Rhododendron ponticum (rhododendron)

Grazing is not a method for control. R. ponticum is unpalatable to most herbivores due to the large amounts of toxic chemicals, particularly 'free' phenols and diterpenes in the leaves (Esen, 2000). The diterpenes, known as grayanotoxins, are not found in all species of Rhododendron but occur in the leaves, flowers and nectar of R. ponticum. Phenols are mostly concentrated in the young tissues, such as young emergent leaves and buds providing a primary defense against herbivores before the tissues have acquired the added deterrent of physical toughness found in older tissues. Young emergent leaf buds have the additional protection of a sticky exudate, which also contains phenols. This also gives protection to the buds against attack by invertebrates.

Heavy machinery with long hydraulic arms is best for digging up the root material and crushing the branches. However, heavy machinery in certain habitats such as ancient woodlands compacts the forest floor causing problems such as increased erosion (Esen, 2000). Operator knowledge of the structural growth of the plant is essential to enable full eradication of the plant without leaving underground suckers. The surface rooting characteristics of R. ponticum should allow for the shrub to be lifted entirely from a site with most of the main rooting structure intact. R. ponticum removal is best suited to autumn and winter in order to make, and take advantage of, a clean uncontaminated seedbed in the spring. Due to the spread of the shrub, a systematic approach is best suited to the removal of the weed at each stage. If R. ponticum is just cut back, this will be followed by vigorous growth from the stumps during the next growing season. An approach of methodical cutting, mechanical removal and burning of root material and stumps is the preferred means to achieve a R. ponticum-free site.

Spraying cut stems with ammonium sulfamate is effective in controlling the growth of R. ponticum though surrounding vegetation may be effected. Glyphosate is commonly used to control R. ponticum in the UK after the bulk of the biomass has been removed by burning or mechanical removal, and triclopyr and imazapyr at differing concentrations have also been used with varying success (Lawrie and Clay, 1989).

Until now, biological control has not been taken into consideration as part of the management strategy for R. ponticum. However, this may offer a potentially simpler, cheaper and safer alternative approach to chemical and mechanical control, or an additional tool in the battle against this weed in the UK. Due to the horticultural importance of the genus Rhododendron, with over 500 spp. currently in cultivation in the UK alone (Mabberley, 1998) the risk associated with the classical approach using coevolved, exotic natural enemies from the native range is considered to be too high (Evans, 2000). However, a number of natural enemies have already been identified in the native range, including a rust fungus from Portugal (Farr et al., 1996). Instead, the proposed strategy is based on the evaluation of natural enemies associated with the weed in the UK, focusing on fungal pathogens for use in an inundative approach. This will involve the mass production and formulation of a selected pathogen and, in the case of R. ponticum, application of such a proposed fungal product or mycoherbicide as a site-selective stump treatment. Potential pathogens include a species of Cryptosporiopsis which was associated with dieback in Berkshire, UK (Shaw, 2003). This strategy has been implemented safely and successfully against woody invasives in the Netherlands, where BioChon, a product based on local strains of the wood-rotting basidiomycete Chondrostereum purpureum, has been marketed for the control of alien Prunus spp. as well as Populus spp. (Jong, 2000). Evaluation of this strategy against similar woody targets is also underway in both Canada and New Zealand.