European holly – Puget Sound Forest menace

English holly female with red berries
Ilex aquifolium female

By Richard Olmstead

“Deck the halls with boughs of holly, fa la la la la…”

Holiday season is upon us and it’s a good time to rethink our attitude towards that venerable holiday decoration – holly. English holly, Ilex aquifolium, to be specific, is now a Class C noxious weed in Washington.

English holly is a European species introduced to the Pacific Northwest as an ornamental and as a commercial crop for seasonal greenery.

There are some 400 species of holly worldwide, but none are native to the PNW. Unlike most flowering plants, hollies have separate male and female plants; each have clusters of small white flowers, but only females bear the familiar bright red berries. All have shiny, dark green leaves with sharp teeth along the margins. The berries are attractive to birds, which eat the fleshy outer fruit and disperse the seeds throughout the environment.

English holly male in flower
Ilex aquifolium male

Research conducted at UW Bothell in St. Edwards State Park (Stokes et al., 2014; Church et al., 2025), found that wild holly is spreading exponentially, with doubling times of approximately 5-6 years! In addition to being dispersed by birds, holly also spreads horizontally by root shoots and stem layering resulting in individual plants forming dense thickets.

In St. Edwards, individual thickets sometimes exceeded 500 square feet and were so dense and dark that almost no native plants were found growing underneath; especially affected were salal (Gaultheria shallon) and sword fern (Polystichum munitum). And while hundreds of trees were counted, only a single dead one was found, showing that they are long-lived and have few, if any, pathogens. The largest trees were about 50 years old, dating to the time that development began to surround St. Edwards monastery, implicating use of holly in landscaping as the source for the invasion.

What can you do?

First, don’t plant holly as part of your home landscaping.

Second, consider removing any holly on your property and substituting other, non-invasive, trees or shrubs. Washington State Noxious Weed Control Board suggests three holly alternatives: Meserve hybrid hollies (Ilex x meserveae cultivars), Osmanthus or false holly (Osmanthus heterophyllus) or our native tall Oregon grape (Mahonia aquifolium).

If you opt to remove holly, small trees can be dug up to remove the roots; for larger trees, cut the trunk near the ground and apply an herbicide to the cut surface that will be absorbed into the roots without harming other plants in the vicinity (Whatcom County Noxious Weed Board provides a fact sheet, or see your local garden center for a safe herbicide). Be careful not to mistake our native Oregon grape species, which have superficially similar leaves, for holly.

Alas, how will I decorate our fireplace mantle this holiday season? Listing English holly as a noxious weed came after years of debate between advocates for its listing, who pointed to its spread and negative impact on native forest communities, and commercial growers, who depend on the harvest of berry-laden boughs for holiday season decorations. A compromise was reached to permit commercial growers to grow holly, and only the escaped, or feral, holly designated as a noxious weed. So, enjoy your holiday holly, but make sure to properly dispose of any remaining berries to help prevent the spread of this forest menace.

References

Stokes, D.L., Church, E.D., Cronkright, D.M., & Lopez, S. 2014. Pictures of an invation: English holly (Ilex aquifolium) in a semi-natural Pacific Northwest forest. Northwest Science 88: 75-93. Nonnative English holly (Ilex aquifolium L.) associated with reduced cover and diversity of common native understory plant species

Church, E.D., Stokes, D.L. & Gold, W.G. 2025. Nonnative English Holly (Ilex aquifolium L.) Associated with Reduced Cover and Diversity of Common Native Understory Plant Species. Natural Areas Journal 45: 198-208.