Selection of Drought-Tolerant Shrubs for Temperate Mediterranean Climates

Western Oregon (WO) has a temperate, Mediterranean climate with characteristic warm, dry summers and mild, wet winters. The region is near the northern perimeter of this climate type, with 800 mm precipitation which falls mainly from October to May and 12-16 days when high temperatures exceed 32 degrees C. Therefore, WO has reduced heat and drought stress compared to areas further south, allowing Mediterranean-climate plants to be grown without irrigation in the landscape while retaining satisfactory appearance. The main challenge for growing some Mediterranean-climate plants in WO is lack of cold hardiness. Typical winter temperatures in the region range from -6 to -12 degrees C, however periodic winter freezes of greater severity can damage marginally hardy plants. To expand the selection of shrubs for low-input landscapes in the region, evaluations of several genera have been conducted under the name 'Northwest Plant Evaluations'. This program has focused on evaluating plants for cold hardiness, attractive form and drought tolerance. Evaluations of Ceanothus, Cistus and Halimium were conducted from 2003-2009. Arctostaphylos and Grevillea evaluations were initiated in 2009. Plants are collected as stem cuttings, rooted, and grown-on before planting out in a replicated field plot. After establishment, plants are not irrigated, fertilized or pruned. Data on establishment, growth, flowering, foliage quality, cold-hardiness and pest problems are recorded. Cultivars selected from these evaluations as being superior in cold hardiness, attractive form, and drought tolerance include Ceanothus 'Blue Jeans', Cistus xobtusifolius, C. 'Snow Fire' and Halimium lasianthum 'Sandling'.