Introduction
Rockrose (Cistus and Halimium) are genera of evergreen, woody shrubs of the family Cistaceae native principally to the Mediterranean basin. The name Cistus is derived from the Greek word kisthos or kistos, classical names for various species. These days they are commonly known as Rock Roses, for the resemblance of the flowers to old-fashioned, single roses and for the preferred habitat of the plants, which is rocky, relatively poor soil.
The genus is the source of ladanum, an aromatic, viscous substance which is exuded from the leaf and stem hairs of some species, notably C. ladanifer, on warm summer days. Ladanum has been used since antiquity as a component of incense and perfumes. It is still harvested and used in this way in Crete, and small amounts are exported elsewhere even today. For the gardener, the value of ladanum is that long after the flowers are finished, the scent of the plants on warm, still summer nights has a unique ornamental appeal
The 20 species of Cistus and seven species of Halimium have remained as separate genera since the 18th Century. The hybrid of these two genera (×Halimiocistus.) contains three species. 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. In Cistus, flowers are unscented and are usually white, pink or purplish.
In Halimium, the flowers are also unscented and are commonly yellow, or they may be white.
The spent flowers develop into reddish or dark brown capsules, which split open to reveal numerous small dark brown or blackish seeds. The flowers in some species and cultivars in both genera may also have a spot at the base of each petal, which lends an additional ornamental appeal to the flowers. In Cistus, only one species, C. ladanifer, has a reddish or dark brown blotch at the base of each petal, and has conveyed this characteristic to the various hybrids which have blotched petals.
In Halimium, two species, H. halimifolium and H. lasianthum, have a reddish or dark brown blotch, and some selections of other species also have this feature.
The climate in which these plants originated is Mediterranean, with mild, rainy winters and warm to hot, dry summers. Throughout this region, Cistus and Halimium form an important component of the xeric shrub community, commonly known as maquis. All of the species are evergreen shrubs, which range in habit from prostrate to sprawling or, in some cases, erect large shrubs. Although all are evergreen, they are considered to be drought semi-deciduous (malacophyllous), possessing the ability to develop different types of leaves in summer and winter, and to drop leaves during prolonged periods of drought (Acosta et.al., 1997). As a result, Cistus and Halimium are well adapted to growing in hot, dry situations, and in relatively poor soils.
Rockroses in the Pacific Northwest
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 (Nahal, 1981). Nevertheless, few Cistus, Halimium or ×Halimiocistus taxa are grown or used extensively in landscapes in western Oregon or Washington. The most commonly grown are C. ×hybridus and C. ×purpureus, both of which may be used as tall groundcovers or specimen shrubs.
The genus has suffered from a reputation of being short-lived and tender. This reputation may partly be a result of the limited selections cultivated, but may also be attributed to plants being grown in landscapes that are watered regularly in summer – a common practice in summer-dry areas such as western Oregon.
The objective of our study was to determine growth and relative hardiness of Cistus and Halimium species and cultivars under western Oregon conditions and also to identify superior selections for local landscapes.
Note: Recently, Demoly (2006) has proposed assigning all the Halimium and ×Halimiocistus to Cistus. As the plants in this evaluation were obtained and evaluated under their previous names, those will be retained for the purposes of this evaluation.
Cold hardiness of Cistus and Halimium
Published studies on hardiness of Cistus or Halimium are rare and tend to be anecdotal in nature. Johnson (1947) commented briefly on the relative hardiness of some common Cistus in the U.K. after cold weather in Feb. 1947. Mulligan (1953) evaluated approximately 18 species and cultivars of Cistus and ×Halimiocistus in Seattle, WA from 1949 to 1953 and found significant variations in hardiness and ornamental qualities. Mundie (2001) evaluated Cistus in northern England from 1997-2001. However, most research on response of either Cistus or Halimium to winter conditions has focused on physiological responses of the plants to cool winter temperatures, not necessarily their specific cold hardiness. Various studies have demonstrated changes in chlorophyll content (Nunez-Olivera et al., 1994), pigments and antioxidants (Garcia-Plazaola et. al., 2000) and photosynthesis (Oliveira and Penuelas, 2004). These studies do not typically relate these changes to hardiness, although in a study of physiological responses of eight Mediterranean shrubs to winter stress, Varone and Gratani (2007) found C. incanus (syn. C. creticus) to be one of the least cold hardy and suggested that in addition to summer drought, winter temperatures might represent an additional limitation to Mediterranean species productivity.
Literature cited
Demoly, J-P. 2006. Notes taxonomiques, chorologiques et nouveautés nomenclaturales pour le genre Cistus L. élargi, incluant Halimium (Dunal) Spach (Cistaceae). (Taxonomical, chorological and nomenclatural new data about the genus Cistus L., including Halimium (Dunal) Spach (Cistaceae)). Acta Bot. Gallica. 153:309-323.
Garcia-Plazaola, J.I., A. Hernandez and J.M. Becerril. 2000. Photoprotective responses to winter stress in evergreen Mediterranean ecosystems. Plant biol. 2:530-535.
Johnson, A.T. 1947. Cistuses and frost. J. of the Royal Hort. Soc. 72:248-249.
Mulligan, B.O. 1953. A Cistus collection in Seattle. J. Calif. Hort. Soc. 14:131-141.
Mundie, A. 2001. Cistus trial 1997-2000. Northern Gardener. 55:50-51.
Nahal, I. 1981. The Mediterranean climate from a biological viewpoint. In: Di Castri, F. D. Gooddall and R. Specht, (eds.). Mediterranean-type shrublands. Ecosystems of the world 11. Elsevier, Amsterdam, pp 63-86.
