Kaczmarek-Derda1, A.-K. Holm1,2, L. O. Brandsæter1,2, K. A. Solhaug2 and I. S. Fløistad1
1Norwegian Institute of Bioeconomy Research (NIBIO), Biotechnology and Plant Health, Department Invertebrate Pests and Weeds in Forestry, Agriculture and Horticulture, Høgskoleveien 7, Ås, Norway
2Norwegian University of life Sciences (NMBU), Norway
Japanese knotweed (Reynoutria japonica), giant knotweed (R. sachaliensis) and their hybrid (R. ×bohemica) are tall, robust perennials arising from strong, woody below-ground shoots that form a dense rhizomatous system. The species native to Asia are invasive weeds in many European lands, including Norway. Reynoutria infestations threaten biodiversity by changing the habitat and species composition and are common along railway lines, roads and riverbanks. Since the species primarily reproduces clonally in Norway, the most important control measure is preventing the spread of vegetative fragments. Knowledge about how long the rhizomes can live in soil is crucial to develop control methods, monitoring locations after control and handling of infested soil masses.
Two field studies were established in autumn 2015 in southeastern Norway (59°N 10°E) to investigate survival time of rhizome systems in Reynoutria taxa. Experiment 1 (Exp. 1) examined the survival time of intact rhizome systems when above-ground shoot production was prevented by covering with geotextile, while Experiment 2 (Exp. 2) investigated if fragmentation of rhizomes, either alone or combined with removal of below-ground shoot clumps before covering can reduce the survival time of the rhizomes. In Exp. 1, five stands of R. japonica and R. x bohemica were cut down. Shoots were counted within 2 × 2 m quadrats before the stands were covered with geotextile. In each stand, one of these quadrats were uncovered in early June 2016, 2017 and 2018. A new quadrat was uncovered each year. In Exp. 2, a stand of R. x bohemica was cut down and divided into nine treatment rectangles (4 × 6 m), each containing two counting quadrats (2 × 2 m) where shoots were counted at the start of the experiment. The treatments in Exp. 2 included a) cutting + covering, b) cutting + fragmentation (50 cm depth) + covering and c) cutting + fragmentation (50 cm depth) + removal of shoot clumps + covering. Half of the counting quadrats were uncovered in 2016, the other half in 2017. In both experiments (Exp. 1 and 2), the shoots that had sprouted under the cover were counted and harvested at the day of uncovering. The quadrats were left open for one month, and the regrown shoots were counted and harvested before the quadrats were covered again. The harvested plant material was dried at 60°C for one week for dry matter (DM) determination.
For Exp. 1, preliminary results showed that the number of shoots increased within all stands in 2016 and 2017, but decreased within four out of five stands in 2018, compared to the start of the study in 2015. The mean dry matter of shoots one month after uncovering decreased, by more than 70% in 2017 and by approximately 98% in 2018, compared to 2016. The increase in shoot number may be an effect of the cutting at the start of the experiment. For Exp. 2, without fragmentation, the number of shoots one month after uncovering increased in 2016, but decreased in 2017, compared to at the start in 2015. The treatments that included fragmentation reduced the number of shoots in both years. All treatments resulted in a higher decrease of shoot biomass in 2017 than in 2016. Fragmentation contributed considerably to reduced regrowth in 2016, especially when combined with removal of shoot clumps, but the differences were less pronounced in 2017.
The experiments show that the rhizome systems survive longer than two years and none of the treatments allowed for complete control of Reynoutria taxa within this period. However, a longer duration of covering resulted in a higher reduction of new shoots and dry matter. Covering combined with fragmentation and removal of shoot clumps contributed most to the reduction of regrowth.