Graham Paul, Technical Specialist at Sherriff Amenity, offers some advice on two invasive species that are particularly difficult to exterminate!
SPACE INVA
Concern for our environment has never been more ‘in the public eye’ than it is today. Changes brought about by global warming are just one aspect of our concern. However, of equal importance, are changes caused by mankind to our natural environment in the range of plants, birds, insects and animal species that are found in the UK. There is quite a long list of plants that can be found in Britain today that originated from outside these shores. Most of these were introduced in the 18th and 19th century by explorers and botanists who, at the time, had no knowledge of their potential impact on our natural flora and fauna. Today we are seeing the dramatic spread of some of the more aggressive introductions which are displacing our native plant-life and also the insects, birds and animals that live on them. Two of these
non-native species giving cause for concern are Japanese Knotweed (Fallopia japonica) and Indian Balsam (Impatiens glandulifera) also known as
Himalayan Balsam. In this article I shall look in some detail at these two weed problems and highlight the strategies that have been developed to deal with them.
Japanese Knotweed (Fallopia japonica)
Japanese Knotweed originates from East Asia (Japan, Taiwan, China and parts of Korea) and was first introduced into Britain in the 1850s when a Dutch nurseryman sent the plant to Kew Gardens. From here it has spread throughout the UK - initially from the sale and distribution of the plant to landscapers and gardeners but, in more recent times, it has spread in soils contaminated with small fragments of
the rhizomes e.g. soil movement during building works and from fly tipping. Japanese Knotweed is a hairless
perennial producing an extensive rhizome system that can reach up to seven metres from the plant down to a depth of three metres beneath the surface. It is this rhizome system that makes this plant so aggressive and difficult to control. The stems rapidly grow to a height of 2-3 metres and are hollow, ‘bamboo-like’ and flecked with red pigment. In the winter the stems will die off but remain erect and new shoots will sprout from the rhizomes in the following spring. The large, fresh green leaves are oval-triangular, truncated at the base and pointed with a red leaf stalk. Numerous small greenish/white flowers approximately 3mm in size are produced late in the season on branched spikes. Once established it can quickly dominate large areas
squeezing out all
other plant life.
Japanese
Knotweed is a dioecious plant - it has male and female organs on separate plants. In the UK we have
only female plants so propagation by seed is not possible and, therefore, all our Japanese knotweed specimens are clones that have grown from rhizome fragments or from cuttings. We are now also aware that the rhizome fragments can spread by travelling down rivers and streams.
Only a small piece of rhizome (less than one gramme) is needed to generate a new plant. It is an extremely invasive and hardy plant. In its natural habitat in East Asia it can survive in harsh conditions in volcanic regions on poor, thin, acid soils with a pH of less than 4.0 and extremes of temperature, which in mountainous regions can fall below -35°C for long periods during the winter. It is, therefore, not surprising that it does so well in our comparatively gentle climate. The British Government has been
aware of the invasive problems caused by this species for some time; it is one of the weeds listed in Schedule 9 and subject to controls specified in section 14 of the Wildlife and Countryside Act of 1981. Whilst there is no statutory requirement under this act to remove Japanese Knotweed from your land, it is an offence to plant or cause the species to grow in the wild. The plant, prunings and soil contaminated with its rhizomes are classified as controlled waste and must be disposed of at ‘licensed sites’ or rendered safe by burning.
Control of Japanese Knotweed
This is not an easy weed to control, due to the extensive rhizome system that sustains the plant. Because there is no viable seed from UK plants we need to concentrate our efforts towards killing these underground rhizomes. Chemical control is the best method of eradication, although this may take several years to fulfil. Choosing the most suitable chemical depends on the situation; in sensitive areas near water, trees and shrubs we should use non- residual
translocated herbicides such as 2,4- D amine (e.g.
Depitox) or glyphosate (e.g.
Roundup
Pro-biactive) taking care to
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