Nunez-Olivera, E., J. Martinez-Abaigar and J.C. Escudero. 1994. Chlorophyll content of a Mediterranean shrub (Cistus ladanifer L.) over a latitude and altitude gradient in the Iberian Peninsula. Photosynthetica. 30:133-042.
Oliveira, G. and J. Penuelas. 2004. Effects of winter cold stress on photosynthesis and photochemical efficiency of PSII of the Mediterranean Cistus albidus L. and Quercus ilex L. Plant Ecol. 175:179-191.
Varone, L. and L. Gratani. 2007. Physiological response of eight Mediterranean maquis species to low air temperatures during winter. Photosynthetica. 45:385-391.
Materials and methods
Plants were from two main sources. A total of 98 accessions were obtained either from nursery sources in California or from the United Kingdom. Fifteen of these were obtained as stock plants from nurseries in California in Sept. 2003. These plants were grown on through early October, at which time 4 inch tip cuttings were taken. Unrooted tip cuttings of the other 83 accessions were obtained from the National Collection of Cistus and Halimium held by Robert Page in Leeds, UK in Oct. 2003. These cuttings, and those removed from the stock plants, were all struck in early Oct. 2003.
Five of the taxa failed to root in sufficient number and were not included in the evaluation. Rooted cuttings were potted into 6 in square nursery pots in Mar. 2004.
Plants were grown on until planting in the evaluation site on June 22.
The evaluation site was a 0.5A south-facing slope at NWREC at 155 ft elevation. The soil series was a Willamette silt loam. Prior to planting, the site was subsoiled to a depth of 1 foot, then disked to break up and level the soil. The trial was divided into 12 rows 185’ long, spaced 10 ft apart, orientated north to south. Individual plants were randomly assigned to rows and planted at an in-row spacing of 5 feet.
After planting, plants were watered in by hand, then received periodic overhead water through the end of Aug. 2004. Plants received no supplemental irrigation after this time.
No fertilizer was applied after planting for the duration of the trial. Plants were not pruned. Because irrigation was not provided in summer, the plot remained mostly weed-free and weed management involved occasional hand weeding. The exception to this was a one-time spot application of herbicides to control a crabgrass (Digitaria spp.) infestation in summer 2004 caused by the irrigation during establishment.
Data collected included plant height, flowering season, cold hardiness evaluation and plant form and foliage quality. Cold hardiness evaluations were done in early spring after mild weather allowed for full symptom development from any prior cold injury. Data were collected only in Apr. 2006 and in Apr. 2009, as there was no apparent damage following the winters of 2004/05, 2006/07 and 2007/08. Cold hardiness was rated on a 0 to 5 scale, with “0” meaning no cold injury, and “5” indicated the plant was killed. Foliage quality and overall plant form were rated on a 1 to 5 scale, with “1” indicating a sparsely-foliated plant with poor form and dieback and“5” meant a flawless canopy with dense foliage. Flowering was also rated on a 0 to 5 scale, where “0” meant no flowers present and “5” meant the plant was completely covered with flowers.
Figures 1-4 show the flowering period of all cultivars based on a rating of “2”.
Results
In general, the plants became established, grew, and filled in the rows by 2007. Photos below show the growth of the planting from 2004-2008.
There were some plant losses over the course of the trial affecting some cultivars in particular. All plants of C. ‘Enigma’ died shortly after planting, a fact which was attributed to the very small size of the plants at planting compared to other cultivars. Other than the loss of all plants of this cultivar, other losses occurred sporadically from undetermined causes. Among the other Cistus, three ‘Silver Ghost’ as well as two each of C. ×pauranthus, C. albidus var. albus, C. ×argenteus ‘Silver Pink’ and C. ×dansereaui ‘Jenkyn Place’ were lost, as well as individual plants of C. heterophyllus, C. ×ralletii, C. ×aguilarii, C. monspeliensis, C. ×dansereaui ‘Decumbens’, ‘Victor Reiter’ and C. ×heterocalyx ‘Chelsea Bonnet’. Among the Halimium, two H. atriplicifolium and one each of H. ×pauanum, and ‘Susan’ died over the course of the evaluation.
Although aphids can occasionally be seen on Cistus, no serious infestations of these pests were observed. The only noticeable disease problem was sooty mold, which was observed on the foliage of some Cistus cultivars in spring, being especially noticeable on the previous season leaves as the bright green new foliage was emerging. The sooty mold did not appear to be growing on honeydew caused by an aphid infestation, instead it was apparently attracted to the ladanum which certain plants produce in abundance. The affected varieties were C. ×ledon, C. ×purpureus, C. ladanifer var. sulcatus f. bicolor, C. ladanifer cultivars including ‘Bashful’ and ‘Blanche’, C. ×cyprius ‘Troubadour’ and C. ×stenophyllus.
Despite this, most of the affected plants continued to grow and flower well and this could be regarded as more of an aesthetic problem. A search of the literature did not reveal any prior research linking ladanum with sooty mold.
Plant growth
By fall 2006, many plants had filled their allocated space and some were beginning to grow into one another (Tables 1 through 4). The results show that Cistus and Halimium have a wide range of vigor and growth habit and can be divided into three broad groups based on growth habit: 1) upright shrubs with similar height and width that exceeds 100 cm; 2) mounding shrubs that are wide spreading with height is between 80 and 130 cm and width approximately twice that; and 3) low spreading groundcovers with height that does not exceed 80 cm and width that exceeds twice that.
Of the upright shrubs, the tallest in the evaluation was C. ×verguinii ‘Salabert’ which produces many ascending, sparsely foliated stems and forms a very open V-shaped shrub.
Somewhat shorter than this, but forming a much denser shrub, is C. ×aguilarii (often sold in North America erroneously as Cistus ‘Blanche’), which has thick, wavy-edged leaves and from an ornamental standpoint is far superior to C. ×verguinii.
