Invasive Ivy

Irish or Atlantic Ivy

"Creeping where no life is seen,
A rare old plant is the ivy green."

-Charles Dickens


Helix hibernica (formerly H. helix 'Hibernica') is a widespread invasive ivy encountered in woodlands crossing ground eradicating all native plants in its path & climbing up into trees where it eradicates epiphitic native plants such as licorice ferns. It is in just about all o our public parks, & there is scarsely a neighborhood that doesn't have it in great abundance. Education on these matters being far too limited, many people who have this stuff all over their property don't even know they should get rid of it.

Here in Washington state 'Hibernica' is classified as a Class C invasive. Class C invasives are those which can probably be controlled with a concerted effort, but the law does not thus far demand that Class C invasives be uprooted from garden landscapes. I believe these ivies should be banned, as there are scores of varieties of non-invasive ivies, & just no excuse for keeping or encouraging Hibernica.'

The photo on this page shows a little section of this large-leafed ivy on a hillside a half black from our house. It's one of two yards on our block just thick with the stuff. It has not only covered an extensive hillside, but has climbed up to the top of Pacific madrona & is depriving it of light.

Our own home had a shitload of this stuff in front of it when we bought the house, but with the help of a homeless chap who asked if he could help in the garden for a pittance, the ivy was completely dug out & there is now an elaborate wonderful garden where once only this crappy aggressive ivy flourished.

English Ivy is common throughout western Europe except along the Atlantic coast, where the Atlantic or Irish ivy takes over.

In a garden Irish ivy as well as many large-leafed varieties of English ivy (H. helix) won't share space well with other plants. But what defines it as invasive is not the fact that it is aggressive in the garden, but that it flowers & fruits & birds transport the seeds to woodlands where the ivy out-competes native plants.

In England & Ireland many plants have evolved to cope with the presence of ivy, but in the Pacific Northwest, native plants evolved without this competitor, so are easily overwhelmed when it arrives.

It flowers only in full sun, so if it can be kept in the shade it may never flower, hence never fruit. But spreads of ivy used as roadside groundcover, or which have permitted to climb to the tops of trees to find full sun, inevitably do flower & seed into the broader environment.

The fruit is toxic to many of the birds that eat them, & have been shown to lower bird populations. Again, in its native range birds are adapted to its toxins & can eat the berries without ill effect, but Northwest fauna is at risk from what to them proves to be a poisonous exotic. The one bird that is unphased by the toxicity is the starling, which is itself an alien interloper native of areas of Europe where it evolved to feed on ivy fruits. So while invasive ivies are harmful to our native birds, those same ivies assist interlopers in extedning their population.

The Washington State Noxious Weed Control Board, citing a 1999 masters thesis by Midori Murai at the University of Washington, noted that Murai "evaluated the taxonomic identity of 58 invasive ivy populations in the Pacific Northwest. She found that 80 percent of the 119 samples were derived from H. hibernica 'Hibernica,' while 13 percent were from H. helix cultivars. Murai also examined genetics & growth rates of potentially invasive Hedera taxa. Her findings indicated that H. hibernica 'Hibernica,' H. helix 'Baltica,' H. helix 'Pittsburgh,' & H. helix 'Star' have invasive potential & should be avoided as landscaping plants in the Pacific Northwest."

The popularity of these ivies are understandable since they thrive in dry shade where little else wants to grow, are attractive, & easy to grow without attention. But with hundreds of non-invasive cultivars available, there's really no excuse for 'Hibrernica,' 'Baltica,' 'Star' or 'Pittsburgh.' Only these four ivies are on the Class C list, so it is no burden on anyone to avoid planting them, & replace what one may already have with ivies that are not harmful.

Some experts are of the opinion that H. hibernica 'Hibernica' & H. helix 'Baltica' are actually the same rather than distinct cultivars, but Murai's research lists them as separate. So there may actually be only three varieties to rigorously avoid or remove, vs hundreds of non-invasive cultivars to fulfill the needs of all ivy lovers.

Ivies have been gardened in North America since 1727, but have only been in the Pacific Northwest since about the 1890s. In the Olympic rain forest, the ivy removed from a single tree was estimated at 2,100 pounds of attached vines. This extra weight is responsible for added breakage & storm damage. Again, in its natural range in Europe ivy is part of a healthy ecosystem & climbs mainly into trees already beyond their prime, & provides habitat for animal species evolved to take advantage of ivy's behavior. But as a rainforest invader, it is not part of the natural ecosystem.

When a gardener makes the intelligent & decent decision to get rid of all their invasive ivy, it is almost impossible to do it chemically with the types of herbicides available in stores. Because of a waxy cuticle, ivy repells herbicides, & those chemicals which are the exceptions require special liscensing & training to use, & even then absorption by host trees is likely, & death of all nearby vegetation is inevitable. The main method to be rid of it is mechanical removal of vines from trees, & repeat blow-torching or mowing of the ground-level population which in time, eventually, will wear itself out regrowing.

If one has an extensive amount of this ivy, control can be an overwhelming project. Focus can be made on that which grows in sunlight & so is apt to flower & fruit; that which climbs trees; & removal of any area of ivy that has acquired upright mature portioins which are what flowers. Remove all flowers between December & March, before the seeds are mature.


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