Road salt used for winter de-icing flushes heavy metals out of bioretention gardens and rain gardens in cities, raising contamination risks — especially in older systems and those near busy roads. Plants in the daisy family (Asteraceae) absorb a wide range of these metals and could help clean the soil, but need seasonal pruning so they don't release metals back when they decompose.
1
Metal concentrations in bioretention media were 2–26% lower in summer than winter, pointing to road salt as a seasonal driver that flushes heavy metals out of the soil and toward waterways.
2
Systems in operation for roughly 10 years had copper, zinc, and chromium levels exceeding environmental toxicity thresholds, especially near inlet zones — suggesting periodic removal of the top 2–5 cm of surface sediment as a maintenance practice.
3
Plant species from the Asteraceae (daisy) family accumulated metals at potentially phytotoxic levels across 14 species studied, identifying them as candidates for phytoremediation in high-metal urban rain gardens.
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