Non-native Berberis aquifolium is notoriously invasive in Belgian coastal dunes. With its strong clonal growth through suckers, this evergreen shrub outcompetes native species and affects dune succession. To prevent further secondary spread and mitigate its impact, there was an urgent need for knowledge on the effectiveness of control measures, both at the plant and habitat level. Here, we report on a first control experiment. Individual B. aquifolium clones were subjected to one of four treatments (manual uprooting, foliar herbicide application, stem cutting followed by herbicide or salt application), with regrowth being measured up to one year after treatment. Plants proved most susceptible to foliar herbicide application (5% glyphosate solution), resulting in 77% of the clones apparently killed. We discuss the limitations of our experiment and the potential for actual field application of the different treatments. We present some guidelines for future control that may become further refined as experience builds up.

Experimental treatment of a selection of individuals with limited clonal extension was set up in four heavily infested dune sites (fieldcode region). Individual B. aquifolium plants (fieldcode plant_id) selected for treatment were located (fieldcode x_proj, y_proj) on a certain day (fieldcode date) with a hand-held Garmin Foretrex 401 GPS, photographed and marked with a flag so as to easily relocate them in the field after treatment. Height (cm) (fieldcode height) and diameter (cm) (fieldcode diameter) were measured and the number of stems (fieldcode n_stems) was counted to get an idea of the dimension of each individual or clone. Within the selected plants, clone diameter was 85 cm on average (minimum 10 cm, maximum 6 meter). In April/May 2013, plants were subjected to one of the following management treatments (fieldcode treatment): (1) manual uprooting by digging with shovels (DIG), (2) leaf treatment using a spray bottle with a 5% Roundup Max (450g/l glyphosate) solution (LEAF), (3) stem treatment (cut and paint) with the same glyphosate solution (STUB) or (4) stem treatment with a saturated salt (NaCl) solution (SALT). Each ramet of a clone was treated similarly. We alternated treatments in sequence to different plants. When one plant received a given treatment, we moved to the next plant and applied the following different treatment, making sure plants were sufficiently far apart (minimally 10 meters) so as not to treat the same plant clone and making sure the most closely located plants got different treatments. The 5% glyphosate solution corresponds with the recommended concentration for cut stump treatment of P. serotina which is higher than the recommended concentration (1.5–2%) for leaf treatment (Agentschap voor Natuur en Bos 2013). The salt treatment is regarded as an environmentally friendly alternative because the used quantities of salt, considered on a m² basis, result in soil salt concentrations far below the natural values (Rozema et al. 1983). The direct effect of treatments was compared in terms of categories of stem regrowth (dead, limited regrowth, vigorous regrowth) (fieldcode regrowth), after six months following treatment (November 2013) and after one year (May 2014) (fieldcode date_regrowth). In total, 127 clones were treated. Unfortunately, nine could not be retrieved, probably because flags were removed by site visitors. Nonetheless, the resulting 118 clones measured were more or less equally distributed over sites and treatments.