Slightly smaller than this were ‘Bennett’s White’, ‘Blanche’ and ‘Troubadour’, all of which form well-shaped shrubs, although as previously-mentioned sooty mold did mar the foliage of the latter two cultivars.
The tallest of the pink-flowered Cistus, ‘Victor Reiter’, was only slightly smaller than these three.
The tallest of the Halimium was H. ×pauanum by a considerable margin, followed by H. ×santae and H. halimifolium. The majority of Halimium are low-growing plants.
Among the mounding, spreading shrubs, the most vigorous were C. ×laxus ‘Snow White’, C. ×oblongifolius, ‘Snow Fire’, ‘Jessamy Beauty’, ‘Gordon Cooper’, and ‘Ann Baker’, which vary somewhat in height but which typically will form broad, dense domes. These cultivars would all be effective as tall groundcovers for large areas.
The most vigorous pink-flowered selection of this group was C. ×purpureus.
Among Halimium, the largest plants were H. lasianthum ‘Hannay Silver’ and ‘Sarah’. The former is the most vigorous cultivar of H. lasianthum.
The lower-growing groundcovers in the trial generally form dense mats of foliage between 50 and 80 cm tall, and width of these plants often approached three times their height. Many of these would make good small- to medium-scale groundcovers for dry areas. The tallest is ‘Sunset’, which is a fairly common cultivar. Somewhat lower-growing than this are ‘Grayswood Pink’, C. ×florentinus ‘Tramontane’, C. ×gardianus and H. lasianthum ‘Sandling’.
The other H. lasianthum cultivars in the evaluation are somewhat lower-growing than these, although all of the aforementioned cultivars form dense, wide-spreading groundcovers. The lowest-growing plant in the entire evaluation was ×Halimiocistus sahucii, which was less than 30 cm tall after three years.
Cold hardiness
Plants were rated for cold damage in Apr. 2006 and Apr. 2009 (Tables 1 through 4). Minimum temperatures during winter from 2004-2009 tended to be quite mild, with the exception of cold snaps during Dec. 2005 and Feb. 2006, and again in Dec. 2008. The low temperature on Dec. 15 and 16, 2005 was 20°F and 19°F, respectively. A relatively late freeze occurred on 20 Feb. 2006, when the temperature dropped to 20°F. The coldest temperatures over the duration of the evaluation occurred from 16 Dec. to 17 Dec. 2008, when the minimum temperatures were 17°F and 19°F, respectively.
Cold damage typically shows up as browning of foliage followed by dieback of stems.
Some of the same cultivars showed damage during the evaluation during all these cold events. These include C. creticus ‘Tania Compton’, C. ×pauranthus, C. albidus f. albus and H. atriplicifolium.
Several other cultivars, such as ‘Silver Ghost’, ‘Silver Pink’, ‘Anne Palmer’, ‘Jessica’ and C. ×purpureus nf. stictus, also developed generally poor form and vigor as the evaluation progressed and may have been predisposed by this to cold injury by 2009. H. atriplicifolium also suffers from a reputation for tenderness and was one which displayed poor foliage and vigor.
Other species or hybrids which exhibited injury particularly in 2009 included C. ×lucasii and C. ×ralletii. Both of these hybrids involve tender Canary Island species in their parentage.
Of the remaining plants most exhibited good hardiness during both cold events. The industry standards, C. ×hybridus and C. ×purpureus, showed relatively minor foliar damage in both 2006 and 2009.
Plant Form
Evaluations of foliage quality and plant form were undertaken in response to variations in plant quality which emerged as the trial continued.
Some cultivars which were perhaps showy in bloom would become sparse and exhibit dieback by the end of the summer. Leaf drop is a normal response to summer drought in malacophyllous shrubs like Cistus or Halimium, and the degree of defoliation in response to drought stress varies. The appearance of some Cistus, with bare stems and leaves remaining primarily at the shoot tips, is typical of their strategy for survival in the wild, even if this detracts from their ornamental appeal. Some Cistus, such as C. albidus, have been shown to lose not only leaves, but stem and root tissues as well.
These types of responses may allow the plant to persist from one year to the next, but render them poor specimens in the landscape.
Ratings of foliage and form generally declined from 2006 to 2007 (Tables 1-4), but this may be partly the result of the cold injury suffered in Feb. 2006. Those taxa which received high ratings generally continued to look good through the end of the evaluation in 2009. The best plants overall were C. salviifolius ‘Gold Star’, C. ×laxus and C. ×obtusifolius, all of which exhibited near-flawless foliage even at the end of the summer drought in 2007.
Rated only slightly lower than these were two Cistus, both of which have white flowers with blotched petals, ‘Gordon Cooper’ and Snow Fire’. Pink-flowered Cistus in general seem to display lower foliage quality than white or white-blotched Cistus. Of the former, those with the highest ratings were ‘Grayswood Pink’, ‘Bicolor Pink’ and C. ×gardianus, all of which form dense, low-growing mats of foliage and which make excellent groundcovers.
Among the Halimium, most of the selections of H. lasianthum stand out as having good foliage quality, particularly ‘Sandling’. Others which retained good appearance included ‘Susan’ and H. ×pauanum.
