Control and Disposal of Invasive Japanese Knotweed Reynoutria japonica Houtt. Using Microwave Treatment

Summary

This protocol provides a method for microwave control of Japanese knotweed in the field and disposal of dug-up rhizomes in laboratory conditions. The advantages and disadvantages of both methods are discussed. Future research directions are also suggested to optimize the use of microwaves for controlling Japanese knotweed.

Abstract

The study aims to assess the effectiveness of microwave treatment (MWT) at a frequency of 2.45 GHz and a power of 800 W to control Japanese knotweed (Reynoutria japonica Houtt.) using a self-propelled device that was built in the in-house facility. The MWT was applied in the field population of knotweed in July 2022. First, plants were mechanically moved from the area of 1 m^2, and next, the cut shoots around 4 cm high were microwave-treated for 25 min, 20 min, and 15 min. The control treatments were: 1) only cut plants and 2) rhizomes dug out to 30 cm deep. The effectiveness of the microwave treatments was observed for the next 11 months by counting the number of newly grown shoots. The results showed that a 25 min MWT was 100% effective in Japanese knotweed loss of vitality, while a 15 min MWT microwave treatment stimulated plant growth by around 50%, compared to controls. Rhizomes were dug out in a separate in vitro experiment for laboratory testing. The rhizomes were categorized by thickness and subjected to a 60 s MWT using a commercial microwave, after which their temperature and vitality were assessed. The temperature of rhizomes following MWT depended on their thickness. Those rhizomes that warmed to temperatures above 42 °C were effectively destroyed. Summing up, the time plants are exposed to microwaves plays a major role in the effectiveness of this method. The longer the exposure to MWT, the better control. The thinner the rhizomes, the more effective the in vitro MWT rhizomes disposal.

Introduction

Japanese knotweed (Reynoutria japonica Houtt.) is one of the seven invasive plant species that threaten the natural environment in Poland¹. This plant, outside its original range, exhibits a wide spectrum of habitats from anthropogenic habitats, including railway embankments, roadsides, parks, cemeteries, home gardens, various types of urban and post-industrial wastelands to natural ones, e.g., forest edges, riverbanks, thickets. It can also sometimes be found in agricultural areas. It copes well with various types of soils with various pH, from acidic to slightly alkaline^2,3. It exhibits high tolerance to high temperature, salinity, periodic flooding, and drought². It is also very resistant to soil contamination, including sulfur compounds⁴. Knotweed seriously threatens nature and contributes to the decline in plant species richness. They effectively compete with native species, preventing their regeneration through rapid growth and limiting light access⁵. They affect other plants alleopathically and cause changes in the physical and chemical properties of the soil. In addition, they negatively affect the human economy by limiting visibility along roads, destroying flood protection, reducing the attractiveness of investment and tourist areas, and causing economic losses associated with their control^6,7.

Many attempts have been made to control Japanese knotweed, mainly using synthetic herbicides such as glyphosate or 2,4-D⁸. However, due to unfavorable environmental effects, this method is not recommended for most sites occupied by knotweed. On the other hand, mechanical methods involve regular mowing of plants, which are ineffective due to the deep system of rhizomes from which new shoots emerge⁹. An interesting solution is to use dense nets that limit the growth of knotweed, but this method also has limitations due to possible damage to the net or shoots growing outside its area. Therefore, lateral methods of controlling this species are sought. One such method may be using microwaves¹⁰.

Microwaves are electromagnetic waves with frequencies from 0.3 GHz to 300 GHz and wavelengths from 1 m to 0.001 m. Microwave radiation is invisible to the human eye. The electromagnetic spectrum of a microwave oven falls within the range of infrared radiation and radio frequencies¹¹. Of the wide range of microwave frequencies, only a few are intended for medical or industrial applications. Federal Communications Commission regulations specify the use of specific microwave frequencies. Microwaves are transmitted through electrically neutral materials such as paper, glass, ceramics, and most plastics and are reflected by metals. In the absorbing material, they cause heat to be generated¹². The electromagnetic field at microwave frequencies mainly provides energy to living organisms within its range. The thermal effect consists of increasing the body temperature due to the body's absorption of some energy. The appropriate frequency, field intensity, and the organism's ability to thermoregulate are required to increase the temperature of the tissue. It also depends on the time of exposure and the type of tissue. When a critical tissue heat level is exceeded, protein denaturation occurs¹³.

Microwave radiation has been used in natural sciences for many years. It is used, for example, for heating air in greenhouses¹⁴, disinfecting soil^15,16,17, and drying fruits and vegetables^18,19,20. Microwaves can also destroy insect pests of crop plants^21,22,23 or weeds at a seedling stage²⁴. Recent studies also indicate the high effectiveness of the microwave method in combating invasive Sosnowsky's hogweed^10,25.

The device HOGWEED was built in the Faculty of Forestry of the University of Agriculture in Krakow²⁶. It has its drive and moves on a caterpillar chassis, which can be used in areas with difficult access. Such a drive system does not damage the ground because the rubber tracks exert low unit pressures on the terrain. A radio remote control remotely controls the vehicle. The device was constructed to study the effect of microwaves on invasive weeds in natural ecosystems.

The study aims to determine the effectiveness of microwave radiation with a wave of 2.45 GHz, a power of 800 W, and an assumed operating time (from 15-25 min) for controlling the growth of Japanese knotweed plants (Reynoutria japonica Houtt.) in the field using the HOGWEED device. The study also aims to determine the disposal of rhizomes in laboratory conditions using a commercial microwave device. Disposal is important in the safe management of invasive plant waste so that it does not threaten environmental safety.

