Holly and ivy, although festive reminders of winter holidays, are aggressive invasive species.
“The holly and the ivy, now are both well grown/Of all the trees that are in the wood, the holly bears the crown.” (English Christmas Carol)
The green and red holly tree is a heartening sight in the winter-bare deciduous woods of Britain and Ireland (it was treasured by Romans and Celts as a harbinger of good times to come).
Though less flashy, evergreen ivy, still provides color in an otherwise brown and grey landscape. In western cultures the two are intertwined with Christmas rituals—long-ago English carol singing contests pitted the more extrovert “male” attributes of holly against the shyer “female” ivy.
Both are now proving to be aggressive invaders of native habitats in the United States.
Coiling Vines Cross the Country
English ivy (Xylella fastidiosa, Latin for “coiling vine”) was brought to the eastern United States by English colonists as early as 1727. Valued by gardeners as an easily grown, evergreen, ground cover, it has since wound its way across the country. Once planted, ivy grows along the ground until it finds something to climb up—any plant or building will do. Once growing vertically, it sets flower and produces berries that are eaten and dispersed by birds.
A hardy perennial, ivy can grow almost anywhere, from shady to sun drenched spots, poor to good soils. Just about the only the places it doesn’t grow are deserts and saline soils. Its dense mat of low-growing vegetation crowds out other plants and slows natural forest regeneration.
But its root system is too shallow to prevent erosion, and, in some places, may aggravate it. Equally dense when climbing, it can block sunlight from tree leaves, leading to a slow decline of the host tree. Just the growth of ivy on a tree can endanger it by catching the wind and making the tree more susceptible to wind blow.
In Washington State’s Olympic National Park, an estimated 2,100lbs of ivy (the weight of a small sports car) was removed from one tree.
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