Alien Invasion, Part 9:

English Ivy

Most everyone is familiar with English ivy (Hedera helix) growing as a landscape border or ground cover. It is one of the ivies that blanket the hallowed halls of education (hence the name Ivy League) and other public and private buildings. However, the reader may not be aware that like so many other horticultural imports, English ivy can become a wildland pest weed. It has invaded in many parts of the United States and can be a nuisance.

English ivy (Hedera helix)

Photo courtesy of Barry A. Rice/ The Nature Conservancy

Hedera helix belongs to the plant family Araliaceae (ginseng, etc.) and is obviously a native of Europe and the British Isles. It is well adapted to many soil types and can grow in both sun and shade. It is fairly drought resistant once it is well established. This makes it able to out-compete native plants in the Elfin Forest. There is a large colony in the Southwest corner of the park in a wooded area. It is believed that the leaves produce chemicals that act as pesticide for insects and snails. If so, it may be toxic to the endangered Morro Shoulderband Dune Snail (Helminthoglypta walkeriana).

English ivy produces round black berries, smaller than cherries. Each berry contains a seed that can sprout and produce a vine that produces alternate, lobed leaves. Usually there are three to five lobes in small, immature leaves. As the vine matures, the adult leaves become oval instead of lobed. When the plant flowers, it produces greenish-white blossoms. The ivy can spread both by seed and by runners. It can climb, using small roots to cling to vertical surfaces, making it very tenacious and hard to remove. The vine can envelop and strangle host trees. In a mild climate such as on the central coast, the ivy can grow all year long and out-compete the native plants.

The vine can be uprooted with difficulty, but the soil surface has to be disturbed. Digging may be more effective in removing enough of the root system to retard re-growth. Simply pulling up the vine usually allows re-growth to occur and only slows the spread. Since large vines cannot be pulled out of trees easily, it may be necessary to girdle the vine or cut a section away, to allow the upper section to dry out and die. The surfaces of the cut should be treated with a herbicide.

Given the difficulty in exterminating English ivy vines, chemicals may be more effective than mechanical methods. For home use, glyphosate (Roundup or Brush-Be-Gone) can be used. A water-soluble chemical such as glyphosate cannot penetrate the wax layer on the leaves. Therefore, ensure that the commercial product contains a surfactant to allow the active ingredient to soak into the plant. Glyphosate is more effective if used at high concentration (25%). Another herbicide that is effective on English ivy is 2,4-D (at 2% concentration).

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