The Holm oak or Holly oak is a non-native invasive species in the UK. The first trees were brought to the UK as acorns from the Mediterranean, as their evergreen leaves brought colour to the winter landscapes of stately homes. Since holm oaks produce large quantities of seeds that readily germinate, trees have spread throughout southern UK and have only been halted by the frigid northern climate. Trees can grow up to 28 m in height and 6 m in girth; the dense canopy provides thick shade.
Culture for Holm Oak
Trees make for ideal coastal windbreaks and roadside hedges (if trained early) due to their salt and drought resistance, and air pollution tolerance. Most soil types are tolerated providing it is well drained, not too high in organic matter, and have a pH between 6.5 and 8. Full sun is preferred and partial shade is tolerated.
Concerns about Holm Oak
Holm oaks are sensitive to waterlogging and freezing temperatures. Drought tolerance is at least partly derived from its deep root system and thus correct planting location is imperative. Holm oaks are less susceptible to pest and pathogens than our native oaks but may be susceptible to acute oak decline and chronic oak dieback. Trees can still be affected by aphids, and the blotch and linear holm oak leaf miner. The blotch leaf miner causes pale brown oval discolouration on the upper leaf surface by eating the middle leaf layers. The linear leaf miner causes winding mines that often end in a blotch.
Management Practices for Holm Oak
Holm oak leaf miners are difficult to control, damage already incurred cannot be reversed until that leaf is replaced by the plant. Foliage can be treated with an insecticide and surfactant to increase product penetration. Three generations per year mean multiple spray treatments are required, these should occur in spring, early summer, and autumn to give the best chance of contacting the flying moths before damage is incurred. Continuous defoliation can weaken the tree, so a fertilisation programme is advised. Poor draining soils should be corrected.