Table 1. Plant size, foliage rating, and cold injury rating for blotched, white-flowered Cistus. Mean of n=4 replications with standard deviations in parentheses.
| Plant sizex | Foliage ratingz | Cold injuryy | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Plant name | Height (cm) | Width (cm) | 2006 | 2007 | 2006 | 2009 | ||||||
| C. 'Ann Baker' | 113 | (10) | 196 | (22) | 4.3 | (0.5) | 3.4 | (0.5) | 0.5 | (0.5) | 0.8 | (1.4) |
| C. 'Gordon Cooper' | 104 | (18) | 212 | (18) | 4.8 | (0.5) | 4.5 | (0.6) | 0.8 | (0.5) | 0.5 | (0.6) |
| C. 'Jessamy Beauty' | 128 | (10) | 205 | (17) | 3.8 | (0.5) | 2.8 | (0.5) | 0.5 | (0.6) | 1.0 | (0.8) |
| C. ladanifer var. sulcatus f. bicolor | 125 | (30) | 116 | (29) | 2.8 | (0.5) | 3.0 | (0) | 0.0 | (0) | 0.5 | (0.6) |
| C. 'Ruby Cluster' | 110 | (8) | 179 | (16) | 4.8 | (0.5) | 4.3 | (0.5) | 0.3 | (0.5) | 1.3 | (0.5) |
| C. 'Snow Fire' | 105 | (17) | 216 | (7) | 4.9 | (0.4) | 4.4 | (0.7) | 0.4 | (0.5) | 0.1 | (0.4) |
| C. x aguilarii 'Maculatus' | 138 | (4) | 176 | (9) | 3.0 | (0) | 3.0 | (0) | 0.0 | (0) | 1.0 | (0) |
| C. x cyprius 'Troubadour' | 154 | (13) | 179 | (21) | 3.0 | (0) | 3.0 | (0.8) | 0.0 | (0) | 1.3 | (0.5) |
| C. x dansereaui 'Decumbens' | 78 | (5) | 136 | (9) | 4.0 | (0) | 3.3 | (0.5) | 0.3 | (0.5) | 1.0 | (0) |
| C. x dansereaui 'Jenkyn Place' | 134 | (11) | 181 | (14) | 4.8 | (0.5) | 2.8 | (0.8) | 0.0 | (0) | 1.7 | (1.6) |
| C. x stenophyllus | 134 | (16) | 144 | (27) | 3.3 | (0.5) | 2.8 | (0.5) | 0.5 | (0.6) | 1.0 | (0) |
| C. x verguinii 'Salabert' | 210 | (22) | 210 | (26) | 3.0 | (0) | 3.0 | (0) | 0.3 | (0.6) | 1.7 | (0.6) |
| X Halimiocistus wintonensis | 76 | (11) | 136 | (17) | 3.9 | (0.7) | 3.0 | (0) | 0.3 | (0.5) | 1.7 | (0.5) |
| x Plant size measured in Sept. 2006 | ||||||||||||
| y Foliage rated on a scale from 1 to 5 where 1 = sparse foliage and dieback and 5 = full canopy with no irregularities | ||||||||||||
| z Cold injury was rated on a scale from 0 to 5 where 0 = no injury and 5 = complete plant death. | ||||||||||||
Table 2. Plant size, foliage rating, and cold injury rating for white-flowered Cistus and Halimium. Mean of n=4 replications with standard deviations in parentheses.
| Plant Sizex | Foliage ratingy | Cold injuryz | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Plant name | Height (cm) | Width (cm) | 2006 | 2007 | 2006 | 2009 | ||||||
| C. albidus f. albus | 83 | (18) | 106 | (27) | 3.0 | (0) | 2.5 | (0.7) | 2.0 | (0) | 2.0 | (1.4) |
| C. creticus ssp. creticus 'Tania Compton' | 74 | (8) | 101 | (8) | 3.5 | (0.6) | 3.0 | (0) | 2.3 | (0.5) | 2.8 | (0.5) |
| C. inflatus | 73 | (10) | 141 | (21) | 4.8 | (0.5) | 4.0 | (0) | 1.0 | (0) | 1.0 | (0) |
| C. ladanifer 'Bashful' | 98 | (10) | 134 | (9) | 4.0 | (0) | 3.8 | (0.5) | 0.0 | (0) | 1.0 | (0) |
| C. ladanifer 'Blanche' | 154 | (31) | 149 | (36) | 3.3 | (0.5) | 3.5 | (0.6) | 0.0 | (0) | 0.8 | (0.5) |
| C. ladanifer var. petiolaris 'Bennett's White' | 165 | (7) | 160 | (14) | 3.0 | (0) | 2.5 | (0.7) | 0.0 | (0) | 1.0 | (0) |
| C. libanotis 'Major' | 109 | (10) | 135 | (22) | 3.3 | (0.5) | 2.5 | (0.6) | 0.0 | (0) | 0.3 | (0.5) |
| C. monspeliensis | 111 | (13) | 143 | (19) | 3.5 | (0.8) | 2.6 | (0.7) | 1.3 | (1) | 0.9 | (0.7) |
