The Northwest Plant Evaluation Program

Identifying Good Plants for Pacific Northwest Gardens

By: Neil Bell

Neil Bell is Community Horticulturist for Oregon State University Extension, oversees the Master Gardener program in Marion and Polk counties,…

More From This Author
Author Neil Bell (left) with members of the Great Plant Picks Hebe evaluation team.  Photo: courtesy of Great Plant Picks

Author Neil Bell (left) with members of the Great Plant Picks Hebe evaluation team. Photo: courtesy of Great Plant Picks

Northwest Plant Evaluation is a small program based at the Oregon State University North Willamette Research and Extension Center in Aurora, Oregon. Since 2000, the program has sought to evaluate various woody plant genera for cold hardiness, drought tolerance, and landscape use. It is hoped that trial results will expand the range of plants available for Pacific Northwest gardeners, growers, and landscapers west of the Cascade Mountains.

The program began as a result of a personal experience. A cold spell in late December 1998, when temperatures fell to 13oF, damaged or killed all the hebes in my garden near Monmouth, Oregon. This experience suggested that this genus was not well suited to growing in Willamette Valley landscapes. Yet on a visit to Seattle the following July, I came across a 4-by-4-foot specimen of Hebe ‘White Gem’ in full flower (and evidently free of any winter damage) in the New Zealand garden at the University of Washington Botanical Garden. Obviously, not all hebes are glorified annuals; I discovered I had inadvertently planted tender forms of the genus. I thought it might be worthwhile to gather and plant as many hebes as I could find and let Mother Nature sort out which were hardy and which were not.

Hebe trials at the North Willamette Research and Extension Center in Aurora, Oregon.  Photo: courtesy of Great Plant Picks

Hebe trials at the North Willamette Research and Extension Center in Aurora, Oregon. Photo: courtesy of Great Plant Picks

 

That evaluation established the basic methodology of Northwest Plant Evaluations: gather as many cultivars of a given genus as we can find and accommodate, propagate the plants from cuttings, grow them on in a landscape setting, and see what happens when it gets cold. We quickly realized that in order to provide useful information on a specific cultivar, we needed more than its relative hardiness. So we also began collecting data on ornamental characteristics including plant size, flowering times, pest and disease problems, and what I called “plant quality”—a subjective evaluation of overall plant appearance out of bloom.

In some ways, the choice of Hebe for the inaugural evaluation was a good one, and in other ways it would prove to be a source of frustration. More than 50 species and cultivars were already available in the Pacific Northwest, from small alpine plants to tree-like shrubs—with a range of bloom that covered almost the entire year. I quickly learned that this genus could occupy a lifetime. The taxonomy is complicated and, unfortunately, the names of many cultivars are highly suspect. But few plants are as easy to propagate. I joined the UK-based Hebe Society, becoming one of only a handful of North American members to participate in its members-only cuttings exchange. Cuttings I received would still root even after spending 10 days in the mail. Largely because of the generosity of Hebe Society members, Northwest Plant Evaluation would eventually include more than 300 Hebe cultivars. The first 45 cultivars were planted in the spring of 2000.

Hebe cupressoides ‘Boughton Dome’ is a Great Plant Pick selection based in part on its performance in the Northwest Plant Evaluation trials.  Photo: Richie Steffen/Great Plant Picks

Hebe cupressoides ‘Boughton Dome’ is a Great Plant Pick selection based in part on its performance in the Northwest Plant Evaluation trials. Photo: Richie Steffen/Great Plant Picks

Soon after this initial planting, I began to think about other candidates for evaluation and consider how the program might proceed. The Northwest Plant Evaluation program expanded to test plants that could be grown completely without supplemental water. Publications on drought-tolerant plants for our region often suggest periodic irrigation in the summer, which defeats the goal of limited inputs that drought-tolerant landscapes seek to achieve. I also knew that stopping irrigation would substantially cut back on summer weeds. The Hebe evaluation, which required irrigation, was plagued with weeds during its first two years.

This Arctostaphylos glandulosa SBH 7717 (a collection by Sean Hogan) planted in the 2011 trial withstood temperatures down to 10°F over the winter of 2013 with little to no damage.  Photo: Neil Bell

This Arctostaphylos glandulosa SBH 7717 (a collection by Sean Hogan) planted in the 2011 trial withstood temperatures down to 10°F over the winter of 2013 with little to no damage. Photo: Neil Bell

The Ceanothus evaluation was planted in June 2001, followed by Cistus and Halimium evaluations in 2004, and Arctostaphylos and Grevillea plantings in 2011. These subsequent evaluations were planted in unamended clay loam or silty clay loam—Willamette Valley soil—and were not irrigated once the plants were established. Often, the recommendation is to amendsoil to improve drainage for mediterranean plants like those in our evaluations. But I wanted to avoid the effort and expense and see if the plants would grow in these soils without developing problems such as root rot. To date, we haven’t had problems, although combining summer irrigation and heavy soil may be a different matter.

As evaluations mature, nursery growers are welcome to take cuttings from plants in the program that they feel show promise for the Northwest landscape trade. Both retail and wholesale nurseries have done so, and a number of new plants have been introduced. The Northwest Plant Evaluation program continues in 2014 with trials of future plants for Pacific Northwest gardens.