Holly and Ivy Too Well Grown: Invasive Holiday Plants in the United States

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Bright green ivy leaves. Image by The Equinest.

Aggressive Ivy Threatens To Take Over

From gardens and hallowed halls, ivy has now spread to city parks and natural areas.

Officials in Vancouver, BC, expect that it will take 50 years or more to clear ivy from its parks. Portland, OR, has a “No Ivy League” of dedicated volunteers who devote their time to pulling ivy out of Forest Park—the largest urban park in the country.

In Seattle, WA, ivy covers 700 acres of urban parklands (that’s about 700 football fields).

Bees and birds back in England feast on ivy’s nectar and berries. Bats, birds, squirrels live in the dense growth on trees. But here, few native insects or animals inhabit its dense monoculture, though rats and other urban rodents often find a haven there.

English ivy can also serve as a reservoir for bacterial leaf scorch (Xylella fastidiosa) that can severely damage the leaves (and so the health) of several garden and commercial tree species, including elms, oaks, and maples, in the eastern states.

Eighteen states, and the District of Columbia have reported ivy as an invasive species and several have listed it as a noxious weed in need of control, including both Washington and Oregon. Despite this, it is still marketed as a ground cover plant and sold in many “big box” garden centers.

Holly’s Thorny Side

And what of the lovely holly (Ilex aquifolium, Latin for “evergreen oak with sharp leaves”)? Its red berries are as attractive to birds here as in Europe and holly is rapidly invading natural areas in Seattle, where it is regarded as the city’s most invasive alien tree species. It is slow growing, and dioecious (there are male holly trees and female holly trees) and pollinated by bees, so the female tree must be close enough to a male tree for bees to fly between them for a good crop of berries.

But, holly can also reproduce vegetatively—tree branches that touch the ground can take root. Naturalized populations have been found throughout the western part of the state and it is on the monitor list of weeds maintained by the Washington State Noxious Weed Control Board. King County (in which Seattle is situated) has listed holly as a “weed of concern” that should be controlled in natural areas and protected forests.

Holly is valued as a Christmas decoration and nursery owners have resisted efforts to list it as a noxious weed, which would come with stricter controls on its sale and distribution.

Ivy and Holly Control: What Next?

What can you do about this? If you have English ivy in your garden, get rid of it—it’s not hard to pull it up from small areas, especially if the soil is wet—and replace it with some native plants. At the very least, don’t let it grow up along trees, fences, or buildings, so it won’t flower and set seed.

If you feel Christmas wouldn’t be the same without the red and green of holly, there are some non-invasive cultivars and some native holly trees that are just as pretty as the English variety.

Resources:

City of Portland. No Ivy League. (2012). Accessed December 21, 2012.

University of Wisconsin. English Ivy. (2006). Accessed December 21, 2012.

Anderson, D. The Holly and the Ivy. Hymns and Carols of Christmas. (2012). Accessed December 21, 2012.

Reberg, K. English Holly. Web Pages for Multiple Organisms. University of Wisconsin. (2006). Accessed December 21, 2012.

Richards, D. English ivy is an invasive weed in Pacific Northwest. Oregon State University Extension Service. (2008). Accessed December 21, 2012.

Swearingen, J., B. Slattery, K. Reshetiloff, and S. Zwicker. Plant Invaders of Mid-Atlantic Natural Areas, 4th ed. National Park Service and U.S. Fish and Wildlife Service. (2010). Accessed December 21, 2012.

Swearingen, J., Sandra Diedrich. English Ivy. Plant Conservation Alliance’s Alien Plant Working Group: Least Wanted. (2009). Accessed December 21, 2012.

Seattle Urban Nature. Which Invasive Species is the most extensive on public land in Seattle? The Understory. Vol 1 No 1. (October 2004).

Washington State Noxious Weed Control Board. Garden Wise: Non-invasive Plants for Your Garden (Western Washington). (2008.) Accessed December 21, 2012.

King County Noxious Weed Control Program. Weed Bulletin – English ivy – Hedera helix. (2004). Accessed December 21, 2012.

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