Ceanothus Evaluation for Landscapes in Western Oregon

The landscape evaluation of Ceanothus was established at The Oregon Garden in Silverton in 2001 and continued through 2005. Our interest in evaluating Ceanothus is based on its drought tolerance, wide range of growth habits and also its ornamental appeal. Although many cultivars are available and used in California, few are grown or used extensively in landscapes in western Oregon or Washington. The most commonly grown are C. gloriosus and 'Skylark' (syn. 'Victoria'), which are a groundcover and large shrub, respectively.

Ceanothus gloriosus groundcover in Yachats, ORCeanothus 'Victoria'

The goals of this evaluation were to develop comparative data on hardiness of Ceanothus cultivars and species and identify cultivars that were capable of tolerating typical cold events in a Pacific Northwest winter. In addition to assessing hardiness, other goals were to record flowering and growth information on the various cultivars and species, and also any pest or disease problems.

Cold hardiness of Ceanothus

The genus Ceanothus, also known as wild lilac or simply ceanothus, comprises approximately 55 species of woody shrubs native to North America (Hardig et al., 2000). The distribution of species is concentrated in western North America, particularly in California, where they are an important component of semi-arid forest, oak woodland, and chaparral.

Ceanothus in Northern California

The majority of the species are evergreen shrubs, which range in habit from prostrate to sprawling or erect large shrubs, although some may be found as small trees (Schmidt, 2003). The climate in the California floristic province in which many of the species of ceanothus originated is Mediterranean, with mild, rainy winters and warm to hot, dry summers. As a result, ceanothus is adapted to growing in hot, dry situations, often in relatively poor soils.

Little specific information on the cold hardiness of species of ceanothus cultivars is available. An estimation of winter hardiness can be inferred from the native range of species or individual clones. Fross and Wilkin (2006) provide anecdotal information on a number of cultivars based on experiences in California and the United Kingdom. A field trial of 40 cultivars in Holland, which included both evergreen and hybrid cultivars, found all hardy to at least United States Department of Agriculture (USDA) zone 8b (Hop, 2006). Boorse et al. (1998) studied hardiness of four chaparral shrubs, including C. megacarpus, a species found in southern California and the Channel Islands and C. spinosus, a widespread species found from Eldorado County in the Sierra Nevada foothills south into Baja California. Their research concluded that adult plants of each species showed greater resistance to cold than seedlings. Langan et al. (1997) examined the relationship between cold hardiness and drought stress in C. megacarpus and concluded that both factors needed to be considered as limiting survival and distribution of the species. In studies of potted ‘Autumnal Blue’ ceanothus, plant survival was diminished by increased duration of freezing and smaller container size (Cameron and Dixon, 1997).

Literature cited

Boorse, G.C., F.W. Ewers, and S.D. Davis. 1998. Response of chaparral shrubs to below-freezing temperatures: acclimation, ecotypes, seedlings vs. adults. Amer. J. Bot. 85:1224-1230.

Cameron, R.W.F. and G.R. Dixon. 1997. Air temperature, humidity and rooting volume affecting freezing injury to Rhododendron and other perennials. J. Hort. Sci. 72:553-562.

Fross, D and D. Wilkin. 2006. Ceanothus. Timber Press, Portland, OR. 272pp.

Hardig, T.M., P.S. Soltis, and D.E. Soltis. 2000. Diversification of the North American shrub genus Ceanothus (Rhamnaceae): conflicting phylogenies from nuclear ribosomal DNA and chloroplast DNA. Amer. J. Bot. 87:108-123.

Hop, M.E.C.M. 2006. Ceanothus-sortimentsonderzoek en keuringsrapport [Ceanothus-variety trial and ratings/evaluation report]. Dendroflora. 43:80-123.

Langan, S.L., F.W. Ewers and S.D. Davis. 1997. Xylem dysfunction caused by water stress and freezing in two species of co-occurring chaparral shrubs. Plant, Cell and Environ. 20:425-437.

