Copper migration from treated wood garden boxes into soil and vegetable biomass Part I: The first two growing seasons after installation

Abstract

Pressure treated wood is a commonly used material for constructing garden boxes and concerns about metal leaching into garden soils and garden vegetables persists among the public. This study describes efforts to quantify copper migration from copper azole-treated garden bed frames into garden soil and vegetable biomass. Two garden bed frames were constructed from copper azole 2 x 12-inch nominal Douglas-fir lumber and two were constructed with untreated Douglas-fir lumber prior to filling with a mixture of native soil and compost. An assortment of common garden vegetables was planted in identical patterns in each of the beds for two growing seasons. During this two-year study, we found no difference in copper concentrations between identical vegetables grown in beds constructed with treated or untreated lumber. After one and two years, average copper concentrations in soil 0-25 mm from the bed frames were about 23 ppm and 21 ppm higher than soils in the same location in untreated beds, respectively (p<0.05, Tukey’s HSD). Elevated copper levels were not detected in beds constructed with treated lumber at 76-102 inches from the frames or the bed center, indicating that metal migration was limited. This study shows use of treated wood garden beds did not lead to increases in copper concentrations in vegetables grown in those beds. Treated bed materials did lose some copper to garden soil but increases in copper are limited to about 20 ppm immediately next to the treated wood frames and were not detectable at any greater distances from the wood.