S11 - Session P3 - Evaluation of broadleaved evergreen groundcovers for un-irrigated landscapes in western Oregon

Authors: Neil Bell *, Heather Stoven, Lloyd Nackley

Weeds in landscapes in Mediterranean-type climates are typically managed by mulch application, hand removal, herbicide use, or by the inclusion of plants that provide solid groundcover in the design. The presence of weeds may be reduced by eliminating irrigation, which diminishes the vigor of some common summer annual weeds. However, without irrigation, groundcover plants must be able to tolerate summer drought while retaining vigor and good aesthetic quality. Western Oregon has a temperate Mediterranean climate with a warm, dry summer and mild, wet winter. Since the region is near the northern perimeter of this climate type, rainfall is greater and summer temperatures are not as extreme as farther south in California. This reduces drought stress and allows many Mediterranean-climate adapted plants to be grown without summer irrigation. However, winter temperatures tend to be lower, with cold weather limiting which of these plants can be grown in the landscape. To expand the selection of groundcover plants for un-irrigated landscapes, an evaluation of 78 taxa of broadleaved evergreen groundcovers, representing 38 genera, was planted in September 2019 at the North Willamette Research and Extension Center in Aurora, Oregon, USA. Following establishment, plants were evaluated for growth, flowering, and injury from winter cold and summer drought stress. Minimum temperatures in each winter since planting have been mild, with a low of -5 o C on November 30, 2019, and -4 o C on February 13, 2021, resulting in little observable damage on any plants. Summer temperatures reached 38 o C in August 2020 and ranged from 41 o C to 45 o C on three consecutive days in late June 2021. Plant dieback from the latter heat event occurred on some taxa, including Trachelospermum spp. and Vinca spp. and sunburn was even evident on some Mediterranean taxa like Cistus x florentinus . Data collection will continue in the trial through 2022.