Experimenting with methods to control Tradescantia fluminensis, an invasive weed of native forest remnants in New Zealand

Tradescantia fluminensis, commonly referred to as ‘tradescantia’, is an invasive weed of canopydepleted forest remnants. Previous research suggests that a reduction of tradescantia biomass to ~80 gm-2 (~40% cover) is compatible with native forest regeneration. I assessed herbicide application, hand weeding and artificial shading as methods for the control of tradescantia in two lowland podocarp/broad-leaved forest remnants in the lower North Island of New Zealand. Herbicide spray and hand weeding, applied to separate experimental plots, did not prevent re-growth of tradescantia after three successive treatments. Re-growth of tradescantia and invasion of other weeds were positively related to light availability, which increased in the more canopy-depleted areas, and negatively related to native forest regeneration measured two years after initial treatment. Artificial shading was the most effective method of control. The biomass of tradescantia was significantly reduced in artificially-shaded plots (2–5% full light; 81.3 ± 10.6 gm-2) relative to non-shaded plots (15–27% full light; 597.6 ± 6.6 gm-2; t4 = 17.38, P< 0.001) after 17 months. Native sub-canopy species were planted into tradescantia to achieve natural shading over large areas of forest. After 2.5 years, 61% of the saplings planted had emerged from the surrounding tradescantia.