By Neil Bell

For the last several years, together with my OSU Master Gardeners, I have evaluated species and cultivars of ceanothus, cistus and halimium for adaptability to western Oregon gardens. Ceanothus is primarily a western U.S. genus, though they are best known by the species and selections native to California. However, there are ceanothus found through the midwest to the east coast, principally Ceanothus americanus, a deciduous species that figures prominently in some of the desirable hybrids. Cistus and halimium (and the hybrid between them:
x Halimiocistus) are Mediterranean in origin, ranging from the Canary Islands through the Mediterranean basin to the Caucasus Mountains.

These trials are simple evaluations of growth, flowering and cold hardiness conducted on plants grown in the open. No special soil preparation or other techniques are used, so they are grown in much the same way as a gardener would. The ceanothus trial was conducted at The Oregon Garden in Silverton, Oregon between 2001 and 2005. The cistus and halimium were evaluated at the OSU North Willamette Research and Extension Center, near Aurora, Oregon, from 2004 through 2006. In both cases, the young plants were irrigated only for the first summer until they were established. After that, no irrigation, fertilizing or pruning was done on the plants. Cold hardiness was evaluated in spring by assessing damage from the previous winter. The only real cold damage noted was to the ceanothus, as the result of freezes during the winter of 2002 and 2003, when temperatures dropped abruptly into the low 20s. Significant winter damage to the cistus or halimium has not been observed during the evaluation. The results of these trials—I think—would apply to most gardens west of the Cascades from southwestern British Columbia to northern California.

Ceanothus

Ceanothus is probably known to most Northwest gardeners as one of two fairly common evergreen shrubs. C. ‘Victoria’, as it is commonly sold, is an eight-by-eight-foot shrub with blue flowers in May and June. C. gloriosus (Point Reyes creeper) is a low-growing plant with light blue flowers used as a groundcover. Both of these are good plants that proved quite hardy. There is a vast array of sizes and shapes between these two, however.

Ceanothus can be roughly divided into four groups: 1. Upright-growing shrubs such as ‘Ray Hartman’ and ‘Blue Jeans’; 2. Large, rounded shrubs such as C. thyrsiflorus var. thyrsiflorus ‘El Dorado’, ‘Dark Star’ or C. ‘Victoria’; 3. Mounding, wide-spreading shrubs such as C. ‘Joan Mirov’, C. ‘Joyce Coulter’ and C. thyrsiflorus var. griseus ‘Kurt Zadnik’; and 4. Low, spreading groundcovers such as C. gloriosus, C. hearstiorum and C. ‘Centennial’. Many of these are rarely grown in the Northwest, although they can be found in specialty nurseries. Ceanothus can grow quite large—an important fact to keep in mind!

Initially, some of the plants were not very hardy. A freeze in early November 2002 did serious damage to several cultivars, including most forms of C. thyrsiflorus var. griseus. Some of these plants were, in fact, killed. A striking exception to this was ‘Kurt Zadnik’, which suffered relatively little damage. C. hearstiorum was also heavily damaged, as was C. thyrsiflorus var. thyrsiflorus ‘Snow Flurry’. Both of these partly recovered. Others that suffered lesser damage, and recovered more or less fully included ‘Centennial, ‘Joan Mirov’ and C. maritimus ‘Popcorn’. Other plants in this evaluation may have had some damage but recovered well. The plants that were damaged can be grown in our region, but require a more protected site.

In looking for ceanothus of diverse size and habit for the garden, quite a few performed well in this evaluation. Of the upright-growing plants, ‘Ray Hartman’ was one of the best. It has a vase-shaped habit, and will easily grow to 10 feet tall. It produces light blue flowers from early April to mid-May. ‘Blue Jeans’ is an exceptional plant, forming a dense shrub to about five feet tall and as wide. The small leaves give it an unusual texture, and the light blue flowers are among the first of any ceanothus, appearing in early March. Among the large, rounded shrubs, ‘Dark Star’ and ‘Julia Phelps’ are both eight-by-eight-foot shrubs and are very striking with deep blue flowers from late March through April. For a tall, drought-tolerant ground cover, there are several good choices: ‘Wheeler Canyon’ grows to about six by ten feet, and produces a profusion of blue flowers in late April. ‘Joyce Coulter’ is of similar size and bloom time. ‘Joan Mirov’ grows to five feet tall and twelve feet across and produces deep blue flowers through May. Finally, ‘Kurt Zadnik’ is a very large and striking groundcover, to six feet tall and fifteen feet across and produces large clusters of deep blue flowers from late April through May.

