Ceanothus flowering shrubs are stunning in PNW gardens

Last Updated: 
April 29, 2005

CeanothusCORVALLIS –If you've passed by a garden in the Pacific Northwest in May or June and have been astonished to see an eight-foot-tall, eight-foot-wide shrub completely covered in blue flowers and humming with the activity of hundreds of bees, you have probably encountered a Ceanothus.

Ceanothus is a genus of native North American shrubs with about 60 species. People sometimes call them "California lilacs." These native North American woody plants are not yet common in Pacific Northwest domestic landscapes. Several species have been collected, bred and domesticated in Europe and California. Very few of those varieties have been tested here in the Northwest, until now.

Oregon State University Extension community horticulturist Neil Bell is working with the Oregon Garden to test 45 varieties of Ceanothus to see how they do in Pacific Northwest conditions.

"The goal of our evaluation is to study the growth and flowering of the Ceanothus varieties as well as their cold hardiness in Pacific Northwest Maritime conditions," explained Bell. "We are looking for significant disease or insect pest problems as well."

The name Ceanothus comes from the Greek word keanothos, meaning spiny plant. These shrubs are commonly thought of as native to California. About 36 of the species are found only in California, but there are several Ceanothus species that range over the western, mid-western and eastern United States and southern Canada, including some native to Oregon.

Most of the species from mild areas such as California are evergreen shrubs, while those from cold climates are usually deciduous, explained Bell.

The genus offers a great diversity of plant size, habit and leaf shape and size. Ceanothus species vary from prostrate ground covers to mounding, spreading shrubs to tree-like forms. All are typically free of major disease or insect problems.

Ceanothus are also cultivated for their small but profuse flowers, seen in the Northwest from April through August, depending on the species or cultivar. Flower colors include white and pink, but most people are familiar with the extraordinary blue flowers, which range in color from powder blue to indigo.

In their native habitat, Ceanothus often inhabit exposed, dry slopes in mountains and in coastal regions. These plants can grow in poor soils, thanks to nitrogen-fixing bacteria that form nodules on the roots. As a result, it is tolerant of poor, dry soil. The flowers also attract swarms of beneficial insects, so the plant has ecological value as well.

The Europeans were the first to use Ceanothus as a cultivated ornamental. C. americanus has been in cultivation in the United Kingdom since the early 18th century. In the 19th century, C. thyrsiflorus, a large-growing evergreen species native to California and southwest Oregon, was introduced to Europe, along with other evergreen species from the West. The evergreen forms were much admired for their spectacular bloom, but many proved somewhat tender. It was not long before European breeders were crossing the hardy deciduous forms with the tender evergreen forms in the search for greater adaptability.

The result was C. x delilianus, commonly called the French hybrids. Possibly the best known of this variety are 'Gloire de Versailles' and 'Henri Desfosse.' Further breeding over the years has produced other hybrid forms, which include the pink-flowered C. x pallidus 'Marie Simon.' Most of these offer significantly greater cold hardiness than the evergreen types from which they are derived, often being rated hardy to USDA Zone 6.

In wide use in the United Kingdom and California, the beauty and adaptability of some have made them staples of inhospitable environments, such as highway roadside landscapes and embankments in California.

In the Pacific Northwest, Ceanothus is still relatively uncommon in landscape situations where drought-tolerant evergreens are required. In Oregon's cooler northern climate, it suffers from a persistent reputation for tenderness and a short lifespan, said Bell.

Bell hopes his work testing varieties in Oregon will help increase the use and popularity of the shrub in the Pacific Northwest. So far, his OSU tests of Ceanothus at the Oregon Garden look encouraging.

"The growth rate on many of these test Ceanothus plants has been remarkable," said Bell. "Most of the plants were in four-inch or one-gallon pots at the time of planting in 2001, and in May 2004 there were many that were four- to six-feet tall and wide. The form on many of the plants is excellent."

Based on the trial results, Bell wants to construct a flowering calendar so gardeners can plant a series of Ceanothus varieties that will bloom, in succession, for several months.

The study is ongoing, but the results are showing that so far, there are a number of varieties that are performing well in the maritime Pacific Northwest. With further study, Bell projects that the hardier varieties may deserve greater use in our region.

"The tolerance Ceanothus exhibit for the climate and for poor, dry soils is a significant advantage of these plants, and makes them an attractive option for difficult sites," he said. "The fact that we don't have to give up dazzling ornamental attributes to get that toughness makes them even more appealing."

Author: Carol Savonen
Source: Neil Bell