| C. monspeliensis 'Vicar's Mead' | 116 | (11) | 132 | (28) | 3.3 | (0.5) | 3.3 | (0.5) | 1.0 | (0.8) | 1.0 | (0) |
| C. populifolius ssp. major | 101 | (10) | 151 | (23) | 4.5 | (0.6) | 3.8 | (0.5) | 0.3 | (0.5) | 0.3 | (0.5) |
| C. salviifolius 'Gold Star' | 106 | (27) | 186 | (11) | 3.8 | (0.5) | 4.8 | (0.5) | 1.0 | (0) | 1.0 | (0) |
| C. salviifolius 'Prostratus' | 45 | (12) | 128 | (22) | 4.8 | (0.5) | 3.5 | (0.6) | 1.3 | (1) | 1.3 | (1) |
| C. x aguilarii | 176 | (18) | 185 | (18) | 3.9 | (0.4) | 3.7 | (0.5) | 0.0 | (0) | 0.9 | (0.4) |
| C. x argenteus 'Paper Moon' | 125 | (21) | 120 | (11) | 2.5 | (0.7) | 2.0 | (0) | 0.0 | (0) | 0.5 | (0.7) |
| C. x argenteus 'Silver Ghost' | 69 | (6) | 79 | (7) | 2.0 | (0) | 1.0 | (0) | 0.0 | (0) | 5.0 | (0) |
| C. x canescens 'Albus' | 103 | (6) | 123 | (12) | 3.7 | (1.1) | 3.3 | (0.6) | 0.0 | (0) | 0.7 | (1.1) |
| C. x cyprius var. ellipticus 'Elma' | 130 | (8) | 169 | (13) | 3.0 | (0) | 2.8 | (0.5) | 0.3 | (0.5) | 0.3 | (0.5) |
| C. x dansereaui 'Portmeirion' | 104 | (7) | 153 | (10) | 4.3 | (0.5) | 3.3 | (0.5) | 0.0 | (0) | 0.3 | (0.5) |
| C. x dubius (costei group) | 90 | (4) | 161 | (5) | 4.5 | (0.6) | 3.0 | (0) | 0.3 | (0.5) | 0.8 | (0.5) |
| C. x florentinus 'Fontfroide' | 131 | (13) | 158 | (9) | 4.3 | (1) | 3.3 | (1) | 1.0 | (0) | 0.5 | (0.5) |
| C. x florentinus 'Tramontane' | 63 | (16) | 156 | (20) | 5.0 | (0) | 3.8 | (0.5) | 0.8 | (0.5) | 0.0 | (0) |
| C. x heterocalyx 'Chelsea Bonnet' | 109 | (23) | 162 | (31) | 3.1 | (0.6) | 2.1 | (0.7) | 1.1 | (0.6) | 1.6 | (1.1) |
| C. x hybridus | 99 | (14) | 179 | (9) | 3.5 | (0.6) | 3.0 | (0) | 1.0 | (0) | 0.3 | (0.5) |
| C. x laxus | 88 | (5) | 189 | (44) | 5.0 | (0) | 5.0 | (0) | 0.5 | (0.6) | 0.3 | (0.5) |
| C. x laxus 'Snow White' | 124 | (3) | 228 | (21) | 5.0 | (0) | 4.3 | (1) | 0.0 | (0) | 0.8 | (0.5) |
| C. x ledon | 91 | (9) | 126 | (15) | 4.3 | (0.5) | 3.3 | (0.5) | 0.0 | (0) | 0.5 | (0.6) |
| C. x nigricans | 109 | (10) | 186 | (13) | 4.0 | (0) | 3.0 | (0) | 0.0 | (0) | 0.8 | (0.5) |
| C. x oblongifolius | 115 | (14) | 208 | (15) | 4.5 | (0.6) | 3.5 | (1) | 0.5 | (0.6) | 0.3 | (0.5) |
| C. x obtusifolius | 71 | (3) | 156 | (4) | 5.0 | (0) | 5.0 | (0) | 1.0 | (0) | 0.3 | (0.5) |
| C. x platysepalus | 108 | (32) | 160 | (30) | 4.3 | (1.5) | 3.5 | (1) | 0.0 | (0) | 0.0 | (0) |
| C. x stenophyllus f. albiflorus | 115 | (21) | 131 | (21) | 3.0 | (0) | 2.8 | (0.5) | 0.5 | (0.6) | 0.8 | (0.5) |
| C. x verguinii f. albiflorus | 106 | (18) | 189 | (23) | 3.3 | (0.5) | 3.0 | (0) | 0.3 | (0.5) | 1.0 | (0) |
| H. umbellatum ssp. umbellatum | 49 | (12) | 137 | (20) | 4.0 | (0) | 3.8 | (0.5) | 0.0 | (0) | 0.0 | (0) |
| X Halimiocistus 'Ingwersenii' | 50 | (4) | 116 | (13) | 5.0 | (0) | 3.5 | (0.6) | 0.0 | (0) | 0.0 | (0) |
| X Halimiocistus sahucii | 29 | (10) | 127 | (24) | 4.4 | (0.5) | 4.1 | (0.6) | 0.0 | (0) | 0.1 | (0.4) |
| x Plant size measured in Sept. 2006 | ||||||||||||
| y Foliage rated on a scale from 1 to 5 where 1 = sparse foliage and severe dieback and 5 = full canopy with no irregularities. | ||||||||||||
| z Cold injury was rated on a scale from 0 to 5 where 0 = no injury and 5 = complete plant death. | ||||||||||||
Table 3. Plant size, foliage rating, and cold injury rating for pink-flowered Cistus. Mean of n=4 replications with standard deviations in parentheses.
