English holly is a versatile, hardy plant combining year-round beauty and wildlife appeal. But when it escapes cultivation it can become an uninvited pest.
My first encounter with English holly (Ilex aquifolium) was making holiday decorations in elementary school. Nearly 20 years later, I encountered the plant again as a restoration volunteer with the City of Seattle working to remove invasive plants from parks and greenbelts. Himalayan and evergreen blackberry (Rubus sp.), English ivy (Hedera helix), and English holly were removed and replaced with native sword ferns (Polystichum munitum), salal (Galtheria shallon), and Oregon grape (Mahonia aquifolium).
When visiting my parent’s property in rural Grays Harbor County, I was surprised to find English holly among the second growth Douglas fir (Pseudotsuga menziesii) and Western red cedar (Thuja plicata) forest; these plants were volunteer offspring of an old tree planted on an adjacent property. Seeing the interlopers randomly scattered throughout the forest made me wonder why English holly grew in one area versus another. Since 2009, I have worked to answer that question for my Master’s thesis at the University of Washington’s School of Environmental and Forest Sciences.
English holly is an evergreen plant that can grow as a tree or sculpted into hedges to form a living fence. Its red berries provide a speckling of color in the dark green foliage and provide a valuable food source for birds who find refuge within the dense branches. A shade-tolerant understory in its native European deciduous forests, English holly grows equally well in sun or shade in the Pacific Northwest, where our mild climate provides ideal growing conditions.
Introduced in Oregon in 1874—possibly by a homesick Englishman or woman wanting to recreate a bit of the English countryside—a market developed for the plant’s foliage for holiday décor. Today more than 90 percent of the English holly sold in the United States is grown in the Pacific Northwest with the support of the Northwest Holly Growers Association. Washington farms alone generate $5 million annually, providing employment and tax revenue.
The very traits so prized by gardeners—hardiness, adaptability, and wildlife support—are the same qualities that make English holly a potential problem as birds distribute the seeds throughout parks and forests without respect for property lines. Once a seedling matures into a sapling, survival is almost guaranteed. If this sapling is a female, and if there is a male tree nearby to enable fertilization, more berries are produced and the cycle repeats itself. An extensive, deep root system enables the plant to withstand tough conditions. And like Western red cedar, English holly can sprout new shoots from branches lying on the ground, creating dense, difficult-to-eradicate thickets where it competes for space and nutrients to the detriment of native plant communities.
In 2010, King County submitted a proposal to the Noxious Weed Control board (NWCB) asking that English holly be designated a class C noxious weed so counties could educate citizens about the plant’s potential negative impact on the landscape. Steven Burke, speaking on behalf of King County, presented an Earthcorps survey of Seattle’s urban forests at an NWCB meeting. The survey reported that English holly occupies 320 stems per acre; in the City’s coniferous forests, English holly represents over 60 percent of all tree regeneration. From 2007 to 2009, Seattle Public Utilities spent nearly $90,000 to eradicate English holly at Lake Youngs. Arguments against the noxious weed classification were offered by people living around holly farms who did not find the plant to be invasive and holly farmers who maintained that they could control the plant with chainsaws.
After weighing public testimony, the board declined the designation based on a lack of scientific evidence about the plant’s impact on native areas and in protection of the commercial English holly market, which they felt would be economically damaged by a noxious weed classification. But this wasn’t the end of the English holly debate. In 2011, House Bill 1169 and its companion Senate Bill 5087 sought to amend the laws governing noxious weeds by stipulating that a plant “actively cultivated in Washington as a commercial crop” cannot be classified as a noxious weed. The legislation was revised with language stating that any listed plant must have scientific evidence to support its listing, and the legislation passed.
Debate over English holly has quieted for now, but the study of its effect on the landscape is ongoing. My research focuses upon whether stand characteristics such as tree spacing or volume can be used to predict English holly’s presence within a forest. Other researchers predict that English holly may eventually become the dominant understory species within City parks and forests, which could have long-term implications for the structure of Pacific Northwest forests.
So what’s a gardener to do? English holly is here to stay. Pacific Northwest natural resource managers are more concerned about its presence in natural areas such as state parks or national forests than in urban settings like backyards. Yet the story of English holly is a cautionary tale of the unexpected consequences of introduced plants. While some plants cannot survive without human assistance, others thrive, escaping cultivation. Even though English holly has lived in the Pacific Northwest for over 100 years, only now are we starting to see its effect on our forests and wild spaces. Which prompts the question: How many newly introduced plants will venture on the wind or be carried by a bird beyond the backyard to impact the native environment? And at what cost?