Schmidt, C. 2003. Dr. Cliff Schmidt’s treatment of the genus Ceanothus. 18 June 2008.
http://oregonstate.edu/dept/botany/herbarium/projects/ceanothus/genus.html.

Data Collection

Species and cultivars of Ceanothus were obtained from nurseries in California and Oregon in Oct. 2000 and May 2001. The plants were growing in either 2.5-inch tree band or 1-gal pots. The trial was located at the Oregon Garden, Silverton, OR. The evaluation site was approximately ¼ acre, sloping slightly westward at 440 ft elevation. The soil was coarse construction fill remaining from the building of nearby wetlands. Prior to planting, the site was bladed off using a bulldozer and the top 1 ft was ripped and roughly graded.
Ceanothus evaluation in May, 2001
Soil was classified as clay with a pH of 5.6.

After planting on 25 May 2001, plants were mulched with Douglas fir (Pseudotsuga menziesii) bark to a depth of 3 inches, and were watered individually, by hand, through Sept. 2001.
Ceanothus evaluation, May 2002
Plants received no supplemental irrigation from Sept. 2001 to Oct. 2005. No fertilizer was applied after planting for the duration of the trial. Plants were not pruned. Data collected included plant height, flowering season, and cold hardiness evaluation. 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 Spring 2003 and 2004, as there was no apparent damage following the winters of 2001-02 and 2004-05. Winter cold damage was rated on a 0-5 scale, with 0 indicating no damage; 1= minor leaf damage (browning); 2= leaf and stem damage restricted to the exterior 30% of the plant; 3= leaf and stem damage to the exterior 60% of the plant; 4= plant was killed to the ground and was re-sprouting; 5= plant was killed.

Plant growth and overall performance

Ceanothus cultivars became established, grew, and filled in the rows by 2005. Photos show the growth of the plants each year starting in 2002 through 2005

Ceanothus evaluation, May 2002

Ceanothus evaluation, June 2003

Ceanothus evaluation, May 2004
Ceanothus evaluation, May 2005
A small part of the trial area at the west end was subject to periodic flooding because of runoff from a nearby road after rainfall, resulting in death of several cultivars. Individual plants were lost during the trial for undetermined causes, including two of the three C. velutinus var. velutinus, although the third plant grew and flowered well. This is a montane species and though the origin of the clone was unknown, it may not be particularly well-adapted to clay soils and the climate at low elevation. Individual hybrids of C. impressus; ‘Julia Phelps’ and ‘Dark Star’ died during the study. Plants derived from this species are generally regarded as preferring well-drained soils and possibly were unsuitable for the constructed clay soil on the evaluation site.

Plant height and width data in Spring 2005 are shown in Figures 1 and 2. As with other Mediterranean plants, ceanothus show a distinct pattern of growth during the year that depends on both temperature and moisture availability. The majority of the yearly growth occurs from early spring through mid-summer, when soil moisture is abundant and temperatures are increasing. From mid-summer through fall, growth slows significantly or stops, as soil moisture is depleted (assuming the plants are not irrigated). As rain begins again in fall, then growth will resume, although cooler fall and winter temperatures limit the amount that occurs.

Overall, one can divide ceanothus as a group into four broad groups based on growth habit: 1) upright shrubs; 2) large, rounded shrubs; 3) mound-forming, wide spreading shrubs; and 4) low, spreading groundcovers.

Figure 1. Height of ceanothus after growing for 4 years in a landscape evaluation in Silverton, OR in Spring 2005, mean + SE.

Chart showing height after 4 years

 

Figure 2. Width of ceanothus after growing for 4 years in a landscape evaluation in Silverton, OR in Spring 2005, mean + SE.

