Hedera | Pest Rating Proposals and Final Ratings

California Pest Rating for

**English Ivy, Irish Ivy & Algerian Ivy | Hedera spp.**

Pest Rating: None

PEST RATING PROFILE

Initiating Event:

This plant is subject to a petition to the Secretary to list English ivy (Hedera helix) as a noxious weed.

History & Status:

True ivies (Hedera spp.) are vining, evergreen plants with dark, glossy, (generally) lobed leaves. When they reach something to climb on, they grow upwards using innumerable adventitious roots that sprout from the stems. After growing upward into sun, the plant changes it growth form, producing unlobed leaves and rigid stems that project from the framework. These fertile stems produce many small green flowers in compound umbels. The fruits are black berries (yellow in H. helix subsp. poetarum) that are generally dispersed by birds, sometimes to a great distance from the mother plant. Although a single ivy plant can carpet a ~0.1 hectare area or take over a tree, it is the bird-dispersed seed that account for most of the continuous spread of the plants.

The various species of ivy are difficult to distinguish and they sometimes hybridize. Even in their home range, widespread cultivation of non-native species or cultivars make identification difficult. The most common species in cultivation on the West Coast are English ivy (H. helix), Irish ivy (H. hibernica) and Algerian ivy (H. algeriensis, synonym H. canariensis). They can be distinguished by the types of hairs on the underside of the leaves, as well as by their chromosome number. Nevertheless, because of confusion between the species, all three will be treated together.

Ivies are popular ground covers in gardens, where they can dominate large areas with a weed resistant mat. Many cultivars have been developed over the years and some gardeners form collections of the various forms. As pot plants, they are often trained over frameworks into topiaries.

Worldwide Distribution: Ivies are widespread throughout Eurasia, northern Africa and the Canary Islands (Hedera algeriana). As adventives, ivies are widespread in forested temperate and warm temperate areas of the world. In the United States, ivies have been found as introductions in all western, all eastern and all southern states.

Official Control: Ivies are listed as official weeds in Oregon and Washington. In California, some State Parks and other land management agencies have instituted ivy control in lands under their jurisdiction.

California Distribution: Aside from frequent usage in gardens, ivies are adventive in much of the state in the understory of forests. Ivies are only lacking in the hotter, drier areas.

California Interceptions: Ivies have been collected over 350 times in California, although many of these collections represent plants persisting from cultivation.

The risk ivies would pose to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California.

Risk is High (3) as the plant is naturalized in many localities in CA. It prefers the north where there are extensive forests and higher rainfall, but ivies have proved themselves capable of spreading under favorable conditions (e.g., along rivers and creeks) in many areas of California. In Washington, Irish ivy is more invasive than English ivy and much more invasive than Algerian ivy. However, this relationship may differ in other parts of the West Coast. English ivy may be more invasive in California.

Score: 3

– Low (1) Not likely to establish in California; or likely to establish in very limited areas.

– Medium (2) may be able to establish in a larger but limited part of California.

– High (3) likely to establish a widespread distribution in California.

2) Known Pest Host Range: Evaluate the host range of the pest.

Risk is High (3) as weeds do not require any one host, but grow wherever ecological conditions are favorable.

Score: 3

– Low (1) has a very limited host range.

– Medium (2) has a moderate host range.

– High (3) has a wide host range.

3) Pest Dispersal Potential: Evaluate the natural and artificial dispersal potential of the pest.

Risk is Medium (2). The plants produce large vines that can dominate significant areas. Mature plants can form thousands of bird-dispersed seeds per year.

Score: 2

– Low (1) does not have high reproductive or dispersal potential.

– Medium (2) has either high reproductive or dispersal potential.

– High (3) has both high reproduction and dispersal potential.

4) Economic Impact: Evaluate the likely economic impacts of the pest to California using the criteria below.

Risk is Medium (2) as the plants can lower timber crop value by interfering with tree seedling establishment, damage adult trees by overgrowing them and poison livestock if grazed extensively.

Economic Impact Score: 2

A. The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

G. The organism can interfere with the delivery or supply of water for agricultural uses.

– Low (1) causes 0 or 1 of these impacts.

– Medium (2) causes 2 of these impacts.

– High (3) causes 3 or more of these impacts.

5) Environmental Impact: Evaluate the environmental impact of the pest on California using the criteria below.

