Ivy is a name given to many vining plants, but usually to those belonging to the botanical genus Hedera. Generally speaking, ivies trail their stems across the ground and climb upward if provided support such as a tree, fence or wall. These plants also have two distinct growth periods. When juvenile their leaves are smaller and vines are slender and elongating; when older and "mature" they are much more branching and shrub-like, producing tiny flowers that yield black fruits.
Ivy has about 10 species. Some of the more commonly found ivies in garden settings include Azores ivy (Hedera azorica), Persian ivy (Hedera cochica), English ivy (Hedera helix), Atlantic or Irish ivy (Hedera hibernica), and Japanese ivy (Hedera rhombea). Among these species are evergreen, wood-stemmed, trailing or self-clinging plants.
Although called ground ivy (Glechoma hederacea) or poison ivy (Rhus radicans), these two "ivy-like" plants are in different botanical genera, not closely related to true ivies. Boston ivy (Parthenocissus tricuspidata) also is not related.
Ivies are found growing naturally in bright, airy woodlands or rocky openings in forests in northern Africa, the Canary Islands and Azores, western Europe to the Himalayas and in China, Korea and Japan. None of ivies are native to North America.
True ivies display leaves arranged in an alternating pattern on their vining stems. The American Ivy Society uses the Pierot Classification System to categorize ivy plants based on their leaf shapes. Ivy-types bear "classical" leaves that are flat and have five pointy lobes. Heart-shape types have heart-like leaves or triangular leaves that comprise three lobes. Fan types develop broad, fan-like leaves with many pointy lobes of equal length, almost looking like forward-facing fingers. Bird's foot types have narrowly lobed leaves or willow-like leaves that lack lobes and curly type ivies have ruffled, rippled, or pleated leaves or leaf edges.
As members of the aralia family (Araliaceae), true ivy plants can cause dermatitis on skin if the sap is touched. Those with eye or respiratory allergies can be aggravated by the stem hairs when walking in ivy patches or pulling their foliage and stems in garden maintenance tasks. Eating any part of ivy plants also leads to severe stomach discomfort according to the "A-Z Encyclopedia of Garden Plants."
According to Elizabeth Licata in her May 2010 article in "Horticulture" magazine, the only species that is truly invasive or noxiously weedy in North American gardens is Hedera hibernica (Irish ivy), which is visually very much like Hedera helix (English ivy). Irish ivy plants are often mislabeled as being English ivy, adding to the confusion as to which species really is the problematic one. Licata further comments that this confusion has already resulted in many American states, particularly in the Pacific Northwest, banning English ivy in landscapes, even though the culprit is the misidentified Irish ivy species.