Japanese knotweed is not the only alien invasive weed in the United Kingdom (UK) and certainly not the most widespread (that applies to Rhododendron ponticum), but it is definitely the most difficult and expensive to control.
The species usually found in the UK is Fallopia japonica var. japonica introduced from Japan two centuries ago, which continues to destroy fresh water ecosystems and blot the landscape in a wide range of urban and suburban situations, including parks, sporting facilities and gardens.
Tarmac is no problem for Japanese Knotweed because the stems can force their way through concrete. Specific factors that assist spread are moving water and human activity including soil excavation and movement into landfill and fly tipping.
Dense growth of Japanese Knotweed restricts access to riverbank inspection and increases flood risk through large quantities of dead stems and leaves washed into rivers and streams. Attempts to remove established stands from riparian areas may cause instability in river banks and increased risk of soil erosion.
Attempts to physically control Japanese Knotweed using strimmers, flails or diggers actually aggravate the spread of this weed. Spraying with contact herbicides simply burns off the leaves leaving the rhizomes intact. Use of systemically acting herbicides moving down into the rhizomes is required, but given the sensitivity of situations where Japanese Knotweed grows, whether along water-courses or in built-up areas, there is often a high risk of chemical contamination from spraying.
The 'root' of the problem with Japanese Knotweed is fast growing food-filled rhizomes (underground stems) with dual over-wintering and vegetative propagation functions. Rhizomes may extend 7 metres from parent plants and 3 metres down into the ground. Rhizome fragments as small as 0.7g will generate new plants and pieces of fresh aerial stem readily grow in soil or water.
Plants grow up to 3 metres tall with 6 metres of rhizome growth in a single season. New shoots arise from the tiniest pieces of rhizome and cut stem-sections will root at the nodes, while crowns survive drying or composting and produce new canes once in contact with water.
Other alien invasive weeds (like giant hogweed and Himalayan balsam) require relatively early summer treatment to kill plants before the seed is set. This is not a consideration for Japanese Knotweed which does not set viable seed in the UK, but herbicide application timed to coincide with the strong movement of soluble assimilates from the aerial stem and leaves and down into the rhizomes is essential.
This is why control of Japanese Knotweed is recommended for late summer through to plant senescence in autumn. It is essential to establish and encourage growth of native plants as soon as possible after Japanese Knotweed has been controlled.
As stated above, mechanical-control actually makes the situation worse while overall spraying can cause contamination in environmentally sensitive situations. Rapid long lasting control is best achieved with a short-sharp shock from an 'inside job', whereby systemically-acting herbicide is introduced right into the intact stem by injection or cut stem filling with the underground stems (rhizomes) as targets. Wiping herbicide onto the outer surface of Japanese Knotweed to exploit the strong basipetal (downward) translocation of assimilates into the rhizomes during late summer through to plant senescence in autumn is another option.
Dedicated professional applicators for stem-injection and 'stem cut and fill' are available from Micron Sprayers, who also supply hand-held weedwipers that can be used in sensitive areas without contaminating the environment. Weapon of choice for stem injection is the InjectorDos, hand-operated injection applicator equipped with a 'dial-in' facility that allows operators to dispense a pre-determined metered volume of herbicide formulation. The InjectorDos is ideal for introducing herbicide (usually undiluted herbicide containing glyphosate) into the uncut hollow stems of Japanese knotweed. Generally a single 2 ml shot is made just below the first stem node up from soil level, as near as possible to the underground rhizome.
The InjectorDos comes with a one-litre backpack to replenish the applicator via a length of hose. A funnel is provided for ease and safe re-filling and the backpack is fitted with a comfortable shoulder strap. Liquid supply from the backpack is shut off, disconnected and isolated from the applicator using an in-line tap. The InjectorDos has chemical resistant seals and provides an adjustable metered dose of 0.1 to 2ml/shot.
The InjectorDos meets all the specifications and requirements for a lightweight, highly-portable, easy to use and safe herbicide applicator to control of Japanese Knotweed. By using the InjectorDos there is no loss of product into the environment and the unit is simple to clean and maintain.
An alternative way to treat Japanese Knotweed is 'cut and fill' whereby stems are cut through near to ground level and herbicide introduced via the cut end of the hollow stem. The recommended applicator form Micron for this treatment is the Accudos 25P, a professional applicator for delivery of pre-determined volumes of spray liquid (ie metered doses of herbicide) by using its 'dial-in' facility'.
The Accudos 25P is supplied with 5-litre backpack with filter and wide aperture to facilitate easy filling and cleaning. Liquid flow is shut off using an in-line tap which allows the backpack to be disconnected from the hand-lance fitted at the nozzle (with a non-drip check valve as standard).
Alternatively, direct application of herbicide to the outer surface of Japanese Knotweed plants is possible using the Micron Microwipe, a hand-held 'rope-wick' weed-wiper for wiping herbicide onto flowering stems up to 2 m in height in late summer.
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