A Proliferation of Ivy

March 10, 2022
| Education, Management

Different invasive ivy species may have different habitat preferences and kill techniques. Learn how to identify among different species — then report them to EDDMapS so researchers can access data!

Nearly every place I’ve lived in the eastern U.S. and Midwest (10+ places) has had invasive evergreen ivy vines as part of the landscaping, not planted by me! I always called it English ivy, Hedera helix. But after reading “Increasing liana frequency in temperate European understories is driven by ivy” by Perring et al. (2020) and a response by Boeraeve et al. (2021), I wish that I had examined all that ivy more closely. Maybe some of it was the even more aggressive Atlantic (or Irish) ivy, Hedera hibernica.

Counting Chromosomes

I have been aware that there might be more than one species of ivy out there for a number of years, but some taxonomists have felt that H. hibernica was a variety or subspecies of H. helix, rather than a separate species. The most recent taxonomic treatments find that H. hibernica is a tetraploid derivative of H. helix and should be considered an independent species. Tetraploid means it has four copies of each chromosome as opposed to the usual two copies that diploids have. Not all tetraploids become different species, and it depends on how different physical characteristics are and ability to interbreed. Several recent papers conclude that Hedera hibernica is in fact the most widespread species in North America, followed by H. helix and H. colchica (Persian ivy, an octaploid), with a smattering of H. algeriensis (Algerian ivy, another tetraploid).

Hedera helix ssp. helix L. specimen from University of Washington Herbarium, Vascular Plant Collection.

Hedera hibernica invasive ivy — Hedera helix ssp. helix L. specimen from University of Washington Herbarium, Vascular Plant Collection.

However, distribution varies by range. Green et al. (2013) found that H. helix was more common on the east coast, and H. hibernica more common on the west coast. They also found a few populations of a triploid hybrid between H. helix and H. hibernica where the two species grow together. Southern California also had some populations of H. algeriensis, and there are some naturalized populations in Florida. Again, there may be more populations of these species that have gone unrecognized.

Why is it important to differentiate among ivy species?

Different ivy species may have different habitat preferences. Atlantic ivy is less cold-tolerant than English ivy for instance (McAllister and Rutherford, 1990), but grows more vigorously in common gardens (Green et al. 2013). Tetraploids of other species, including Canada goldenrod (Solidago canadensis) and spotted knapweed (Centaurea maculata), dominate invasive populations. In Europe, English ivy is found most frequently in inland understory forests (Perring et al. 2021). Green et al. (2013) speculate that habitat preferences could be why English ivy is more common on the east coast. Algerian ivy may be more drought tolerant than other ivies. The different ivies may be differently susceptible to different kill techniques, but management literature generally hasn’t distinguished among the ivy species. Accurately reporting where different ivy species occur will help researchers answer these questions.

Dot Your I(vie)s!

Illustration of Hedera hibernica trichomes.

Above image credit: Manual of the Alien Plants of Belgium, CC BY-SA.

I’ve compiled some key differences below, but when it comes time to compare trichomes, the tiny hair-like structures on the leaves and stems, you will likely need 30 – 40x magnification. Trichomes are easiest to see on the young leaves of trailing (rather than climbing) ivy vines. On older leaves you may still see trichomes, but they won’t be as useful for identification. In the following descriptions the leaves referred to are the leaves of trailing (juvenile) vines.

English ivy trichomes detail. — Hedera helix trichomes (clusters of hairs on surfaces of leaves and stems) radiate outward from the leaf in all directions, leading to more fuzzy appearance and texture. Photo © Doug Goldman, CC BY 4.0, from bplant.org.

Hedera helix – Leaves generally less than 8 cm wide or long. More distinct lobes than other species with a pointier central lobe. Usually has raised veins that look silvery. Very young leaves are lobed. Shoot tips often densely hairy. Whitish trichomes more upright and less than 1mm in diameter. Odor of cut stems weak or acrid.

Atlantic ivy trichome detail — *Hedera hibernica* trichomes tightly pressed against the leaf or stem surface. Photo © Doug Goldman, CC BY 4.0, from bplant.org.

Hedera hibernica – Larger leaves up to 10 x 10 cm, shallower lobes with a rounded central lobe. Veins not raised and green in color. Very young leaves are unlobed. Shoot tips sparsely hairy. Pale yellowish-brown trichomes on undersides of leaves lie flat and are less than 1mm in diameter. Odor of cut stems piney and sweet.

Hedera colchica – Larger leaves, can be greater than 10 cm, and are often longer than their width with very shallow lobes. Very young leaves unlobed and folded. Trichomes much smaller (<0.4mm) with more rays, starting out white and turning rusty brown. It may be easier to observe the trichomes of this species on the buds or petioles of young leaves. Cut stems have a strong resinous odor.

Hedera algeriensis – Large leaves, 12-20 cm long, with glossy upper surface and shallow lobes. Leaf bases are relatively straight across (truncate). Upper leaf surface glossy. Scale-like trichomes.

Please, this spring get out those magnifying glasses and compound microscopes and record your ivy observations on EDDMapS!