Invasive Weed of the Month: English Holly
By Cathy McQueeney
English holly (Ilex aquifolium) is a popular evergreen frequently noticed during the month of December. Festive decorators and traditional songs abound, like “deck the halls with boughs of holly.” For many people, the shiny, dark green leaves and red berries of holly are reminders of the holiday season. This popular plant, however, is not native to Oregon. In fact, invasive holly can spread rapidly and degrade our natural areas.
English holly is also known as Christmas holly or simply “holly.” It is native to Western and Southern Europe, Northwest Africa, and Southwest Asia. It was brought to the United States as a landscape plant but quickly proved to be invasive. Now holly spreads along the west coast and into native forest habitat where it thrives in shade, crowding out native species.
Because of the cultural significance of holly, it is actively cultivated in our region. It is not regulated as a state-listed noxious weed and it can legally be cultivated and sold within Oregon. The term “noxious” refers to weeds classified by the Oregon State Weed Board to be “injurious to public health, agriculture, recreation, wildlife, or any public or private property.” This regulatory designation limits the sale and distribution of a plant. Very few culturally and economically significant species receive the noxious weed designation. Although not listed as noxious, holly is widely recognized as an invasive plant in our region.
How Can I Identify English Holly?
Holly is a large, slow growing evergreen, usually found as a shrub or small tree. It has woody stems and its leaves are 2-5 inches long and 1-3 inches wide. The oval, shiny leaves are dark green on the surface, lighter green underneath. Young leaves have three to five sharp spines on each side. Older leaves, and those found higher up in the tree, do not have spines. Flowers of English holly are white, small, and have a sweet scent. This makes them attractive to bees and other pollinators.
Holly berries form on female plants in the fall and can be red, yellow, or orange. The berries contain ilicin, a bitter alkaloid, which make them unpalatable and poisonous to people and pets. Winter frosts soften the berries and make them more tasty to birds and rodents who spread seeds in their droppings. Holly can also spread by suckering and layering.
Why Should I Care About English Holly?
Holly is naturalizing and spreading throughout our area. Coniferous forests are particularly susceptible to invasion. Holly competes with native shrubs and can suppress the natural regeneration of forest trees over time.
Natural resource managers across the Pacific Northwest spend a lot of time and money to treat and control holly in an effort to preserve native forests.
How Can I Control English Holly?
Because of its invasive nature, Clackamas County property owners are encouraged to remove holly plants. Replace them with non-invasive alternatives whenever possible. The ”GardenSmart Oregon Guide to Non-Invasive Plants” lists many ornamental and native alternatives. These plants provide similar function and appearance.
Clackamas SWCD encourages growers who are actively cultivating holly to consider phasing out production. This will help to protect your agricultural operations from losses if holly becomes listed as a noxious weed in the future.
Remove holly when the plants are small, when they can be easily hand pulled or dug up. Be sure to remove all parts of the root to limit re-growth. Check the area regularly to remove any new sprouts.
Removal of larger plants may not be feasible and could cause extensive soil disturbance and erosion. In this case, a cut-stem approach that uses a combination of mechanical and chemical control is more effective and may be less damaging to a site. For additional information on this approach, contact the District’s WeedWise program at 503-210-6000.
Clackamas County residents interested in removing holly from their property may also check out one of the District’s tools from the Weed Tool Library free of charge.
Happy Holidays and happy weed pulling!