If these are simply too large, there are smaller options. As mentioned. C. gloriosus is readily available and low-growing. A really nice, dwarf selection of this species, ‘Heart’s Desire’ grows to only a few inches tall and four feet across. Both C. hearstiorum and ‘Centennial’—though a bit tender—are excellent small-scale ground covers, and look great trailing over a wall. C. maritimus ‘Popcorn’, also a bit tender, nevertheless forms a well-shaped bun only two feet tall and four feet across. Finally, some of the hybrid ceanothus are very adaptable plants. The best, and most showy of these was ‘Henri Desfosse’, which has striking deep blue flowers, and apparently grows to about four feet tall and wide. In our trial, it was grazed enthusiastically by deer, which reduced its size dramatically. Most other ceanothus, by the way, seem pretty resistant to deer.

Cistus and Halimium

Among the cistus and halimium, the most familiar and commonly used by far are C. x hybridus and C. x purpureus, both of which form dense, aromatic shrubs (most cistus are distinctly and pleasantly aromatic) to four feet tall and perhaps five feet wide. Both of these are good plants, although C. x purpureus is a showier plant in bloom, but the foliage quality of C. x hybridus is superior. Actually, in the absence of any significant cold damage, it was plant form and foliage quality that really separated good selections of these plants from poor ones. Some cultivars which were very showy in bloom, such as ‘Victor Reiter‘, ‘Peggy Sammons’ and ‘Silver Pink’, became very sparse and leggy as the trial went on, and make poor specimens after only three years. The following suggestions are all plants which retained excellent form and foliage throughout the evaluation and flowered well.

Many of the Cistus have a mounded habit which makes them good tall groundcovers for sunny sites with dry, poor soil—situations in which they thrive. Several cultivars fit this description. ‘Snowfire’ grows to four-by-six-feet and has white flowers with a red blotch at the base of each petal from mid-May through June. C. x dansereaui ‘Decumbens’ is somewhat smaller, but has similar flowers and flowers later. C. hirsutus and C. x laxus form nicely-shaped domes about four feet tall and six feet wide and have single, white, flowers.

Unlike the many dome-shaped or low-growing cistus, C. x aguilarii is upright. It can reach seven feet after four or five years, and will typically be about five feet across. It produces large, pure white blooms in May. It is often sold as ‘Blanche’, though this is not correct: ‘Blanche is a cultivar of C. ladanifer, a very different plant. Some of the halimiums also have an upright habit, and of these H. x pauanum is probably the best for a combination of bloom and foliage. It grows to six feet tall, features grey-green foliage and has yellow flowers from late May through early July.

For the smaller garden, or even for large containers, there are several plants to consider. The best known of these is C. x pulverulentus ‘Sunset’, which is widely available and forms a mound of grey-green foliage two feet tall and four feet wide. ‘Sunset’ begins producing its brilliant magenta flowers in late May, and virtually alone among the cistus, can have a few blooms as late as October. ‘Grayswood Pink’ is of similar habit to ‘Sunset’, although it seems to maintain a tighter form and not open up the way ‘Sunset’ can do. ‘Grayswood Pink’, as the name suggests, features pink flowers starting in early May.

Finally, of the x Halimiocistus, probably the best overall is x Halimiocistus sahucii, which forms a dense mat barely one foot tall and four feet across, with white flowers in May.

All of these plants not only flower well, they also maintain an attractive appearance the rest of the year and seem tolerant of Northwest weather. You might have to look around a bit before finding some of these cultivars, but once found they will be great additions to the dry garden.  NWGN