| Plant Sizex | Foliage ratingy | Cold injuryz | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Plant name | Height (cm) | Width (cm) | 2006 | 2007 | 2006 | 2009 | ||||||
| C. creticus ssp. creticus 'Lasithi' | 39 | (3) | 109 | (10) | 4.0 | (0) | 3.5 | (0.6) | 0.0 | (0) | 1.5 | (1) |
| C. creticus 'Lasca Select' | 70 | (9) | 148 | (8) | 3.5 | (0.6) | 3.0 | (0) | 0.0 | (0) | 0.5 | (0.6) |
| C. crispus | 55 | (11) | 119 | (21) | 3.5 | (0.6) | 3.5 | (0.6) | 0.3 | (0.5) | 2.0 | (1.2) |
| C. crispus 'Decanso' | 31 | (3) | 116 | (22) | 4.0 | (0) | 3.0 | (1.2) | 0.8 | (0.5) | 2.0 | (0.8) |
| C. 'Doris Hibberson' | 76 | (8) | 93 | (14) | 3.3 | (0.5) | 2.8 | ().5) | 0.0 | (0) | 1.0 | (0) |
| C. 'Grayswood Pink' | 65 | (7) | 154 | (14) | 4.3 | (0.6) | 3.8 | (0.6) | 0.0 | (0) | 0.0 | (0) |
| C. heterophyllus | 80 | (17) | 124 | (19) | 3.3 | (0.6) | 2.7 | (0.6) | 0.0 | (0) | 1.0 | (0) |
| C. 'Bicolor Pink' | 54 | (5) | 152 | (14) | 4.3 | (0.5) | 4.0 | (0.8) | 0.0 | (0) | 0.0 | (0) |
| C. 'Santa Cruz' | 74 | (8) | 101 | (10) | 3.3 | (0.5) | 3.0 | (0) | 0.8 | (0.5) | 0.0 | (0) |
| C. 'Victor Reiter' | 142 | (11) | 161 | (28) | 3.0 | (0.5) | 2.1 | (0.4) | 0.0 | (0) | 0.6 | (0.5) |
| C. x argenteus 'Blushing Peggy Sammons' | 133 | (5) | 175 | (23) | 3.0 | (0) | 2.5 | (1) | 0.0 | (0) | 1.0 | (0.8) |
| C. x argenteus 'Peggy Sammons' | 115 | (30) | 136 | (29) | 3.0 | (0.8) | 1.8 | (1) | 0.3 | (0.5) | 2.0 | (1.4) |
| C. x argenteus 'Silver Pink' | 64 | (5) | 95 | (19) | 2.3 | (0.5) | 1.3 | (0.5) | 0.0 | (0) | 5.0 | (0) |
| C. x argenteus 'Stripey' | 130 | (0) | 167 | (27) | 3.0 | (0) | 2.0 | (0) | 0.3 | (0.5) | 1.5 | (0.6) |
| C. x bornetianus 'Jester' | 98 | (12) | 143 | (26) | 3.3 | (0.5) | 2.3 | (0.5) | 0.3 | (0.5) | 1.5 | (1) |
| C. x canescens | 83 | (17) | 111 | (14) | 4.0 | (0) | 3.3 | (0.5) | 0.0 | (0) | 0.0 | (0) |
| C. x crispatus 'Warley Rose' | 60 | (8) | 124 | (14) | 3.8 | (0.5) | 3.0 | (0) | 0.3 | (0.5) | 0.0 | (0) |
| C. x fernandesiae 'Anne Palmer' | 96 | (19) | 125 | (27) | 3.0 | (0) | 2.3 | (0.5) | 1.0 | (0) | 3.3 | (2.1) |
| C. x gardianus | 66 | (9) | 163 | (12) | 4.0 | (0) | 3.8 | (0.5) | 0.0 | (0) | 0.5 | (0.6) |
| C. x lucasii | 93 | (3) | 155 | (10) | 3.3 | (0.6) | 2.7 | (0.6) | 0.7 | (0.6) | 2.7 | (0.6) |
| C. x mesoensis | 70 | (0) | 127 | (5) | 4.0 | (0) | 3.0 | (0) | 1.0 | (0) | 1.0 | (0) |
| C. x pagei | 99 | (10) | 132 | (25) | 3.8 | (05) | 2.8 | (0.5) | 0.0 | (0) | 0.5 | (0.6) |
| C. x pauranthus | 100 | (0) | 153 | (7) | 2.0 | (1.7) | 3.0 | (0) | 2.3 | (0.5) | 4.0 | (0) |
| C. x pulverulentus 'Sunset' | 78 | (5) | 144 | (8) | 4.0 | (0) | 3.0 | (0) | 0.5 | (0.6) | 1.3 | (1.5) |
| C. x purpureus | 108 | (10) | 175 | (21) | 4.0 | (0) | 3.3 | (0.5) | 0.3 | (0.5) | 1.3 | (0.5) |
| C. x purpureus nf. holorhodos | 124 | (9) | 141 | (8) | 3.0 | (0) | 2.0 | (0) | 0.5 | (0.6) | 1.0 | (0) |
| C. x purpureus nf. stictus | 134 | (5) | 153 | (18) | 3.0 | (0) | 2.3 | (0.5) | 1.0 | (0) | 2.0 | (0.8) |
| C. x ralletii | 113 | (21) | 144 | (9) | 4.0 | (1) | 2.7 | (0.6) | 1.0 | (0) | 2.5 | (0.7) |
| C. x rodiaei 'Jessica' | 56 | (11) | 66 | (3) | 2.8 | (0.5) | 2.0 | (0) | 0.0 | (0) | 5.0 | (0) |
| C. x skanbergii | 98 | (17) | 159 | (42) | 3.9 | (0.6) | 3.0 | (0) | 0.9 | (0.6) | 0.4 | (0.5) |
| x Plant size measured in Sept. 2006 | ||||||||||||
| y Foliage rated on a scale from 1 to 5 where 1 = sparse foliage and severe dieback and 5 = full canopy with no irregularities. | ||||||||||||
| z Cold injury was rated on a scale from 0 to 5 where 0 = no injury and 5 = complete plant death. | ||||||||||||
Table 4. Plant size, foliage rating, and cold injury rating for yellow-flowered Halimium. Mean of n=4 replications with standard deviations in parentheses.
