ADERS
avoid spray-drift onto the foliage of desirable plants nearby. Where there is a need to preserve grasses e.g. to prevent soil erosion, then 2,4-D amine is the best option.
Alternatively, Roundup Pro-biactive can be applied by direct stem injection at the rate of 2ml undiluted product per stem. Specialised injection equipment for this would need to be purchased but is relatively simple to use. A sharp needle attached to the equipment is pushed into the lower part of the stem. The needle is cross-bored a few millimetres from the shank to allow the chemical to move directly into the plant once the trigger mechanism has been squeezed. This method ensures that all of the chemical is targeted on the weed and adjacent grasses and other plants are unharmed. In other situations, where there are no risks to water or valued vegetation, we can use herbicides with a residual action, such as picloram (e.g. Pantheon) or triclopyr (e.g. Timbrel). Of these two residual materials, it is widely accepted that picloram produces the best results against Japanese Knotweed. It should be borne in mind that treatment with picloram will render the soil unsuitable for replanting shrubs and flowers for up to two years and triclopyr for six months, however, grass will be mostly unaffected by the spray. In the initial stages of chemical treatment, the plant may sprout new growth from its rhizomes up to seven metres away from the original plant, so one needs to be ready to re- treat the new shoots as soon as possible after they emerge and before they can extend the rhizome system further. Cultural control of Japanese Knotweed is possible by cutting and burning the aerial growth and carefully excavating the soil to remove all traces of the rhizomes, but remember, only one tiny piece left behind can cause the problem to recur. The environmental cost of removing soil to a depth of three metres and replacing it
with fresh soil makes this exercise impractical. Where it invades grassed areas, control of Japanese Knotweed can be achieved by regular mowing, which gradually weakens the rhizome system and eventually the shoots will stop emerging through the turf.
Indian Balsam (Impatiens glandulifera) - also known as Himalayan Balsam
Indian Balsam, a relative of the busy lizzie, is the tallest annual growing wild in the UK, often reaching a height of three metres. It was first introduced into Britain in 1839 by John Forbes Royle who collected plants from all over India and sent some of them to the Horticultural Society of London. It was originally named Impatiens Roylei after the collector. It is a very handsome plant with dark green leaves and attractive fragrant flowers that range in colour from dark purple through pink to white. It thrives in damp soils so is mainly found along the banks of rivers, ponds and lakes although it will survive on drier soils but with a less vigorous growth habit. The main problem with this plant is that, like Japanese Knotweed, it tends to dominate and squeeze out all other vegetation and, with a relatively shallow root system, there is little to hold the soil together when it dies back in the winter. This makes the
riverbanks more
prone to erosion and can cause flooding in extreme
cases. The plant produces large
numbers of seeds in pods that ‘explode’ on touch, scattering them over a wide area, which allows the plant to spread quickly. Many of the seeds, which remain viable for 18 to 24 months, will be carried downstream creating new colonies wherever they reach the shore. The large number of flowers providing nectar
attracts many bumblebees and wasps from the surrounding area, drawing them away from native species growing nearby, and it has been demonstrated that this can result in a lower seed set for these indigenous species.
Control of Indian Balsam
Indian Balsam is somewhat easier to control than Japanese Knotweed as it is only spread by seed. If we can remove the seed reserves by carefully cutting down the plant or by chemical spraying before flowering then it can be eradicated from an area within three years. However, it is necessary to locate and control the weed upstream first to ensure complete removal of the infestation. Cutting to prevent seed production needs to take place just before the onset of flowering and, to be effective, it should be as low to the ground as possible, taking the lowest leaves and buds to avoid stimulating rapid re-growth. This will need to be repeated each year until the seed reserves in the soil have been depleted. Chemical control with glyphosate or 2,4- D can be undertaken in the early spring when the young shoots are emerging. 2,4- D might prove the best choice as it would leave any grasses behind to stabilise the river banks. Direct stem injection with glyphosate may also be useful - providing access to the stems is possible without unnecessary risk to the operator. Special care should be taken when spraying tall plants with herbicide, as light winds can cause the spray to drift back onto the spray operator or onto other vegetation. Always read the product label before using chemicals and consult the Environment Agency before applying herbicides near water.
Roundup Pro-Biactive is a trade mark of Monsanto plc. Depitox is a trademark of Nufarm UK Ltd. Pantheon is a trademark of Pan Agriculture Ltd.
Illustrations (l-r)):
Japanes Knotweed encroaching on a school playing field. Indian Balsam
Site in East London which was virtually overrun by Japanese Knotweed, but was reclaimed by chemical treatment in just three years.
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