Invasive Plants of California's Wildland

English ivy (Hedera helix) is the familiar vining plant often allowed to grow up building walls. It has two forms: an evergreen woody vine and an evergreen shrub. Both forms have deep green, glossy, leathery leaves. Vining plants do not produce flowers or fruits, and their leaves have lighter-colored veins and three to five lobes. Upright shrubby plants may produce flowers and fruits, and their leaves are ovate rather than lobed. In both forms the leaves may have a strong odor when crushed. The white flowers are in clusters on the ends of stems produced in fall, and the fruits are dark blue or purplish drupes. English ivy may be distinguished from grape vines (Vitis sp.) and Ampelopsis species by its evergreen leaves, which are not hairy or fuzzy (pubescent), and by vines that have no tendrils. It differs from cape ivy (Delairea odorata) in having leaves that are evergreen in all climates, with a deep cleft at the leaf base that makes the lower lobes appear larger than the others. Cape ivy may also be distinguished from this species by its small, yellow composite flowers.

Araliaceae. Perennial, evergreen woody vine to 99 ft (30 m) (juvenile plant) or shrub (adult plant). Stems: creeping juvenile stems have roots at leaf nodes with adventitious rootlets that allow the plant to climb vertical surfaces by adhering to, but not penetrating, bark and brick. Adult flowering stems erect and non-climbing. Leaves: leathery, simple, and alternate. On juvenile plants leaves have 3-5 lobes and are 1.6-4 in (4-10 cm) long and about as wide. Terminal lobe about as broad as long; the two basal lobes may be reduced or absent. Lobes often more pronounced on leaves of climbing stems.

Leaf base cordate and veins markedly lighter in color. Leaves on flowering stems mostly unlobed, ovate to rhombic, base shallowly cordate to cuneate, and veins slightly lighter in color. Petioles on both forms about as long as the leaf. Young shoots, leaves, and peduncles covered with stellate hairs and scales; older shoots and leaves glabrous. Inflorescence: a raceme that appears umbellate. Flowers: bisexual, radial, usually 0.2-0.3 in (5-7 mm) across, with 5 sepals fused at the base and persistent but small. There are 5 separate white to yellowish green petals; stamens usually 5 and alternate with petals; 1 style with 5-lobed stigma; ovary inferior. Fruits: berry-like drupe about 0.24-0.36 in (6-9 mm) in diameter containing 4-5 seeds; drupe usually dark blue to black, lighter on some cultivated varieties. English ivy flowers in fall, and fruits are produced the following spring in April and May (Putz and Mooney 1991).

English ivy is found in northern California forests south to at least Santa Cruz. It has also been observed in Shasta and Butte counties and along the south coast from Santa Barbara County to San Diego. It is a serious problem in the coastal Pacific Northwest from central Oregon into British Columbia. On the eastern seaboard it also spreads into woods, particularly from Virginia north to New York. English ivy is generally found in open forests, especially those with a deciduous component, from sea level to 3,300 feet (1,000 m) elevation. It is especially common in forests near urban areas. It climbs up tree trunks and along branches into the canopy and may also cover the ground. English ivy will invade riparian zones where flooding has disturbed the soil, but it does not grow well in areas where the water table is high and soil is waterlogged (Thomas 1980). It grows well in acid and basic soils.

English ivy is native to England, Ireland, the Mediterranean region, and northern Europe west to the Caucasus Mountains. In its native range English ivy is widespread and usually found in woods and along rocky areas. It is often considered a weedy pest in its native range (Wyman 1954). It was introduced into North America in early colonial times as an ornamental (Wyman 1969). English ivy has been planted to control soil erosion in many parts of the United States because of its habit of rooting at the leaf nodes along the stem. It is perhaps the best known of all evergreen vines in cultivation. Birds disseminate the seeds. Once established, it spreads quickly by vegetative means.

English ivy can alter natural succession patterns in forests. It forms “ivy deserts” of vigorous vines in forests where nothing else seems able to compete. It inhibits regeneration of understory plants, including forest wildflowers and new trees and shrubs (Thomas 1980). By blocking regeneration in forests, it jeopardizes their long-term persistence. English ivy also kills trees in the understory and overstory by shading them out (Thomas 1980). It tends to grow up tree trunks into branches, especially those of deciduous trees. The increased winter light under deciduous trees apparently allows this evergreen vine to grow rapidly upward in winter (Thomas 1980). Once in the canopy, English ivy can shade out deciduous foliage during summer months, suppressing the growth of the tree that supports it. As the tree dies back, its increasingly open crown allows the vine to grow even more (Thomas 1980). In addition to shading, the additional weight of water and/or ice on the evergreen ivy leaves may increase storm damage to trees, especially in the presence of high winds. This effect has been observed in trees infested with Vitis sp. in Connecticut (Siccama et al. 1976).

