Polygonum persicaria, lady's thumb, redshank, smartweed, Jesusplant

This species has fibrous roots with a shallow taproot. Stems are branched, often reddish in color and swollen at the nodes. A thin membranous sheath called an ocrea encircles the stem at the base of each leaf petiole. The ocreas of ladysthumb have stiff hairs arising from the top of the ocrea, which are approximately 2 mm long. Flowers are clustered in terminal spikes at the ends of stems. Individual flowers are small and usually pink in color, but can occasionally be white. Fruit is a black achene.

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

Polygonum persicaria has a high socioeconomic impact in the Great Lakes.
The economic impact of P. persicaria in agricultural production is sufficient for various governments to declare this weed as a noxious pest (Anon., 1996; Anon., 2003). The state of Minnesota in the USA has declared P. persicaria as a secondary noxious weed and several states have categorized it as invasive (USDA-NRCS, 2002).  P. persicaria can be very competitive with crop plants, particularly in moist soils and as such can have significant economic impacts in the requirements for use of increased tillage and herbicides, though exact data on costs due to the presence of this species are not known. Holm et al. (1997) note that it is a weed of 35 crops in 50 countries.  Its common name ‘smartweed’ reflects that this plant contains a chemical compound which causes a burning sensation and mild irritation.

Polygonum persicaria has a moderate beneficial impact in the Great Lakes.
This plant is a source of persicarin which is viewed as a candidate therapeutic for the treatment of severe vascular inflammatory diseases, such as sepsis or septic shock (Kim et al 2013).  It has been been demonstrated to removed >60% of nitrite from ground and surface water, but other aquatic macrophytes are available which are even more efficient at nitrite remediation (Rawat et al 2012).  Its leaves and young shoots may be eaten, but this plant is not grown commercially for this purpose. Polygonum persicaria seeds are important food sources for many birds and mammals, including gamebirds, waterfowl, and deer (DiTomaso and Healy 2003).

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control

Biological
Although P. persicaria plants are susceptible to Arabis mosaic virus, no research has been undertaken on the development of biological control agents, whether viral or fungal. No arthropods known to attack this species specifically have been identified (Plantwise 2013).

Physical
When handling smartweed, do not place hands close to eyes – it contains a chemical which burns.

Management of this species in agricultural settings focuses on elimination of seed as a contaminant and cultivation to prevent seedling establishment.  Tillage and cultivation which disrupts seedlings can be effective.  Repeated mowing to prevent seed production can also reduce populations over time (Plantwise 2013).

Solarizing black plastic will kill seeds, as will burning and acetic acid. 

Chemical
P. persicaria seems relatively quick to develop resistance to herbicides – most resources discourage use of chemical control or encourage rotation of herbicides with differing modes of action (Plantwise 2013). 

Pre-emergent herbicides, especially with the chemical Dichlobenil, are effective. Use on-going for up to one year. Systemic herbicides to kill the fibrous roots can also work well. Other options for herbicide control  are non-selective, contact herbicides. Products containing dicamba work best over others containing 2,4-D and glyphosate (Easy Butterfly Garden 2013).

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.