This article is part of a publication series based on the Northwest Plant Evaluation Program at Oregon State University. This research program has evaluated flowering shrubs for their suitability in landscapes in western Oregon since 2000. These publications highlight plants that were successfully grown without irrigation in the trials as well as techniques for growing and establishing drought-tolerant plants in the landscape.

The research trial evaluating the genus Ceanothus occurred during 2001 to 2005 in Silverton, Oregon. The trial compared 38 species, cultivars and hybrid selections of California lilac, which were irrigated only during establishment in the summer of 2001, and were not pruned or fertilized. Information collected about the plants included bloom time, size, and cold hardiness. We have selected the top 5 performing plants for western Oregon landscapes based on our research and describe them below.

Overview of California Lilacs

The genus Ceanothus, also known as wild lilac, California lilac, or simply ceanothus, comprises approximately 55 species of woody shrubs native to both eastern and western North America. Most of these species are concentrated in western North America, particularly in California, but at least 9 species are also native to western Oregon, where they are an important component of summer-dry forest, oak woodland, and chaparral. Most species are evergreen shrubs, which range widely in habit from prostrate to sprawling or large erect shrubs, with some even forming small trees. Ceanothus are also nitrogen-fixing shrubs and do not require nitrogen fertilizers. Because of this they are best utilized in summer-dry, lean, well-drained soils.

The flowering time for the many species is spring into early summer and flower color may be white or pink, but most are various shades of blue. Since blue is a difficult color to find for the garden, Ceanothus are unique in that they allow gardeners to introduce this rare color on a grand scale into the landscape. As a bonus, the flowers of Ceanothus may be lightly scented and are highly attractive to a wide range of pollinators.

In selecting Ceanothus for your landscape, a few things to be mindful of are size, pruning and watering regimes. Some Ceanothus grow very quickly to become large shrubs, which can easily outgrow their allotted space. Carefully check the size of the Ceanothus at purchase and provide it with sufficient space to grow unimpeded to maturity. Pruning these plants excessively to control size, especially evergreen types, often leads to dieback. The semi-deciduous hybrid cultivars are much more forgiving of pruning. Also keep in mind that as mentioned, Ceanothus native to the west are adapted to summer-dry environments and so are naturally summer-dormant. They should only be planted in un-irrigated locations in the garden. Watering them in summer leads to rampant growth and possible root disease.