English Holly

English holly is an invasive species in the Puget Sound lowlands. Birds eat the berries and deposit seeds in forest understory where the seedlings establish in deep shade and form dense thickets that exclude native vegetation. It was planted commercially for the cut-branch holiday trade and has escaped extensively. In existing landscapes, the main diseases are Phytophthora leaf and twig blight in wet winters and holly leafminer, which is cosmetic. The plant is dioecious; removing male pollinators within a quarter mile would stop fruit set, but that is impractical in most neighborhoods. At minimum, prevent berry-laden branches from reaching ground where birds scatter them. For new plantings, there is no horticultural justification for English holly when native alternatives like Mahonia aquifolium or evergreen huckleberry serve the same design functions without the invasive risk.

— Chris Welch, ISA Certified Arborist