In mild parts of the Pacific Northwest and along the California coast, English ivy grows too well. It crawls into the woods from adjacent gardens, spreads by bird-transported seed, and starts from trimmings gardeners dump at the forest's edge.
In Oregon, the problem is so serious that the Department of Agriculture there may forbid the sale of plain English ivy next year (they've already declared it a noxious weed). Sale of numerous small-leafed and variegated cultivars would still be allowed, and gardeners who have ivy could keep it.
To protect woodlands, keep ivy indoors, or outdoors only as a potted plant or in a contained area. If your ivy blooms, cut off flowers and seed heads, and consider replacing the ivy with a noninvasive ground cover like Epimedium.
Also keep ivy out of trees. It can weaken them by causing crown rot and by overtaking and killing leaves. To remove ivy from a tree, cut vines off at the base. After the vine above the cut dies, strip it off by hand.