Can a reduced rate of herbicide benefit native plants and control ground cover weeds?

The use of herbicide to control weeds in natural areas can cause non-target damage to resident native plant communities and compromise native restoration goals. We tested 'full' and 'reduced' (half) rates of herbicide (rates based on previous glasshouse trials) on the ground cover weed species tradescantia (Tradescantia fluminensis), plectranthus (Plectranthus ciliatus), and climbing asparagus (Asparagus scandens) to determine whether the reduced rate would cause less non-target damage to natives and achieve sufficient control of the weeds. We also included a manual removal (hand-weeding) treatment, and experimental control (non-treatment). These four treatments were applied to dense ground cover weed infestations at six lowland forest sites. Subsequent responses of the ground cover weed, native and other exotic plant species were monitored for 24 months. Two months after treatment, biomass of all three weed species was reduced to extremely low levels across all treatments relative to controls. Twenty-four months after treatment, biomass of plectranthus remained low, but tradescantia and climbing asparagus had recovered to near pre-treatment biomass levels across all treatments. Recovery of tradescantia was positively correlated with canopy openness. The reduced rate of herbicide gave a similar level of weed control to the full rate, across all three weed species, however repeat treatments appear necessary for sustained control of tradescantia and climbing asparagus. At sites invaded by plectranthus, the reduced rate of herbicide also increased native species richness. However, herbicide treatments had no effect on native plant abundance or native species richness at sites invaded by tradescantia and climbing asparagus. Manual removal sometimes benefited native plants, but all treatments increased abundance and/or species richness of other exotic plant species at some sites. Generally, native plant abundance and native species richness decreased with increasing canopy openness, while the opposite was true for exotic species. This study provides evidence that, although repeat applications are likely necessary, a reduced rate of herbicide can benefit native plants and achieve ground cover weed control. Further research to fine-tune these results and extend them to other ground cover weed species would be invaluable.