Chart showing width after 4 years

Cold hardiness

Despite a reputation for being too tender to grow in Oregon, most of the cultivars tested showed good hardiness in this evaluation. The most serious damage occurred after temperatures in early Nov. 2002 dropped from average night temperature of 37°F to 22°F. This was an early freeze that occurred while many plants were still growing. A similar freeze occurred on 1 Nov. 2003, when the temperature again dropped quickly from mild lows of 37°F to 25°F. Again, because of the relatively severe temperature early in the winter, serious damage occurred on a wide range of plants. The chart below shows cold injury to all cultivars in the evaluation.
The most commonly grown cultivars of ceanothus, ‘Skylark’ and C. gloriosus, showed relatively little, or no cold damage in this trial. Similarly unaffected were the semi-evergreen cultivars ‘Marie Simon’ and ‘Henri Desfosse’, probably because of hardiness inherited from their C. americanus lineage Several of the most desirable cultivars, such as ‘Remote Blue’, ‘Blue Jeans’, and ‘Wheeler Canyon’ showed relatively minor leaf and twig burn from the cold events described above and quickly recovered.
With the exception of ‘Kurt Zadnik’, which is a vigorous shrub, most cultivars of C. thyrsiflorus var. griseus, popular as groundcovers in California, showed severe cold damage.

Cold damage to C. thyrsiflorus var. griseus
C. hearstiorum also had considerable tip dieback, although the plants recovered. C. thyrsiflorus var. thyrsiflorus ‘Snowflurry’ is a white-flowered cultivar from the mild Big Sur coast of California and was much more susceptible to cold damage than other cultivars of var. thyrsiflorus. Plants of this cultivar recovered poorly from successive cold spells in 2002 and 2003, and would thus be best suited for coastal areas. Another cultivar which showed serious cold damage in 2002 and 2003 was C. maritimus ‘Popcorn’, which had dieback, although plants eventually recovered.

Cold damage to C. maritimus 'Popcorn'
Another cultivar of this species, ‘Point Sierra’, had relatively little cold damage in both years and with its blue flowers and compact habit, would make a good small-scale groundcover.

Table 1. Cold hardiness ratings of ceanothus evaluated in Silverton, OR on 3 Apr. 2003 and 29 Apr. 2004 and an average over years with the standard deviation (SD).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Cold hardiness rating (0-5 scale)z

Plant Name

2003

2004

Average

SD

C. x delilianus ‘Henri Desfosse’

0.0

0.0

0.0

0.01

C. gloriosus ‘Hearts Desire’

0.0

0.0

0.0

0.87

C. x pallidus ‘Marie Simon’

0.0

0.0

0.0

0.01

C. sanguineus

0.0

0.0

0.0

0.01

C. velutinus

0.0

0.0

0.0

0.01

C. integerrimus

0.0

0.7

0.3

0.58

C. cuneatus (Nipomo Mesa)

0.0

1.0

0.5

0.71

C. cuneatus

0.3

0.0

0.2

0.29

 Wheeler Canyon

0.3

0.0

0.2

0.29

C. lemmonii

0.3

0.7

0.5

0.87

C. gloriosus

0.3

1.3

0.8

0.87

C. thyrsiflorus var. thyrsiflorus ‘El Dorado’

0.7

0.0

0.3

0.29

‘Skylark’

0.7

0.0

0.3

0.58

 ‘Italian Skies’

0.7

0.7

0.7

0.87

 ‘Cynthia Postan’

1.0

0.0

0.5

0.50

 ‘Dark Star’

1.0

1.0

1.0

1.41

 ‘Remote Blue’

1.0

1.3

1.2

0.96

 ‘Blue Jeans’

1.2

0.2

0.7

0.77

 ‘Autumnal Blue’

1.3

1.3

1.3

0.58

C. maritimus ‘Point Sierra’

1.3

1.3

1.3

0.87

C. ramulosus ‘Rodeo Lagoon’

1.5

0.0

0.8

0.35

 ‘Concha’

1.7

0.0

0.8

0.76

 Julia Phelps’

1.7

0.0

1.0

0.76

Ray Hartman’

1.7

0.0

0.8

0.29

C. thyrsiflorus var. griseus ‘Kurt Zadnik’

1.7

1.3

1.5

0.87

Puget Blue’

2.0

1.0

1.7

0.01

C. thyrsiflorus var. thyrsiflorus ‘Arroyo de la Cruz’

2.0

2.3

2.2

0.29

‘Joyce Coulter’

