English Ivy

English Ivy

Some background info, Hedera helix also known commonly as English Ivy, has a native habitat is around Eastern Europe, Western Asia, and Northern Africa.  This has not stopped this pervasive plant from expanding their range significantly into the United States.  Common uses for English ivy were as an ornamental plant, or commonly as an alternative to lawns due to some of its 'better' properties of being relatively pest free, cold hardy, and fast growing, and low maintenance.  You may already be drawing some conclusions as to why this plant happens to be invasive, and I'd say you'd be right, those properties help along English ivy, letting it spread easily along wide ranges of latitudes.

English Ivy overgrowing a forest, tress and all.

English Ivy is considered invasive because it can overtake under-story plants and kill trees, and as stated above, the English ivy can be pest free, this is because there are no natural predators.  Ivy does spread easily a well, the seeds are dispersed by fructivorous birds, while the vines grow and spread locally.

Original Figure, Spread of English Ivy in the U.S. Click to Expand