1. Hedera helix L. (English ivy)
Pl. 219 c; Map
910
Plants lianas,
less commonly bushy shrubs. Stems to 20 m or more long, the climbing phase
sparsely branched, becoming anchored by adventitious roots, the young stems
densely stellate-pubescent. Leaves alternate in the climbing phase, sometimes
opposite or whorled in the shrubby phase, evergreen, simple, the blade 2–12 cm
long, ovate to nearly orbicular, entire or more commonly palmately 3- or
5-lobed, rounded to pointed at the tip, rounded to cordate at the base, the
margins entire or slightly wavy, leathery, dark green on the upper surface,
whitened along the veins, yellowish green on the undersurface,
stellate-pubescent when young, becoming glabrous or nearly so at maturity.
Inflorescences thus far not produced in Missouri material, solitary from the
branch tips or in a short raceme, consisting of 1 or more simple umbels each
with a stalk 2–8 cm long. Sepals 5 minute teeth or a low, 5-toothed crown.
Petals 2.5–3.5 mm long, elliptic-ovate, greenish yellow. Style 1. Fruits 7–10
mm long, globose or somewhat obovoid, black, with 3–5 stones. 2n=48.
July–October.
Introduced,
uncommon, known thus far from Madison County and St. Louis (native of Europe;
widely cultivated, escaping sporadically in the eastern U.S.). Mesic upland
forests; also old homesites and railroads.
English ivy is
commonly cultivated as an evergreen ground cover or trellis plant, sometimes
also to hide fences and old walls. The heavy growth and the adventitious
rootlets by which the plant becomes fastened as it climbs can damage trees,
wooden structures, and mortared walls. Plants persist for many years after
abandonment and can also occasionally spread from stem pieces that become
rooted. The creeping/climbing phase does not produce flowers and generally has
lobed leaves, but older plants may sometimes produce stiff, branched stems
forming bushy shrubs, which become fertile at the branch tips and have unlobed
leaves (Robbins, 1957). This shrubby phase is not known outside cultivation in
Missouri. Steyermark (1963) noted that all parts of the plant are poisonous to
humans if eaten and can cause dermatitis when handled in some people.
A number of
infraspecific taxa have been described for H. helix; the few specimens
from outside cultivation in Missouri appear to represent the diploid var. helix
(2n=48), which is characterized by having whitened veins on the upper
leaf surface. However, the tetraploid var. hibernica G. Kirchn. (2n=96),
which is known as Irish ivy or Atlantic ivy and differs most notably in its
green veins (Lawrence, 1956; McAllister and Rutherford, 1990), also is
cultivated widely and may eventually be recorded as an escape in Missouri.
Numerous cultivars are sold commercially as well.