Potential of pyrolysis liquids to control the environmental weed Heracleum mantegazzianum

Replacement of synthetic pesticides with biochemical alternatives and other biological and mechanical control methods represents a future need in plant protection. We investigated if slow pyrolysis liquids (PL) originating from hardwoods, which contain a wide range of organic compounds, can be used to control giant hogweed (Heracleum mantegazzianum) either by (i) spraying directly on the seeds (Carum carvi seeds used as substitute) and seedlings or by (ii) covering seedlings with PL-containing mulching material (PLM). The effectiveness of the methods was evaluated in laboratory and greenhouse experiments using seedlings of various ages, PLs produced from aspen (Populus sp.), birch (Betula sp.) and willow (Salix sp.) and various PL/PLM application doses. In addition, the biodegradation of birch-derived PL was investigated. All tested liquids inhibited C. carvi seed germination effectively when used at > 20% concentrations and only slight differences existed among PLs produced from different biomasses. Direct spraying of PL on H. mantegazzianum seedlings was ineffective. PLM (containing 7.5%–40% of PL) inhibited seedling development effectively. Birch PL was readily biodegradable. Further product development is needed because the chemical composition of the PLs in PLM and their modes of action are poorly understood. The weed-inhibiting effect of PLM likely results from the PL and the mechanical barrier constituted by PL-bound peat fibers