2.3

0.0

1.4

0.76

C. foliosus ‘Berryhill’

2.3

2.0

2.2

0.29

C. thyrsiflorus var. griseus ‘Louis Edmunds’

2.5

1.5

2.0

1.41

‘Centennial’

2.7

1.3

2.0

0.87

Joan Mirov’

2.7

2.0

2.3

0.76

C. cuneatus var. rigidus ‘Snowball’

2.7

2.0

2.4

0.29

C. maritimus ‘Popcorn’

2.7

3.0

2.8

1.00

C. thyrsiflorus var. thyrsiflorus ‘Snow Flurry’

3.0

3.0

3.0

0.87

C. hearstiorum

3.5

1.0

2.3

1.06

C. thyrsiflorus var. griseus ‘Yankee Point’

4.0

N/A

4.0

0.01

C. thyrsiflorus var. griseus (prostrate selection)

4.0

N/A

4.0

0.00

zRating system was as follows: 0=no visible damage; 1=minor leaf damage; 2=leaf and stem damage restricted to outer 30% of shrub; 3=leaf and stem damage restricted to outer 60% of shrub; 4=killed to ground, re-sprouting; 5=killed, not recovering.

Flowering

Most cultivars in the trial flowered well starting in 2002. The chart below shows flowering duration, from 5% to 100% bloom for 2005, when plants were mature. The earliest-flowering cultivars were consistently C. cuneatus, C. maritimus selections (‘Point Sierra’ and ‘Popcorn’), and ‘Blue Jeans’. The most commonly grown ceanothus in the Northwest, C. gloriosus and ‘Skylark’, are early- to mid-season and mid- to late season flowering cultivars, respectively. The flowering season can be extended several weeks both earlier and later by selecting other cultivars.
Among the better early-blooming cultivars was ‘Blue Jeans’, which typically began flowering in early March and continued through early- to mid-April. C. cuneatus is a species widespread in California and Oregon and the form in this trial, although of unknown origin, was a shapely, medium-sized gray-leaved shrub that produced white flowers also starting in early March. Although this species is rarely cultivated in the Northwest, it is a hardy, attractive shrub of distinctive appearance because of the gray leaves. Several cultivars or hybrids of C. impressus, ‘Julia Phelps’, ‘Dark Star’ and ‘Puget Blue’, were among the showiest of the trial flowering from April through mid-May. C. thyrsiflorus var. griseus ‘Kurt Zadnik’ was one of the most distinctive in bloom, with deep blue flowers throughout May on a tall, wide-spreading groundcover.
Several cultivars flowered from late May well into July, but because of disease problems, or deer predation, are not included in the chart. These are ‘Autumnal Blue’, ‘Marie Simon’, and ‘Henri Desfosse’. Unfortunately, ‘Autumnal Blue’ had severe leaf spot (Cercospora sp.) causing major defoliation, thus reducing its ornamental appeal and vigor. Both ‘Marie Simon’ and ‘Henri Desfosse’ are excellent, late-flowering plants, but are very attractive to deer and were repeatedly defoliated. If this can be prevented, both of these cultivars are ways to extend the flowering of ceanothus into mid-summer.

Figure 3. Average flowering time and duration of ceanothus grown in a landscape evaluation in Silverton, OR, 2005, duration + SE.

Chart showing average flowering time

Pest and Disease problems

No serious insect pest problems were noted during the trial and disease problems were infrequent. The most significant disease problem was found on ‘Autumnal Blue’, which grew well early in the trial, but severe defoliation was observed starting in 2003. This resulted in the loss of all but the current season’s leaves, leaving this cultivar sparsely foliated and unacceptable for landscape use.

Defoliation due to leaf spot on 'Autumnal Blue'
The pathogen Cercospora sp. (possibly C. ceanothi) was isolated from the affected leaves and was assumed to be the cause.