English ivy may replace species used by native wildlife. Its leaf litter adds nitrogen to the soil, which may disadvantage native species that compete best under lower nutrient levels (Tremolieres et al. 1988). The sap can cause dermatitis in some people, and both berries and leaves are toxic (Hickman 1993).

While vegetative reproduction is a key to the success of English ivy, the plant also reproduces prolifically by seed. English ivy flowers in fall, and fruits are produced the following spring in April and May. The juvenile period is long, often ten years or more, but when it becomes reproductive, it produces large numbers of bisexual flowers in fall that are attractive to pollinating bees. Seeds ripen the following year, and on average about 70 percent are viable (Dirr and Heuser 1987). English ivy seed has a hard coat that must be scarified before it can germinate, a condition easily met as the seed passes through the digestive systems of birds that disperse the fruits. The fruits are eaten by several species of birds. English ivy reproduces vegetatively by adventitious roots along the stem and may regenerate from stem fragments if they remain in contact with the soil. The vines can persist a long time; there have been reports of a vine that was 433 years old (Putz and Mooney 1991).

English ivy grows as a woody vine when young, becoming increasingly shrubby as the plant becomes reproductive. Because its leaves are evergreen, it can photosynthesize year-round and will grow rapidly if unchecked. Its ability to climb structures using adventitious roots suggests that it is well adapted to establishment in late successional forests (Carter and Teramura 1988).

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The root system is shallow. Growth in the adult shrub form is slower. English ivy tolerates shade, but its growth is stimulated by light. Thomas (1980) found that in heavy shade (4 to 7 percent of full sunlight) English ivy survived but began to slowly decline, while under 65 to 68 percent of full sunlight the plant flourished.

Control of English ivy has not received sufficient attention or research. Research in the past has focused on establishing new cultivars rather than on controlling or eliminating the plant.

Manual/mechanical: The best method for controlling English ivy may be hand removal of vines using pruners to cut the vines and then pulling the plants up from the forest floor and down from the trees. Removing and killing vines that spread up into trees is especially important because the fertile branches grow primarily on upright portions of the vine. If vines are cut at the base of the tree the upper portions will die quickly but may persist on the tree for some time; vines on the ground around the tree should also be removed to prevent regrowth up the tree. Care should be taken to minimize disturbance during removal. If the forest floor becomes disrupted, appropriate native species should be planted on the site to inhibit reinfestation by English ivy or another invader (Humphries et al. 1991).

Prescribed burning: An extreme method that has been used with some success is to burn ivy plants and resprouts with a blow torch at regular intervals; the energy used by the plant to regrow will eventually be depleted. Obviously, this approach requires considerable caution. No other attempts to use fire to control English ivy have been reported.

Insects and fungi: There have been no attempts to introduce biological control agents, and it is extremely unlikely that such agents will ever be used. English ivy is an important landscape plant and has strong support from the horticultural community, including a society dedicated to its study and promotion (the American Ivy Society).

Grazing: The palatability of English ivy to grazing animals is unrecorded.

English ivy is tolerant of preemergence herbicides (Derr 1992). Its waxy leaves make effective application of post-emergent herbicides difficult, even when a surfactant is added. Glyphosate (as Round-up®) applied at a rate of 2.7 lb/acre effectively controlled young plants, especially in early spring (Neal and Skroch 1985), but tests on more mature plants suggested that adult upright English ivy is tolerant of this herbicide. This was true even when surfactants, high application rates (4 lb/acre), and second applications were used (Derr 1993), although growth may be retarded as much as 60 percent.

Workers at the Washington Park Arboretum in Seattle have had some success using a string trimmer to remove most of the leaves and young stems and then immediately spraying triclopyr (as Garlon 4®) at a rate of 6.5 oz/gal plus a surfactant. Two years after application the treated plants were dead, although the area was being reinfested from surrounding populations. In smaller infestations the herbicide can be brushed onto cut stems. Check with a certified herbicide applicator for concentrations of these herbicides currently registered for use in California.