Persicaria hydropiper (L.) Delarbre - syn.Polygonum hydropiper L. - Polygonaceae - water-pepper, Wasserpfeffer

Annual herb, up to 80cm tall; leaves short petioled or sessile, lanceolate to narrowly rhombic, without dark triangular or lunate blotch adaxially; flowers 1-3 per fascicle, perianth greenish proximally, white or pink distally.
http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=242100096

„… a cosmopolitan plant, found in Australia, New Zealand, temperate Asia, Europe, and North America. It has some use as a spice because of its pungent flavor.“ https://en.wikipedia.org/wiki/Persicaria_hydropiper

„All parts of Persicaria hydropiper have an acrid, pepperlike taste. The plants have a long history of medicinal use in Europe, and the oily exudate produced in multicellular glands can cause skin irritation, hence the common name smartweed (R. S. Mitchell and J. K. Dean 1978). Some Native American tribes used P. hydropiper as a drug to treat a variety of ailments, and the Cherokee and Iroquois consumed it as food (D. E. Moerman 1998).“ http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=242100096

„The pungency of Persicaria hydropiper (L.) Spach (formerly Polygonum hydropiper L., synonym water pepper) is produced by polygodial, a 1,4-dialdehyde derived from drimane terpenoids.“
[Comparison of volatile constituents of Persicaria odorata (Lour.) Sojak (Polygonum odoratum Lour.) and Persicaria hydropiper L. Spach (Polygonum hydropiper L.)., Starkenmann, C., Luca, L., Niclass, Y., Praz, E., Roguet, D., Journal of agricultural and food chemistry, Vol.54(8), 2006, 3067-3071]

polygodial

Main components of dichlormethane extracts of leaves a of P.hydropiper were polygodial (44.2%), neophytadiene isomers I, II, III (21.0, 7.6, 3.2%), (E)-β-farnesene (7.7%), α-humulene (6.2%), caryophyllene (6.1%), α-bisabolol (1.1%), and (E)-nerolidol (0.5%). The drimanes isodrimenin and drimenin were present at 0.65% and 0.19% resp.
[Comparison of the chemical composition of three species of smartweed (genus Persicaria) with a focus on drimane sesquiterpenoids., Prota, N., Mumm, R., Bouwmeester, H.J., Jongsma, M.A., Phytochemistry, 108, 2014, 129-136]

Lindman,C.A.M., Bilder ur Nordens Flora, vol.2 t.356 (1922-1926)
http://plantgenera.org/species.php?id_species=770860

water-pepper, CC BY-SA 3.0, Author: Andreas Kraska