Hedera - general info:
Hedera is a genus of 15 species of climbing or ground-creeping evergreen woody plants in the family Araliaceae, native to the Atlantic Islands, western, central and southern Europe, northwestern Africa and across central-southern Asia east to Japan. On suitable surfaces (trees and rock faces), they are able to climb to at least 25-30 metres above the basal ground level.

They have two leaf types, with palmately lobed juvenile leaves on creeping and climbing stems, and unlobed cordate adult leaves on fertile flowering stems exposed to full sun, usually high in the crowns of trees or the top of rock faces. The juvenile and adult shoots also differ, the former being slender, flexible and scrambling or climbing with small roots to affix the shoot to the substrate (rock or tree bark), the latter thicker, self-supporting, and without roots. The flowers are produced in late autumn, individually small, in 3-5 cm diameter greenish-yellow umbels; the fruit are small black berries ripening in late winter. One of the plants that produce berries that attract birds.

Hedera Ivies are very popular in cultivation for their evergreen foliage; many cultivars with variegated foliage and/or unusual leaf shape have been selected. They are particularly valuable for covering unsightly walls. Much has been argued as to whether ivy climbing trees will harm the tree or not; the consensus in Europe is that they do not harm trees significantly, though they may compete for ground nutrients and water to a small extent, and trees with a heavy growth of ivy can be more liable to windthrow. Problems are greater in North America, where trees may be overwhelmed by the ivy to the extent they are killed; this could be because ivy in North America, being introduced, is without the natural pests and diseases that control its vigour in its native areas. A more serious problem is that ivy creates a vigorous, dense, shade-tolerant evergreen groundcover (precisely the characteristics for which it is often cultivated) that can spread over large areas and outcompete native vegetation.

Similar concerns are expressed about damage to walls. It is generally considered that a soundly mortared wall is impenetrable to the climbing roots of ivy and will not be damaged, and is also protected from further weathering by the ivy keeping rain off the mortar. Walls with already weak or loose mortar may however be badly damaged, as the ivy is able to root into the weak mortar and further break up the wall. Subsequent removal of the ivy can be difficult, and is likely to cause more damage than the ivy itself. Modern mortars that contain portland cement and little lime are stronger than older mortar mixes that were largely composed of just sand and lime. Most mortar mixes changed to contain portland cement in the 1930s. Soft mortar is still used when laying softer brick.

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Zones: 7-10

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