Protocol

We conducted the field experiment using a field population of invasive Japanese knotweed (Reynoutria japonica Houtt.) localized in Kraków (50.04 N, 19.63 E) under the written agreement with and supervision of the Kraków Municipal Greenery Board, which manages this area.

1. Field control of Japanese knotweed using a specialized device emitting microwaves

1. Build the microwave emitter using a magnetron, which generates waves at a frequency of 2.45 GHz and a power of 800 W. Keep the aperture area of the horn antenna at 0.024254 m² (134 mm x 181 mm) and the microwave power density at 32.8 kW/m². Make the waveguide and antenna of four 1 mm thick brass sheets and join them with soft solder. Ensure the inner side of the sheet plate is silver coated to increase the conductivity of the metal surface²⁶.
2. Conduct microwave control of Japanese knotweed during its intensive growth period when plants are around 0.5-1.0 m high.
3. Count the number of above-ground knotweed shoots per 1 m². Cut all above-ground parts of the plants using a hand mower approximately 4 cm above the ground surface.
4. Mechanically remove dry leaves from the surface using a leaf blower to prevent burning during microwave treatment.
5. Record the temperature of the prepared surface before treatment with the thermal camera.
6. Place the microwave emitter on the prepared surface on the shoots in its center and push it slightly to adhere tightly to the ground. Emit microwaves by pressing a button on the machine and carry out the treatment for 25 min, 20 min, and 15 min for a surface of 268 mm x 362 mm dimensions.
7. Record the temperature of the treated surface with the thermal camera.
8. For controls, use surfaces on which the above-ground parts are only mechanically cut off using a hand mower at around 4 cm above the ground (control 1 - mowed), and the rhizomes are dug out to an approximate depth of 30 cm (control 2 - dig out). To help dig out the rhizomes, use a mobile compressor with a narrow-stream nozzle first and then pull out the visible rhizomes with the help of a metal cutter.
9. Check the growth of plants in the research area monthly and compare it with both control areas during the next several months until the month of intensive growth of plants, e.g., from July to May. Manually count and document photographically the number of new knotweed shoots.

2. In vitro disposal of Japanese knotweed rhizomes using microwaves

1. As a microwave source, use a commercial chamber device with a frequency of 2.45 GHz and a power of 800 W, with an electrically controlled capacity of 28 L.
2. Dig out Japanese knotweed rhizomes from a depth of up to 30 cm and cut them into 28 cm sections using shears.
3. Divide rhizome into three thickness classes, using a caliper to measure the largest diameter of the rhizome. Give the measurement result in centimeters to two decimal places. Use a drawing ruler to calculate the result in centimeters to one decimal place. Class I up to 1.00 cm; Class II 1.01-2.00 cm; Class III above 2.01 cm.
4. Choose ten representative rhizomes per thickness class. Weigh the rhizomes' fresh mass with a balance. Express the results in g to two decimal places.
5. Place the rhizomes in a microwave oven and microwave them for 60 s. Immediately after microwave treatment, take a thermogram with the thermal imaging camera to determine the temperature to which a given rhizome had heated up.
6. Weigh the microwaved rhizomes again after microwave treatment when they cool down to room temperature.
7. Take an additional eight rhizomes to determine their moisture and dry mass. Weigh rhizomes before placing them in the laboratory dryer at 105 °C for 2 days. After that time, weigh them again.
8. Determine the temperature of the rhizomes based on the thermograms of the thermal imaging camera. Determine the average, maximum, and minimum temperature of a marked area or section. In this experiment, each rhizome was divided into 5 equally spaced points-ellipses of an area of around 2 cm that did not extend beyond the outline of the rhizome. Then, calculate each rhizome's average, maximum, and minimum temperature from the 5 points.
9. Place the microwaved rhizomes individually on trays lined with sterile cotton wool. Ensure one tray contains rhizomes of the same thickness class. Use separate trays for the control rhizomes.
10. Water the trays with tap water. Cover with colorless food foil to reduce water loss. Place the trays in a shaded area, monitor them, and top with water when needed. Perform monitoring until new shoots are observed or a visible decay of tissues occurs, e.g., for 14 days.
11. For the rhizomes that survive and develop new shoots, perform additional analysis of their temperature along the entire rhizome length.

Results

Field control of Japanese knotweed using a specialized device emitting microwaves
The average number of shoots per 1 m² of the microwave-treated area was 27. Figure 1 shows the average number of shoots per 1 m² that grew after microwave treatment for 11 months after microwave exposure. No new knotweed shoots appeared in areas treated with microwaves for 25 min. Compared to the control area, a decrease in the number of above-ground shoots was noted...

Discussion

The effectiveness of Japanese knotweed (Reynoutria japonica Houtt.) control using the constructed device and knotweed rhizomes disposal using a commercial microwave oven were demonstrated. Both devices emitted microwaves at a frequency of 2.45 GHz and a power of 800 W.

It was observed that the longer the exposure of knotweed plants to microwave radiation, the lower and later their regeneration. In Japanese knotweed, 25 min microwave treatment effectively destroyed 100% of plants in th...

Materials

Name	Company	Catalog Number	Comments
AXIS BTA2100d	AXIS Sp. z o.o.		balance
CompAir C50	LECTURA GmbH Verlag		mobile compressor
FLIR E60	FLIR Systems, Inc.		thermal imaging camera
FLIR Tools	FLIR Systems, Inc.		software to analyse the temperature from the thermogram
HDL_ANT version 3b4 program	PC Software by W1GHZ		software
Heraus UT 6120	Heraeus		laboratory drier