Improved control of rhododendron ponticum for environmental management

Rhododendron ponticum is an invasive non-native shrub that represents a significant threat to productive land use, biodiversity conservation in priority habitats and potentially other ecosystem services in Britain and Ireland. Management actions that reduce the spread and impacts of invasive species require knowledge of their ecological characteristics as well as best control methods. A series of glasshouse experiments was carried out to investigate the effect of microhabitat on R. ponticum germination and seedling survival rates. Light environment in the form of different shade treatments, watering regime and seedbed substrate were manipulated in a factorial design to test their effects and interactions. Short-growing moss, bare soil and sown lawn grass produced a combination of high germination and seedling survival rates, with much lower rates in tall growing moss, leaf litter and grassland turf. Periodic droughting had a large negative effect on R. ponticum seed germination, especially in the more exposed seedbed substrates. The presence of additional canopy shade above a vegetated ground layer was very detrimental to R. ponticum establishment and this effect was found to be due to a reduction in light quality (ratio of red to far red light) and not light quantity (photosynthetic photon flux density). The use of chemical control methods with herbicides, in particular glyphosate, dominates current practice in the removal of R. ponticum from invaded habitats. In order to optimize the efficacy of glyphosate, a series of experiments were designed to improve knowledge of the effect on glyphosate absorption and translocation, of its dose in combination with application at different times of the year in the field, and of foliar application and light level in the glasshouse. Glyphosate applied to the foliage of R. ponticum plants less than 1.5 m in height, was most readily translocated when the plants were more metabolically active. Plants treated in August and May showed greater rates and extent of crown damage than those treated in November and February. 14C-labelled glyphosate applied to the lower leaf surface of R. ponticum plants was rapidly absorbed over the first six hours with maximum absorption after three days. Translocation to other parts of the plant (measured as glyphosate concentration) was greatest to the stem adjacent to the treated leaves and then to the roots, concentrations were lower in untreated leaves over 30 days. Supplemental lighting did not increase the foliar absorption of the 14C but did increase the amount translocated to the other parts of the plant. This study has brought a new understanding to the invasion dynamics of R. ponticum that can be used in identifying habitats vulnerable to invasion and in developing a post clearance management strategy for preventing reinvasion of sites. It has also brought a basic knowledge to glyphosate absorption and translocation patterns that could have profound implications in improving practical application techniques of invasive woody plant species