Herbicide Residue in Soil: Bioassay with Peas Before Planting Shrubs

Concern

Unknown weed killer/pesticide history in soil. Safe to plant ornamental shrubs? Remove soil?

Only Herbicides Matter

Insecticides/fungicides rarely affect woody plants. Herbicides can persist months to years.

Simple Bioassay

1. Sow pea seeds in the suspect soil (in pots or in ground).
2. If they germinate and grow normally (no twisting, stunting, chlorosis), soil is safe for shrubs.
3. Peas are sensitive to common residual herbicides (clopyralid, aminopyralid, picloram, sulfonylureas).

If Peas Fail

Wait, activate soil microbes (water, OM, warmth), retest. Or replace top 12–18 inches.