CROP PROTECTION AND PRODUCTION STRATEGIES FOR HORTICULTURAL CROPS
Location: Application Technology Research Unit
Title: Growth, Flowering and Cold Hardiness of Rockrose in Western Oregon
Submitted to: HortTechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 25, 2009
Publication Date: June 1, 2010
Citation: Bell, N., Altland, J.E. 2010. Growth, Flowering and Cold Hardiness of Rockrose in Western Oregon. HortTechnology. 20(3): 652-659.
Interpretive Summary: Rockrose [Cistus (L.) and Halimium Spach. (Dunal)] are genera of evergreen, woody shrubs of the family Cistaceae native principally to the Mediterranean basin. The distribution of Cistus ranges from the Canary Islands to the Caucasus Mountains. Halimium are confined to the Western Mediterranean with an outpost in Greece. The climate in which these plants originated is Mediterranean, with mild, rainy winters and warm to hot, dry summers. Western Oregon is characterized by a climate with a mild, wet winter and a pronounced summer drought and is comparable to the cool humid Mediterranean climate characteristic of southern France. Nevertheless, few Cistus, Halimium or ×Halimiocistus taxa are grown or used extensively in landscapes in western Oregon or Washington. The objective of our study was to determine growth and relative hardiness of Cistus and Halimium species and cultivars under western Oregon conditions. A total of 98 accessions were obtained either from nursery sources in California or from the United Kingdom. Accessions were planted at the North Willamette Research and Extension Center in Aurora, Oregon for evaluation. Based on ratings of foliage and bloom time, as well as hardiness, several Cistus are recommended as drought-tolerant groundcovers, including C. × gardianus, C. × obtusifolius, C. × laxus, C. inflatus, C. ‘Gordon Cooper’, C. ‘Ruby Cluster’ and C. ‘Snow Fire‘. Several Halimium are recommended for landscape use, including H. lasianthum ‘Sandling’, H. ‘Susan’, H. × pauanum and ×Halimiocistus ‘Ingwersenii’.
This research offers the nursery and landscape industry guidance on which selections of Cistus and Halimium are likely to perform well in arid U.S. climates. These plants, in particular the recommended selections, offer a wide range of ornamental characteristics for xeric landscape designs.
Ninety-three species, cultivars, and hybrid selections of Cistus spp. L., Halimium spp. Spach. (Dunal) and ×Halimiocistus spp. Janch. were evaluated for growth, flowering, and cold hardiness in a landscape trial in Aurora, OR, from 2004 to 2009. Plants were irrigated to aid establishment when planted in summer 2004, but thereafter were not watered, fertilized or pruned throughout the trial. Cold damage was recorded following freezing events in Feb., 2006 and Dec., 2008, in which low temperatures were 20 oF and 17 oF, respectively. Those taxa which consistently suffered the most cold damage were H. atriplicifolum, C. creticus ssp. creticus ‘Tania Compton’, C. × pauranthus, and C. albidus f. albus. Other taxa showed cold damage related to poor vigor. The length of the flowering period and foliage quality varied widely among taxa in the evaluation. The taxa with the longest flowering period were H. × pauanum, C. inflatus , C. × pulverulentus ‘Sunset’ and ×Halimiocistus ‘Ingwersenii’, all of which flowered for more than 55 days. Plant form and foliage quality declined drastically with some taxa during the evaluation. Those that retained the best foliage quality included C. × obtusifolius, C. × laxus, C. salviifolius ‘Gold Star’, C. ‘Gordon Cooper’ H. lasianthum ‘Sandling’, H. ‘Susan’ and ×Halimiocistus sahucii. Based on ratings of foliage and bloom time, as well as hardiness, several Cistus are recommended as drought-tolerant groundcovers, including C. × gardianus, C. × obtusifolius, C. × laxus, C. inflatus, C. ‘Gordon Cooper’, C. ‘Ruby Cluster’ and C. ‘Snow Fire‘. Several Halimium are recommended for landscape use, including H. lasianthum ‘Sandling’, H. ‘Susan’, H. × pauanum and ×Halimiocistus ‘Ingwersenii’.