Ivy is a woody stemmed, self-clinging climber that can grow quickly into the canopy of a tree. Where it grows as a trailing, ground-cover plant it roots in at many points and its stems extend over a wide area.
The botanical name for ivy is Hedera and it includes the native climber English ivy described here (Hedera helix). Ivy is identifiable as a dense-leaved, evergreen climber. In its climbing and ground cover states it has three- to five-lobed glossy leaves, frequently with prominent veins. Once it reaches the upper parts of a tree’s canopy, ivy often produces shrubby (arboreal), mature growth which bears yellow-green flowers followed by black berries. The leaves are then heart-shaped.
Somewhat to my surprise, when I started researching ivy, I found the words “Common ivy is a popular ornamental, valued for its ability to thrive in shady places, provide excellent ground cover and cover unsightly walls, sheds and tree stumps”. I have seen it in garden centres, most often the variegated forms in hanging baskets where it does look quite pretty. In the ground, I have usually found it invasive and hard work to get rid of! However, it will grow in full shade and being vigorous with a dense growth pattern is undoubtedly useful to cover ground where the object is to crowd out weeds. As ground cover, it spreads horizontally and reaches about 8 inches in height. However, due to its holdfasts, it can also climb, allowing it to reach heights of 50 feet or more.
A common sight in geocaching 
Ivy is not a parasite, and regular trimming can prevent ivy becoming too heavy, a problem that can be exacerbated by the additional weight of rain and snow. Its root-like anchors do not penetrate the bark or roots of trees, they are solely anchors. Its presence on the trunk is not damaging and where it grows into the crown this is usually only because the trees are already in decline or are diseased and slowly dying. It usually only grows up established or mature trees which can tolerate the competition for water and nutrients. As a vigorous vine, it may cause problems to the trees it climbs by blocking sunlight in the canopy and the added weight can be a problem for stability.
Whether or not ivy damages healthy trees is a much debated topic and I have no intention of trying to settle the argument here! Removing it is best done by cutting it at the roots and allowing the vine to die before pulling it away. Control is often applied on trees that are old or damaged, and on ash and larch trees both of which have a naturally thin, open crown so can may suffer heavy infestation. Trees grown for decorative bark are also often kept free from ivy!
It has various other benefits. Ivy can be grown on slopes to control soil erosion. As the Ivy vine grows, it sets roots into the soil deeply and densely, and creates a network of roots which binds the soil, helping to prevent erosion and slippage on slopes.
It is a valuable plant for wildlife and is recommended by the RSPB and loved by Bill Oddie, particularly for providing berries for birds in winter when most others have been eaten. Many insects also feed on the nectar of its white flowers, which are an invaluable late nectar source. As ground cover in woodland, ivy greatly lessens the effect of frost, enabling birds and woodland creatures to forage in leaf litter during bitter spells. Growing on trees, it provides hiding, roosting, hibernating and nesting places for various animals, birds and insects (including butterflies), particularly during the winter months and in areas where there are few other evergreens.
Although ivy will eventually bear greenish flowers and fruit, it is mostly known and grown for its evergreen leaves. It is collected for winter decorations; ivy is associated with Christmas and frequently features in festive designs. In the Christmas carol, "The Holly and the Ivy,” we see a symbolism with these two evergreen plants that derives from pagan times. While the druids thought of holly as masculine and suggestive of a male deity, English ivy plants for them represented female divinity. In the ancient Greco-Roman world, ivy was sacred to the god, Dionysus (Bacchus in Latin).
The berries are moderately toxic to humans, but are very bitter, so poisoning is rare. Ivy foliage contains substances capable of inducing an allergic reaction (contact dermatitis) in some people. People who have this allergy are also likely to react to carrots and other members of the Apiaceae family as they also contain falcarinol.
The name ivy has also been used as a common name for a number of other unrelated plants, including Boston ivy (Japanese Creeper, in the family Vitaceae), Cape-ivy or German-ivy (in the family Asteraceae), poison-ivy (in the family Anacardiaceae), and Swedish ivy (in the family Lamiaceae).