Japanese knotweed sucks more than you think

Background/Question/Methods

Japanese knotweed is a known invasive plant in the United States.  It is present in 40 states and is listed as a top invasive plant or banned in several states.  It grows pervasively along stream banks, but can also be found along roads and parking lots far removed from any streams.  Previous research has shown that the plant can locally significantly reduce stream flow during the hottest part of the summer.  Here we report on further research to assess the impact of the plant on the hydrology of two streams in northern New Jersey.  During the summer of 2012 we measured transpiration with an Infra-red Gas Analyzer (LiCor 6400) across 4 stands throughout the day to estimate total water loss from each stand on a daily basis.  We also mapped the distribution of knotweed stands along two streams using on-the-ground measurements linked to GIS measurements. 

Results/Conclusions

Average water loss per m² of ground area occupied by the plant across three of the stands was 7.4 liters.  This is the result of Japanese knotweed’s high leaf area and high transpiration rate.  Using GIS and stream flow data we estimate that knotweed stands along streams reduce total water flow by about 10% during the summer.  For a different site that was not located along a stream, but was in full sunlight, water loss per m² of ground occupied was 14.5 liters, suggesting that this plant’s impact on ground water content may be significant as well. Given climate change scenarios for the northeastern United States and known water shortages in New Jersey that are already occurring, we propose that the presence of Japanese knotweed will have an exacerbating negative effect on our water supply and stream ecosystem function in the future.