There was some damage from grazing deer (Odocoileus hemionus ssp. columbianus). In general deer to not show much interest in ceanothus, but they will feed on the foliage of the semi-evergreen hybrid types as well as selections or hybrids of C. arboreus, which has a large, soft leaf. Most other cultivars, at least in this evaluation, have relatively tough leaves which do not seem to attract deer In this evaluation, ‘Marie Simon’ and ‘Henri Desfosse’ were heavily grazed as was ‘Cliff Schmidt’.

Deer feeding damage on C. 'Cliff Schmidt'
No other ceanothus cultivars suffered any noticeable feeding damage.

Recommended cultivars

Of the upright shrubs, ‘Ray Hartman’ is not commonly available in the Pacific northwestern U.S., but blooms heavily and proved more cold hardy than its parentage (C. arboreus x C. thyrsiflorus var. griseus) would suggest. ‘Ray Hartman’ was not affected by wind or snow and retained its form and foliage.

Ceanothus 'Ray Hartman'
‘Blue Jeans’ was noticeably smaller than ‘Ray Hartman’ and was of distinctive appearance, with small leaves on upright stems and very early blooming.

Ceanothus 'Blue Jeans'

Although they were decimated by deer in this trial, early on the hybrid, semi-deciduous cultivars showed a lot of promise. ‘Marie Simon’ and ‘Henri Desfosse’ are only two of the many hybrids of the eastern North American C. americanus, C. herbaceus and various evergreen western species.

Ceanothus 'Henri Desfosse'
They are usually listed as cultivars of C. x delilianus or C. x pallidus. There are several common ones including ‘Marie Simon, ‘Gloire de Versailles’ and ‘Topaz’, to name a few. They offer a more or less upright habit and various shades of blue flowers or in some cases (like ‘Marie Simon’) pink..

Ceanothus 'Marie Simon'
Because they are hybrids with the eastern species they seem quite happy in a range of soils and even moisture levels, so if you are planning on mixing them into an irrigated site, these might be your best choice. They also flower on new growth, meaning they are among the latest Ceanothus to flower. This characteristic means they also tolerate being cut back in early spring to shape and reduce the size of the plants without sacrificing flowers.

Ceanothus 'Gloire de Versailles'

Among the large, rounded shrubs the industry standard, ‘Skylark’, grew and flowered reliably.

Ceanothus 'Skylark'
‘Italian Skies’ is not as commonly grown as ‘Skylark’, but also grew well, although the foliage of this cultivar is not as attractive as ‘Skylark’.

Ceanothus 'Italian Skies'
‘El Dorado’ is a variegated cultivar that has a tendency to revert, resulting in variegated and green shoots, although it continues to grow, flower, and retain its form.

Ceanothus 'El Dorado'
The cultivars and hybrids of C. impressus are in this group and, although some of them died during the trial, they are among the best foliage and flowering plants in the genus.

Ceanothus 'Puget Blue'
‘Concha’ also typically forms a large shrub, but in this evaluation lacked vigor.
The third group, mounding, spreading shrubs, contains several well-known cultivars used as groundcovers in California, including ‘Joyce Coulter’ and ‘Joan Mirov’.

Ceanothus 'Joyce Coulter'

Ceanothus 'Joan Mirov'

Another excellent, wide-spreading groundcover is ‘Emily Brown’.

Ceanothus 'Emily Brown'
All of these are distinctive in appearance and would make excellent tall and wide-spreading groundcovers on hot, dry banks in western Oregon. The Oregon native, C. cuneatus, has gray foliage and white flowers and as a native shrub is a good choice as a specimen of tall groundcover.

Ceanothus cuneatus
Other cultivars which would be similarly useful include C. thyrsiflorus var. griseus ‘Kurt Zadnik’.

Ceanothus 'Kurt Zadnik'
and ‘Wheeler Canyon’

Ceanothus 'Wheeler Canyon'
They are all very wide-spreading and likely of sufficient height and density to suppress most weeds.
The lower-growing groundcovers in the trial performed inconsistently. Weed suppression with the lowest-growing cultivars, such as ‘Centennial’ and C. hearstiorum, was poor, because of lack of height and loss of foliage due to cold injury. Low-growing weeds such as dandelions (Taraxacum officinale) and quackgrass (Elytrigia repens) were able to take advantage of these openings and became difficult to eradicate.

