Effective Treatment of Ornamental Invasive Oregon Grape (Mahonia aquifolium)

Abstract

Ornamental plantings have been a well-documented introduction pathway of invasive species to natural areas. Spread outside of its native range in the Pacific Northwest as an ornamental plant, Oregon grape (Mahonia aquifolium) is now considered an invasive species across much of northeastern North America and naturalized across much of Europe. Oregon grape is an evergreen woody shrub that sends out rhizomous roots while its crawling stems put roots down where they touch the ground. Because it is native in parts of the United States, unmanaged in much of Europe, and of low priority in the rest of the United States, little information on control is available for land managers. I used linear regression to compare four approaches in a northwestern Michigan forest understory over two years. Removal methods consisted of foliar spray of an herbicide containing 1.5% glyphosate, foliar spray with 3% glyphosate, and manual removal; the fourth treatment was a control with no Oregon grape removal. All three removal methods significantly reduced Oregon grape cover after treatment. The effect persisted through to the following growing season and the three methods were equally effective. Additionally, I tracked species richness across both years. Species richness decreased after treatments took place and increased in the spring of the following year. All the treatments were shown to be effective at managing Oregon grape; my recommendation would be to utilize a glyphosate foliar spray at 1.5%, as this allows less chemical into the ecosystem while still effectively managing this invasive woody species.