Though it has an unrivaled range of blue flowers and includes plants suitable for many gardening and landscaping situations, this North American native genus has long been underutilized. Its species range from Canada south through southern Mexico and from coast to coast, and there are forms from ground-hugging mats through shrubs to trees. A complete horticultural and botanical treatment of the genus aimed at both gardeners and botanists, this book finally givesCeanothus — with so many plants that tolerate sun and shade, thrive in arid conditions, and bear a profusion of beautiful, fragrant flowers — the recognition it deserves.
Growing Ceanothus
Ceanothus is best suited to gardens where conditions match or approach those of where the plants are found in the wild. In many of the native California habitats of Ceanothus there are a number of ecological similarities, whether plants grow on the coastal bluffs of Mendocino County, sandstone ridges of the Santa Lucia Range, or brushy slopes above the San Gabriel Valley. In all these sites, the soils drain well and the summers are dry. Ceanothus grows in a range of soil types but often is found on steep slopes in soils with low or marginal fertility. Nitrogen-fixing bacteria are found in nodules on their wide-spreading roots, helping the plants adapt to dry, nutrient-poor conditions. Considering their natural habitats, it is not surprising that most California species will endure long periods of drought, are intolerant of summer-wet environments, and require little or no fertilization when planted in gardens.
Many of the selections originating from California species can be grown successfully outside their native habitats and have demonstrated broad cultural tolerances particularly in regions that are cooler and moister than the Mediterranean climate portions of California. British gardeners have long used a diverse range of evergreen and deciduous cultivars as specimens, screens, and ground covers. Ceanothus is frequently espaliered along south-facing walls and included in the perennial borders of England, Ireland, and northern France. Species and cultivars are typically longer-lived and grow larger in these environments than is typical in California gardens.
Other areas with moderate winter temperatures, like the Pacific Northwest of North America, and New Zealand, have been successful growing selected evergreen cultivars such as the hybrids 'Concha', 'Edinburgh', and 'Skylark'. In the eastern United States, Ceanothus americanus has been valuable in woodland gardens and tolerates drier conditions than many other woodland shrubs. The deciduous hybrids, such as C. ×delilianus 'Gloire de Versailles' and C. ×pallidus 'Marie Simon' are also effective in the warmer portions of the eastern United States or with frost protection in colder regions. Most cultivated species and hybrids originated in the Mediterranean climate portion of California with its extended summer drought. The length and severity of this seasonal drought is moderated in the northern portion of the state by a shorter rainless period, and in coastal areas by persistent summer fog that reduces the evapotranspiration rates of plants found in those habitats. Species that occur at higher elevations receive some dry-season moisture from occasional summer thunderstorms, but as a group, most California species receive little or no precipitation when evapotranspiration rates are highest, in summer. The primary requirement for garden success with these plants is a well-drained soil. Numerous fungal organisms (water molds, rots, and wilts such as Fusarium, Phoma, Phytophthora, Rhizoctonia, and Vertcillium dahliae) damage or kill plants in poorly drained soils. Frequent summer irrigation and warm soil temperatures favor these pathogens. Occasional summer watering is acceptable for established plantings and may improve the appearance of most selections, but soils should be allowed to dry between waterings.
Most Ceanothus species and cultivars follow a similar pattern of growth and flowering. During their first few years, plants grow rapidly and flowering is modest. This is typically followed by a period of slower growth with exceedingly heavy flower production. From 7 to 12 years in age, they continue to flower but with less profusion, and the size of the flower clusters decreases in a number of cultivars. Plant structure often begins to open during this period, and specimens develop a more pronounced woody character. This is a generalized time line, and it may vary, depending on the site, species, or cultivar. Gardening practices also influence plant development. In favorable sites, many cultivars and species will continue living many years.
