“The Day of the Knotweed?”
Tales of the Japanese Knotweed’s destructive power are legendary and its invasion can be traced back to the 1840s when botanist Phillip von Siebold acquired a specimen from Japan.
By 1848, his Netherlands based horticultural firm was supplying cuttings to the trade across Europe and in 1850 he sent a specimen to the Royal Botanic Gardens at Kew.
The plant was much admired for its showy flowers and for its ability to grow and thrive in any soil condition. It was readily accepted in country estates as well as nurseries.
However, things began to turn sour when in 1898 William Robinson, a Victorian gardener, warned that Japanese Knotweed was out of control and grows unabated.
By the 1930s, it had acquired the nickname “Hancock’s Curse” after a property in Cornwall was so badly blighted, £100 was slashed from its asking price.
The Knotweed thrives on riverbanks, railway embankments, roadside verges and in Japan it has even been known to grow in still hot volcanic ash, and even concrete doesn’t seem to be a barrier to it.
The main problem is that when the plant dies back in winter, the rhizomes which lie deep underground extend the root of the plant and they have been known to go 3 metres deep and 14 metres wide.
Swansea is particularly hard hit with over 100 hectares of land badly affected.
Surprisingly, the plant cannot produce seeds and every plant in Europe and North America is an offshoot of a single female plant and the smallest trace of stem or rhizome and the plant goes off again.
These tiny fragments can easily be transported by water or dumping of garden waste and the plant’s party trick is that these fragments can remain dormant for years before they show up again.
The UK only recognised Knotweed as a problem in 1981, and declared it illegal in 1990.
It is possible to kill knotweed with herbicides but can sometimes take years to deal with, and the cost to the UK economy is estimated at approximately £165 million per annum, and this has done little to stop the spread.
One of the concerns is hybrids – a cross between Japanese and Giant Knotweed – which is even tougher and more invasive and, to make matters worse, it does produce seeds which thankfully do not germinate in the fields in the UK (yet).
So how is this plant kept in check in Japan?
Well the answer is quite naturally, tiny insects known as psyllids which do not kill the plant but by continually munching its way through the new growth, keep the plant contained.
In the UK, extensive trials and studies are now being conducted with psyllids in quite large numbers and it is already known that the nymphs from the hatching eggs do the damage and stunt the plant’s growth. Although it can take years to get the results, things are looking hopeful.
In the meantime, it is becoming quite a problem in the UK property market, with Lenders and Insurers being very wary of knotweed near to a particular property. The risks are obvious.
In the commercial property market, where values can be significant, a new desktop report has come to market. This will identify and plot any known knotweed in the vicinity of the property or proposed development site.
Costing less than £100 and supplied by Groundsure, the report will definitely not get rid of any knotweed problem but, compared to the clean up-costs, could well fit into the mantra “Prevention is better than cure”