Risk is High (3) as the plant can dominate forest understories and forest margins, excluding native plants and lowering biodiversity, and can exclude cultural plants from a landscape. They can also affect threatened or endangered species by overgrowing their food plants, e.g., covering rocky slopes supporting the host plant of the San Bruno elfin butterfly (Callophrys mossii bayensis).

Environmental Impact. Score: 3

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered species.

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment programs.

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

– Low (1) causes none of the above to occur.

– Medium (2) causes one of the above to occur.

– High (3) causes two or more of the above to occur.

Consequences of Introduction to California for true ivies:

Add up the total score and include it here. High (13)

–Low = 5-8 points

–Medium = 9-12 points

–High = 13-15 points

6) Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. (-3)

–Not established (0) Pest never detected in California, or known only from incursions.

–Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

–Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

–High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (10)

Uncertainty:

The dry climate has limited ivies’ spread in CA. However, they are still spreading in shady forest and riparian habitats, as they are in OR & WA.

Conclusion and Rating Justification:

These are very bad forest weeds. The magnitude of their planted and escaped populations preclude regional control and ensure their continued widespread distribution. Currently, classifying true ivies as noxious weeds would not significantly impact its current or future distribution, as they would continue to spread despite regulation. Many birds feed on ivy berries and can travel long distances from existing residential and naturalized sources.

Similar invasive and widespread plants have been given a C rating. Only a small number of noxious weeds are C rated weeds; However, the California Food and Agriculture Code section 5004 states that “In determining whether or not a species shall be designated a noxious weed for the purposes of protecting silviculture or important native plant species, the director shall not make that designation if the designation will be detrimental to agriculture.” As ivies are largely weeds of forest areas and are mainstays of the nursery agricultural sector, they would fall under this stricture.

Classifying Hedera spp. as noxious weeds would harm agriculture by preventing the sale of popular nursery plants. At this point in their invasion curve, newly planted ivies represent a very small contribution to the existing populations. Any regulation of this plant would have little or no consequence in limiting its invasiveness or reducing the costs of ivy management. Therefore, given the economic and horticultural importance, no rating is recommended for English ivy, Irish ivy and Algerian ivy at this time.

References:

Ackerfield J. & J. Wen. 2002. A morpholometric analysis of Hedera L. and its taxonomic implications. Adansonia 24:197–212.

Aizpuru I., C. Aseguinolaza, P. M. Uribe-Echebarría, P. Urrutia, e I. Zorrakín. 2003. Claves ilustradas de la flora del País Vasco y Territorios Limítrofes. Eusko Jaurlarizaren. Vitoria-Gasteiz, Spain.

Baldwin, B. G., D. H. Goldman, D. J. Keil, R. Patterson, T. J. Rosatti, and D. H. Wilken, editors. 2012. The Jepson Manual: Vascular Plants of California, second edition. University of California Press, Berkeley, CA.xw

Bramwell, D & Z. Bramwell. 1994. Flores Silvestres de las Islas Canarias. Editorial Rueda. Madrid, Spain.

Consortium of California Herbaria (ucjeps.berkeley.edu/consortium/).

King County: English Ivy, Hedera helix. Accessed 2/15/2017. http://www.kingcounty.gov/services/environment/animals-and-plants/noxious-weeds/weed-identification/english-ivy.aspx

Midori M. C., S. H. Reichard, & C. W. Hamilton. 2006. Prevalence of different horticultural taxa of ivy (Hedera spp., Araliaceae) in invading populations. Biol. Invasions 8:149-157.

USDA Plants; Hedera helix. Accessed 2/15/2017: https://plants.usda.gov/core/profile?symbol=HEHE

Vargas P., H. A. McAllister, C. Morton, S. L. Jury & M. J. Wilkinson. 1999. Polyploid speciation in Hedera (Araliaceae): phylogenetic and biogeographic insights based on chromosome counts and ITS sequences. Plant Systematics and Evolution 219:165–179.

Webb, D. A. 1968. Hedera in Flora Europaea Vol. 2: Rosaceae to Umbelliferae. T. G. Tutin, V. H. Heywood, N. A. Burges, D. H. Valentine, S. M. Walters, & D. A. Webb, eds. Cambridge University Press. United Kingdom.

Responsible Party:

Dean G. Kelch, Primary Botanist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 403-6650; plant.health[@]cdfa.ca.gov