The reputation of Ceanothus as short-lived is often based on poor site selection rather than an inherent problem with the genus. Poorly drained soils combined with frequent summer irrigation will kill plants in a few years. Some species and cultivars, such as C. hearstiorum and the hybrid 'Dark Star', are naturally shorter-lived than other selections. Ceanothus arboreus and C. thyrsiflorus var. griseus, however, can live 25 years or more in a garden setting. Most of the California species (and resulting cultivars) have evolved in fire-active plant communities such as chaparral and coastal scrub. These communities are subject to periodic fires that remove senescent stands, activate the dormant seed bank, and cover the charred earth with fresh young seedlings. In Ceanothus ×pallidus 'Marie Simon', Knoll Gardens, Dorset, England their native habitats, many species lose vigor and decline as they age, based on an evolved adaptation to fire or as part of community succession. This natural successional process renews the older stands in rhythmic fire cycles that might have 20-, 35-, or 50-year cadences.
In addition to its stunning spring flowers, Ceanothus fulfills many practical landscape and garden requirements. The broad diversity found in the genus offers a multitude of sizes and forms from which to choose. Larger selections are useful as quick screens, growing to 10 feet tall or more in a few years, or as solitary specimens. They can serve equally well as small trees or even formal hedges when properly and routinely pruned. With detailed attention they can also be espaliered on walls or fences. Medium-sized shrubs are useful as informal hedges, and with knowledgeable cultivar selection flowering can extend from winter to midsummer. Many new ground-cover selections have been added since the 1980s to an existing palette of well-known favorites. They range from low, creeping types like the hybrid 'Centennial', suitable for use in the dappled shade of a woodland garden, to large, sprawling forms like C. thyrsiflorus var. griseus (at the time called C. griseus var. horizontalis) 'Yankee Point', commonly used to cover sunny, south-facing slopes. Some of these lower-growing types are useful in rock gardens or spilling over walls. Although uncommon and labor intensive, a number of ceanothuses can be pruned into small hedges or trained to climb a trellis.
An annual trimming of the new growth will maintain a more compact form and improve the appearance of most species. The removal of spent flowers and fruit improves the vigor of many cultivars and will produce a tidier form. Taller species can be trained into small trees with early pruning, and the removal of interior dead wood as plants age produces a cleaner appearance. Once this arborescent character is achieved it is easily maintained and requires minimal effort. Shearing for hedges and formal effect is tolerated by most species and cultivars if cutting into woody tissue is avoided. Prune immediately after flowering, and only back to the new year's flush of growth.
Fast-growing selections sited in fertile soils and receiving typical garden irrigation can be subject to "wind rock," especially in winter. Specimens become sail-like and blow over in the wind. This contributes to the reputation of the genus as short-lived. Pruning the young stems to reduce size, encourage a denser form, and reduce the amount of available water may help avert this problem. Heavy pruning into woody tissue can lead to branch dieback, which is caused by the fungal disease apricot dieback (Eutypa armeniacae). Air-borne spores spread this pathogen primarily during the rainy season, and entry to the plant is gained through open wounds that expose xylem tissue. Heavy pruning with multiple cuts provides the ideal condition for the disease and should be avoided especially during the rainy season. If the disease is present, remove the infected wood well below any cankers and close the wound with a sealing compound. Always disinfect pruning tools after working with diseased specimens.
Ceanothus attracts myriad insects. Many are beneficial and significantly improve garden habitats. Bees, both native and domestic, and other pollinating insects cloud the plants when in bloom, and the larval forms of a number of butterflies and moths use Ceanothus species for food. Ceanothus silk moth, California tortoiseshell, California hairstreak, and artful dusky wing skipper are just a few of the California native insects that depend on the genus. Some farm operations have begun using Ceanothus species in windrows to attract beneficial predatory insects as part of integrated pest management programs. As a result, birds are attracted to these insect populations and use Ceanothus for cover. Quail, rabbits, chipmunks, and ground squirrels consume the small hard seeds, and many other animals use the shelter they provide.
Disease and Pests
Insect pests can pose a number of problems for Ceanothus in garden settings if left uncontrolled. Aphids, mealybug, scale, and whitefly are the most common and can disfigure and distort populations. Stem gall moth (Periploca ceanothiella) can pose a potentially serious problem in some regions of California. Swollen stems and branches indicate the presence of larvae that will often girdle the distal portion of the infected tissue. Portions of the state are relatively free of the pest, while in other areas it would be prudent to avoid the use of the preferred larger-leaved species in subgenus Ceanothus such as C. thyrsiflorus var. griseus and C. arboreus. Environmentally friendly controls are not available, but close monitoring of irrigation and fertilization to prevent off-season (summer–fall) flushes of growth will help avoid serious damage.
