The December Weed of the Month is English holly (Ilex aquifolium), a fitting invasive for the season when we “deck the halls with boughs of holly.” While English holly does make a festive decoration, it is not welcome in natural areas where it crowds out native plants.
English holly is native to western and southern Europe, northwest Africa, and southwest Asia. It was brought to the United States as a landscaping plant and is still grown commercially for decorations and floral arrangements. It has since spread to coniferous forests, where it thrives in well drained soils in shady to sunny habitats. It is not regulated as a state-listed noxious weed, but it is widely recognized as an invasive plant in our region.
English holly is a large, dense, slow-growing evergreen tree or shrub that can grow as a single tree or a multi-stemmed thicket. It’s dark green, thick, glossy leaves are wavy and usually have sharp spines along the edges, although older leaves higher up on the plant are often smooth edged. Insect pollinated flowers are small and white and produce red, yellow, or orange berries in the winter. There are both male and female plants. English holly reproduces mainly through the spread of seeds when birds eat the berries, although it can also spread vegetatively. The berries contain alkaloids and are unpalatable and poisonous to humans and pets.
English holly can invade forest habitats and form dense thickets that suppress germination and growth of native trees and shrubs. The leaves also produce a flammable vapor when heated that can ignite easily, possibly increasing fire risk. In areas without English holly, remove any encroaching plants as soon as possible to avoid future infestations. Once it is established and widespread, it is much harder to remove.
It is easy to remove English holly when the plants are small and can be easily hand pulled. Be sure to get all the root to avoid it resprouting. Removing larger plants mechanically will cause significant soil disturbance, so it may be better to use a combination of mechanical (cut the stump) and chemical (apply herbicide using a brush or by injecting it in the cut stump). Monitor the treated trees for several years to remove any regrowth from the roots.
There are several alternatives to English holly that are native. Tall Oregon grape (Mahonia aquifolium) grows up to 10 feet tall, has bright yellow flowers in the spring and blue to black berries in the summer. False holly (Osmanthus heterophyllus) looks like English holly with variegated, evergreen leaves, but it does not produce berries. It can get over 10 feet tall. Both make good screening and hedging plants.
For more information on English holly, including best management practices, visit www.columbiagorgecwma.org or contact the Wasco County Soil and Water Conservation District in the Dalles, Hood River Soil and Water Conservation District in Hood River, or Underwood Conservation District in White Salmon.