| Plant Sizex | Foliage ratingy | Cold injuryz | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Plant name | Height (cm) | Width (cm) | 2006 | 2007 | 2006 | 2009 | ||||||
| H. atriplicifolium | 64 | (14) | 71 | (16) | 2.1 | (0.6) | 1.9 | (0.4) | 1.5 | (0.8) | 3.2 | (0.8) |
| H. calycinum | 46 | (5) | 106 | (11) | 4.3 | (0.5) | 3.5 | (0.6) | 0.5 | (1) | 0.0 | (0) |
| H. calycinum (CA clone) | 66 | (13) | 138 | (28) | 3.5 | (0.6) | 2.8 | (0.5) | 0.3 | (0.5) | 2.3 | (1.9) |
| H. halimifolium f. maculatum | 93 | (21) | 138 | (13) | 3.0 | (0) | 2.3 | (0.6) | 0.3 | (0.6) | 2.0 | (0) |
| H. lasianthum | 58 | (8) | 170 | (10) | 4.3 | (0.8) | 3.8 | (0.8) | 0.0 | (0) | 0.0 | (0) |
| H. lasianthum 'Concolor' | 60 | (14) | 103 | (4) | 4.0 | (0) | 3.5 | (0.7) | 0.0 | (0) | 0.0 | (0) |
| H. lasianthum 'Formosum' | 50 | (10) | 154 | (18) | 4.0 | (0.8) | 3.5 | (0.6) | 0.0 | (0) | 0.0 | (0) |
| H. lasianthum 'Hannay Silver' | 126 | (9) | 158 | (18) | 4.0 | (0.8) | 3.3 | (0.5) | 0.0 | (0) | 0.5 | (0.6) |
| H. lasianthum 'Sandling' | 66 | (8) | 151 | (12) | 4.5 | (0.6) | 4.0 | (0) | 0.0 | (0) | 0.3 | (0.5) |
| H. lasianthum ssp. alyssoides 'Farrall' | 58 | (10) | 110 | (17) | 3.3 | (0.5) | 3.3 | (0.5) | 0.0 | (0) | 0.0 | (0) |
| H. ocymoides | 86 | (8) | 166 | (44) | 3.5 | (0.5) | 2.5 | (0.6) | 0.0 | (0) | 1.0 | (1.2) |
| H. 'Sarah' | 101 | (15) | 175 | (19) | 4.3 | (0.5) | 3.3 | (0.5) | 0.0 | (0) | 0.5 | (1) |
| H. 'Susan' | 45 | (5) | 118 | (24) | 4.3 | (0.6) | 4.0 | (0) | 0.0 | (0) | 1.3 | (1.2) |
| H. x pauanum | 151 | (15) | 154 | (20) | 4.0 | (0) | 4.3 | (0.6) | 0.0 | (0) | 0.3 | (0.6) |
| H. x santae | 133 | (5) | 167 | (27) | 3.3 | (0.5) | 2.3 | (1) | 0.0 | (0) | 2.0 | (1.4) |
| x Plant size measured in Sept. 2006 | ||||||||||||
| y Foliage rated on a scale from 1 to 5 where 1 = sparse foliage and severe dieback and 5 = full canopy with no irregularities. | ||||||||||||
| z Cold injury was rated on a scale from 0 to 5 where 0 = no injury and 5 = complete plant death. | ||||||||||||
The effective flowering period for the three genera is late April to late July (Figs. 1-4). The earliest plants to bloom are H. calycinum, particularly the selection from California, which commenced blooming in the third week of April, at approximately the same time as H. umbellatum ssp. umbellatum.
In the first week of May, other early-blooming cultivars like ‘Grayswood Pink’ and ‘Bicolor Pink’ begin flowering, followed by ‘Snow Fire’ and ×Halimiocistus wintonensis. The majority of flowering for these genera occurs from mid-May through the end of June. A few cultivars continue flowering quite strongly well into July, including H. ×pauanum and H. halimifolium f. maculatum, C. inflatus, ‘Sunset’, ‘Ruby Cluster’ and ‘Jessamy Beauty’.
The duration of the flowering period varied dramatically. The shortest bloom duration was that of C. ladanifer var. sulcatus f. bicolor, which flowered for only a few days. A number of taxa flowered for only 10 days or less, including C. ×verguinii, C. ×argenteus ‘Paper Moon’, ‘Peggy Sammons’, ‘Blushing Peggy Sammons’ and ‘Stripey’, C. salviifolius ‘Gold Star’, C. libanotis ‘Major’, C. ladanifer ‘Blanche’, C. ×cyprius ‘Elma’ and C. ×rodiae Jessica. The industry standards, C. ×purpureus and C. ×hybridus, flowered for 22 days and 13 days, respectively.
The longest bloom period belonged to H. ×pauanum, which bloomed for 79 days. A number of other taxa bloomed in excess of 50 days, including C. ×canescens, C. ×pulverulentus ‘Sunset’, C. inflatus, C. ×florentinus ‘Fontfroide’, C. ‘Jessamy Beauty’, C. ‘Ruby Cluster’, C. ‘Snow Fire’, H. halimifolium f. maculatum and ×Halimiocistus ‘Ingwersenii’. These long-blooming plants will exhibit flushes of flowering within these periods where heavier or lighter bloom may be observed. All of these, however, can be counted on for a very long season of interest.
Figure 1. Average flowering time and duration of yellow-flowered Halimium grown in a landscape evaluation in Aurora, OR, 2005-2006. Plants were considered in bloom when several open flowers were distributed through the shrub
Figure 2. Average flowering time and duration of white-flowered Cistus grown in a landscape evaluation in Aurora, OR, 2005-2006. Plants were considered in bloom when several open flowers were distributed through the shrub
Figure 3. Average flowering time and duration of pink-flowered Cistus grown in a landscape evaluation in Aurora, OR, 2005-2006. Plants were considered in bloom when several open flowers were distributed through the shrub
Figure 4. Average flowering time and duration of blotched, white-flowered Cistus grown in a landscape evaluation in Aurora, OR, 2005-2006. Plants were considered in bloom when several open flowers were distributed through the shrub
Recommended cultivars
Differences in foliage quality, bloom time and hardiness were readily evident throughout the evaluation. Some taxa excelled in some areas but lacked considerably in others, so their desirability is based on how important their particular strengths are. For example, C. salviifolius ‘Gold Star’ had excellent foliage ratings and would make a very good tall groundcover, but it has a very abbreviated bloom period. This is a similar problem with ‘Grayswood Pink’ and ‘Bicolor Pink’, both of which have good foliage and would make good small scale groundcovers, but bloom time is less than two weeks, short by the standards of this genus. Other taxa, such as H. halimifolium f. maculatum, have very long bloom periods, but relative poor habit and foliage quality. And there were a few taxa, such as H. atriplicifolium and ‘Tania Compton’, which have long bloom periods but which are lacking in cold hardiness.