Boring insects are a potentially serious problem. Sycamore borer (Synanthedon resplendens) and Pacific flatheaded borer (Chrysobothris mali) are particularly widespread. Shallow, irregular mines or burrows just under the bark of the main trunk or larger branches indicate these pests (Dreistadt et al. 1994). The area above the burrow is dark-colored and often has sap exuding from the wound. If left untreated, borers will usually kill the plants; a systemic insecticide is the only effective control. Early detection and treatment will moderate the problem. Like stem gall moth, the occurrence of borers has a regional pattern and is problematic in some areas yet uncommon in others.
Deer present a challenge when gardening with Ceanothus as they often browse heavily on plants, often deforming or even killing young plants. Large-leaved species and cultivars of subgenus Ceanothus are particularly subject to deer predation. Some of the smaller-leaved types, and those of subgenus Cerastes, are less likely to attract deer, but any will be eaten when fresh from nursery containers or during periods when other browse material is scarce. Protection of young plants is strongly recommended in deer-prone areas. After establishment, deer are less likely to kill the plants but will browse lightly even on older, well-established plants. Mice, rabbits, and rats can also be a problem, especially with young plantings, which may require protection.
The numerous fungal organisms that attack Ceanothus in the garden are usually a result of moist conditions common in many summer gardens. Summer moisture provides a favorable environment for these pathogens. One strategy that can limit or prevent these problems in the landscape is the selection of resistant species and cultivars, careful irrigation, and the recognition that in some situations another plant might be more appropriate. Inappropriate spacing of ground-cover selections can create two problems commonly encountered in large-scale plantings. First and most obvious is planting Ceanothus on 3-foot centers for "quick cover." After a few years such a stand is 5 feet tall, covering walkways and climbing up the fence or adjacent vegetation. This mass then provides ideal conditions for a second problem — fungi that can cause twig and stem dieback as well as plant disfiguration. Proper spacing, appropriate cultivar selection, and a small measure of patience will prevent both problems.
Cultivar Development
Opportunities remain for additional cultivar testing and selection. Garden-tolerant plants that perform well in summer heat, such as that of interior valleys, would be desirable, as would additional selections of the delightfully fragrant and slower-growing members of subgenus Cerastes. Smaller shrubs and lower ground covers would be suitable for the smaller gardens typical of new housing developments, as many popular selections of Ceanothus are simply too large for many residential gardens. Most hybrids common in the nursery trade are a result of crosses that would not have occurred outside of garden or nursery conditions and that would not be possible in the wild. Intentional hybridization holds enormous potential for new cultivar development, although little has been achieved to that end as yet.
Propagation
Most ceanothuses are readily propagated from seed, and many can be grown successfully from cuttings. Cultivar development since the 1950s has focused most commercial efforts on cutting propagation. Restoration practitioners, botanical gardens, collectors, and some native nurseries concerned with site specificity and the genetic diversity of wild populations favor seed propagation.
Seed maturation depends on the weather, but fruits are typically ripe 4–8 eight weeks after flowering. Warm or hot weather hastens the ripening process and should be monitored carefully near the time of expected maturation. Check fruits for ripeness as the color begins to change from ruby or burgundy to dull brown or black. Collect fruit into a closed container when 2–5 percent have shattered on a specimen or in a selected population. As the fruits dry and dehisce, the seeds are projected a considerable distance. Place the drying fruit in containers or trays covered with shade cloth or fine screen to contain the seed. Seed treatment depends on the species. Fresh seed typically produces the best results. Stored seed requires scarification and in some cases stratification, especially species from higher elevations. For production purposes a hot water treatment — placing seeds in water at 180–200°F (82–93°C) and left to cool for 24 hours — is often more practical than scarification.
