Preemergence herbicides are commonly applied to nursery containers for control of annual weeds in the production of ornamental plants. Pine bark is a popular container growing medium because it is inexpensive, drains well, is easy to transport, and supports acceptable nursery crop growth. However, little is understood about leaching of herbicides through pine bark. The downward movement of these herbicides through container media may inhibit root growth in sensitive nursery crops and also reduce herbicidal efficacy. Four experiments were conducted at two different irrigation volumes to evaluate depth of pendimethalin movement in packed columns of pine bark and field soil. After 17.5 cm of water was applied over 7 d, pendimethalin moved downward into the 6 to 9-cm depth in 100% pine bark, whereas no movement was detected below the 0 to 3-cm depth in a Tetotum loam soil, as determined by a large crabgrass bioassay. Doubling the irrigation volume to 35 cm of water applied over 14 d did not significantly increase pendimethalin movement in pine bark or field soil. However, it did decrease pendimethalin persistence in the top 0 to 3-cm depth in pine bark. The pine bark had a higher cation exchange capacity than did the field soil. However, the physical characteristics of pine bark, a large volume of void space and low bulk density, resulted in higher hydraulic conductivity rates than in field soil. These factors may be the principal reasons that pendimethalin leached to a greater extent through pine bark than the field soil.

Nomenclature: Pendimethalin; large crabgrass, Digitaria sanguinalis (L.) Scop.