Poplars are ideal for phytoremediation because of their high water usage, fast growth, and deep root systems. We measured in 2002 and 2003 the sapflow of hybrid poplars (Populus nigra L. x P. maximowiczii A. Henry 'NM6') planted in 1999 for phytoremediation of a landfill in Rhinelander, WI, USA (45.6 degrees N, 89.4 degrees W). Mean sap velocity per tree was 100 +/- 10 and 120 +/- 10 mu m s(-1) for 2002 and 2003, respectively. Mean sapflow per tree was 1.4000 +/- 0.1698 and 5.6760 +/- 0.2997kgh(-1) for 2002 and 2003, respectively. Sapflow was negatively correlated with temperature, wind speed, precipitation, and vapor pressure deficit for both years (0.002 < r(2) < 0.61). Sapflow increased as mean sapwood area increased from 43.8 +/- 2.6 to 122.3 +/-7.6 cm(2) for 2002 and 2003, respectively (r(2) = 0.88). Individual-tree extrapolations using the mean tree approach were: 34 and 136 kg tree(-1) d(-1) for 2002 and 2003, respectively, and 612 and 2 448 kg tree(-1) 18-d-1 for 2002 and 2003, respectively. Extrapolations to the stand were 2.8 and 11.3 mm d(-1) (28.3 and 113.3Mgha(-1)d(-1)) for 2002 and 2003, respectively (assuming 833 trees ha(-1)), and 354 and I 416 mm yr(-1) (3.54 and 14.16 Gg ha(-1) yr(-1)) for 2002 and 2003, respectively (assuming a 125-d growing season). Thus, we believe NM6 and other superior-performing poplar genotypes exhibit great potential for phytoremediation applications where elevated water usage is critical. (c) 2006 Elsevier Ltd. All rights reserved.