C.'Centennial' with dandelions
C. gloriosus ‘Hearts Desire’ was more cold hardy than ‘Centennial’ or C. hearstiorum, but was still very low-growing.

Ceanothus 'Heart's Desire'
These three cultivars might be most effectively used as small-scale groundcovers in limited plantings.

Ceanothus hearstiorum
Taller growing plants such as C. gloriosus, and C. cuneatus var. rigidus ‘Snowball’ tend to be more effective groundcovers, particularly for weed control. Other low-growing shrubs which were hampered by cold injury included C. thyrsiflorus var. griseus and C. foliosus ‘Berryhill’.

Managing Ceanothus in Oregon landscapes

Probably the two most common complaints about Ceanothus as landscape shrubs in the Pacific Northwest is that they get too big, or that they are short lived. These two complaints may not be unrelated! That some Ceanothus get big is true. There are a lot of cultivars that can easily exceed 8 feet in height and width including ‘Victoria’, the most commonly used Ceanothus, and the probable main culprit for this perception! Other large cultivars include ‘Concha’.

3 mature plants of C. 'Concha'

However, there are a lot of Ceanothus that are far smaller than this as well, it’s just that they are not as common as ‘Victoria’.

Also, many times you can observe Ceanothus planted in landscapes that are irrigated, which should be strongly discouraged. For the most part, they are native to regions that receive little or no summer water, and providing irrigation causes them to grow at a time of the year when they should be dormant. As a result they become huge. This often results in aggressive pruning, to which these plants often respond poorly.

Dieback in sheared Ceanothus
Watering also predisposes the plants to root diseases, which can kill the plant outright.

Dieback in sheared, irrigated plants
Some ceanothus, for example ‘Centennial’, do seem to be short-lived, but most of them seem to be quite long-lived shrubs, given the correct environment

Besides, watering these plants, especially in the Northwest, is not necessary. Our drought period and high temperatures impose only modest stress compared to what they would endure in California. A shrub planted in September or October, and then watered in for a short time until the soil remains reliably moist, will usually not even require irrigation the following summer, regardless of how hot it is.

The best use for ceanothus is combining them in un-irrigated landscapes with other plants that require the same treatment. This includes some drought-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 Rockrose (Cistus or Halimium spp.), Rosemary (Rosmarinus officinalis), Manzanita (Arctostaphylos spp.) or Jerusalem sage (Phlomis spp.), to name only a few.

Mixed planting with Ceanothus, Cistus, Phlomis and Artemesia
 

Books and websites

The most complete reference work on ceanothus cultivars, species and cultivation is Ceanothus (Fross, D and D. Wilkin. 2006. Timber Press, Portland, OR. 272pp.). For information on species characteristics, identification and distribution, then the best source of information is Dr. Cliff Schmidt’s descriptions on the OSU Botany website: http://oregonstate.edu/dept/botany/herbarium/projects/ceanothus/. Information on species distribution in the Pacific Northwest is available in Plants of western Oregon, Washington and British Columbia (Kozloff, E. 2005. Timber Press, Portland, OR. 512pp)

A number of other books on California native plants also describe ceanothus for the landscape, and several of these are listed below.
California native plants for the garden. Bornstein, C., D. Fross and B. O’Brien. 2005. Cachuma Press, Los Olivos, CA. 271pp.

Complete garden guide to the native shrubs of California. Keator, G. 1994. Chronicle Books, San Francisco, CA. 314pp.

Native treasures. Smith, N. University of California Press, Berkeley, CA. 278pp.

Garden use of ceanothus native specifically to the Pacific Northwest is covered in Gardening with native plants of the Pacific Northwest (2nd ed.) (Krukeberg, A. 1997. University of Washington Press, Seattle, WA. 282pp.

Website creator:
Neil Bell
Community Horticulturist
OSU Extension Service
Marion and Polk Counties
3180 Center Street NE #1361
Salem, OR 97301
Phone: 503-361-2671
neil.bell@oregonstate.edu