Copper Migration from Treated Wood Garden Boxes into Soil and Vegetable Biomass Part II: the Third and Fourth Growing Seasons after Installation

Concerns about the safety of preserved wood as a garden box frame material persist in the public stemming from fears about chemical contamination of homegrown food. This study describes the third and fourth years of a long-term study to measure copper migration from copper azole-treated garden bed frames into garden soil and vegetable biomass (Presley and Konkler 2024). Garden bed frames made of Douglas-fir lumber untreated or pressured treated with copper azole were planted with common garden vegetables over two growing seasons. Vegetables and soil samples were collected and analyzed for copper concentration. Average copper levels in vegetables collected from treated or untreated beds were not distinguishable, except for radish roots grown in year 4 which contained higher copper levels (7.5 PPM) when grown in untreated wood beds compared to those grown in treated wood beds (3.7 PPM; p < 0.05 Tukey's HSD). At the end of each growing season, average copper concentrations in soil were significantly elevated in soil in direct contact (0-25 mm) with the treated wood (77.8-101 PPM) over copper levels found at equivalent locations in untreated wood beds (21.1-33.2 PPM) (p < 0.05, Tukey's HSD). No differences in average copper concentrations were observed in soils taken from any other sampling location, indicating that measurable copper accumulation was limited to within 25 mm of the bed edge. This study shows that the use of treated wood as a bed frame material has no impact on vegetable copper content and the impacts of copper migration are small and spatially limited.