly, his industriousness sowed a pestilential mistake on the British Isles.
Neat handwriting in the Kew Inwards Book notes, somewhat sternly, that only six of von Siebold’s plants are thought to be new to the Gardens. One of these is Item 34: ‘Polygonum Sieboldii’, the single female plant that every present day Japanese knotweed plant in Britain is descended from.