If foliage quality, length of bloom time and hardiness are considered, then a few of the many taxa evaluated are superior to the others. Many of these are those which have either a low, spreading or a mounding habit and would make good groundcovers for dry areas. In terms of increasing size, these taxa are C. ×gardianus, which forms a flat groundcover and features pink flowers.
Similar in some respects is C. ×pulverulentus ‘Sunset’, which is already well-known in the Northwest and is popular for its magenta flowers and long bloom period.
C. ×obtusifolius forms a near-perfect dome of foliage and would make an excellent substitute for C. ×hybridus, which is the standard white-flowering Cistus in the Pacific Northwest. The flowering period for C. ×obtusifolius is nearly four times as long as for C. ×hybridus, and it is considerably smaller in size.
Unfortunately, C. ×hybridus is often massed as a groundcover in situations where it quickly outgrows its space, requiring repeated pruning to maintain its size, which it, like most Cistus, resents. Similar in size to C. ×obtusifolius is C. inflatus, which has a bloom period that approached 2 months, exceeded only by H. ×pauanum.
Somewhat larger than the forgoing is C. ×laxus, which along with C. ×obtusifolius, received the highest foliage rating of all taxa in the evaluation.
C. ×canescens has a tighter habit than C. ×laxus, with soft gray leaves and pink flowers over a long season.
The cultivars with similar size are ‘Gordon Cooper’, ‘Snow Fire’ and ‘Ruby Cluster’, all of which form fairly large domes of foliage and which have blotched, white flowers.
All of these had excellent foliage and long bloom periods and would make excellent specimen plants or tall groundcovers.
Among the Halimium, H. lasianthum ‘Sandling’ distinguished itself as the best of the cultivars of the species, with longer than average bloom and best foliage rating of that species.
‘Susan’ received similarly high ratings for foliage and had a bloom period of one month, longer than most Halimium.
Of those Halimium with an upright habit, the clear favorite was H. ×pauanum, which had a very long bloom time.
Of the ×Halimiocistus, the best was ×Halimiocistus ‘Ingwersenii’, which featured good foliage and had an exceptionally long bloom period.
Managing Cistus and Halimium in Oregon landscapes
Both of these genera are Mediterranean and so are accustomed to a climate which is mild and wet in the winter and warm and dry in the summer. Like other Mediterranean shrubs, the plants naturally go dormant in the summer when soil moisture becomes limiting and stay dormant until rain begins in the fall, when they resume growing.
The climate in western Oregon and Washington is similar in pattern to this, although the drought period is not as long, and temperatures in both summer and winter are somewhat lower. However the similarity of the climate offers an excellent opportunity for gardeners and landscapers to utilize these plants where they make the most sense and are most at home: in an un-irrigated landscape.
One of the biggest benefits of not watering a landscape is reduced water bills of course, but in addition, summer weed problems are much reduced. In particular, many of the common summer annuals are reduced, or eliminated, when water is unavailable for growth.
Unfortunately, Cistus and Halimium are often combined heedlessly with shrubs or perennials (or even annuals!) in landscapes that receive regular summer water. The result, besides the weed problems, is that they continue to grow through what would otherwise be their normal dormant period, and as a result become unnaturally big. The result of that is they often subsequently require pruning to restrict that excess growth, and most Cistus are not tolerant of repeated or hard pruning. Cutting back into bare stems often results in poor regrowth.
Both Cistus and Halimium make excellent components of an un-irrigated landscape and they combine well with other plants that have a similar requirement, This includes some rought-tolerant native plants like Oceanspray (Holodiscus discolor), snowberry (Symphoricarpos alba) or flowering currant (Ribes sanguineum). It will also include complimentary Mediterranean plants from California and other areas of the world, assuming these are hardy in our area. Some examples include Wild lilac (Ceanothus spp.), Rosemary (Rosmarinus officinalis), Lavender (Lavandula spp.); Manzanita (Arctostaphylos spp.) or Jerusalem sage (Phlomis spp.), to name only a few.
If the plants are situated well then they should perform well for many years. Besides the appeal of the flowers, Cistus in particular offer other ornamental attributes. Assuming one of the better forms is used, the foliage and habit of the plant is attractive. Many of the available cultivars are quite aromatic, and so even after the bloom period is over, the pleasant aroma of the foliage can be detected on warm summer days. The flowers are also attractive to bees, both bumblebees and European honeybees. They seem attracted to the flowers of most cultivars, but particularly the flowers of C. ladanifer.
Books and Websites
The literature devoted to either of these genera is not extensive, but besides the journal articles mentioned earlier, there are a couple of essential references. The most thorough treatment of Cistus in book form is:
Cistus, A guide to the collection at Chelsea Physic Garden. Dr. Paul Bygrave, Robert, G. Page, Ed. 64pp. Available from the National Council for the Conservation of Plants and Gardens.
An up-to-date listing of species and cultivars of both Cistus and Halimium can be found at The Cistus and Halimium Site, maintained by Robert G. Page
Website creator:
Neil Bell
Community Horticulturist
OSU Extension Service
Marion and Polk Counties
3180 Center Street NE #1361
Salem, OR 97301
503-361-2671