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Plant tests bring "Arboretum All-Stars" to Home Gardeners

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Plant tests bring "Arboretum All-Stars" to Home Gardeners

Bletilla striata - Chinese ground orchid
Echeveria Imbricata - Hen and chicks


By S. Karrie Reid

California’s climate both charms and challenges gardeners. Its charm includes rainless summers with warm, sunny days and mild nights, and brief, mild winters. But these charms also have challenges. Long hot summers with no precipitation require irrigation and increase pest susceptibility. The brief mild winters can deprive some plants of needed seasonal chill and allow many pests to survive and multiply from one year to the next. Because so many commonly used landscape plants are ill adapted to these climatic conditions, large inputs of water, pesticides and fertilizers are needed to keep them looking their best.

So how does one create lovely landscapes with such difficult challenges?

The obvious answer is simply to garden with sustainable plants that have greater drought-tolerance, fewer pest problems and an adaptation to milder winters. These plants, usually native to California or other areas of the world with similar Mediterranean-type climates, require little supplemental water and no chemicals to look their best.

The horticultural industry thrives on a constant supply of new and beautiful plants to tantalize its customers. Despite the growing demand, plants in the sustainable category have been relatively few and slow in coming to the mainstream nursery market.

One solution to this problem is a statewide, sustainable plant introduction program.  Five years ago such a program began in California and to date, more than 100 waterwise, heat-tolerant ornamental plants have been recommended to gardeners under the label “Arboretum All-Stars.”  (Click here to view them all http://arboretum.ucdavis.edu/arboretum_all_stars.aspx.)

Master Gardeners in San Diego and around the state helped make this possible by monitoring these plants in local test gardens. We can’t thank them enough for their on going contributions.

Select and Test

Arboretum All-Stars began as a collaboration between UC Davis Arboretum staff and the UC Davis Department of Plant Sciences.  Its goal is to provide the gardening public with a list of recommended beautiful plants that perform well in a variety of the state’s climate zones with a minimum of water and chemicals.

The program also is designed to provide the commercial green industry with a constant source of new, interesting plants with proven public appeal that are environmentally conscious and easy to produce. To meet this goal, the California Center for Urban Horticulture (http://ccuh.ucdavis.edu) joined the team to connect with industry partners.

The program was launched with staff of UC Davis Arboretum selecting garden-worthy California natives or other Mediterranean-climate adapted species that had performed well in the Arboretum’s hot, interior climate for several years.  The plants – all natives and everything but annuals - were picked for various features:

  • Attractive for at least 3 seasons of the year, with interest in the off-season
  • Low water use
  • Pest tolerance and disease resistance
  • Bonus feature: attractive to wildlife, including bees, birds, butterflies, and beneficial insects

These plants were grown and evaluated in irrigated fields in Davis for two years. After a year of regular irrigation to establish the plants, each species was given one of our levels of irrigation during the “irrigated growing season” (roughly May to October) of the second year to assess their performance during the dry season on different levels of water.  The levels varied from every 12 days to only twice during the hottest months. Plant growth measurements were taken monthly, and the plants were rated in 5 categories: foliage appearance, flowering, pest resistance, disease resistance and overall vigor.

The program is now in its fifth year, and the third round of trials has begun.  We have recently added a shade structure over a ¼-acre field, which allows us to evaluate plants for dry shade in addition to those for full sun.

Acca sellowiana (Feijoa sellowiana) - pineapple guava
Heuchera - Coral Bells


Zone Trials with Master Gardeners

The All-Stars Team

This third aspect of the trials is where the Master Gardeners become invaluable.  In order to assess the plants’ ability to do well in a variety of climate zones throughout the state, plants were placed in demonstration gardens tended by Master Gardeners in 13 counties that stretch from Redding to San Diego and Palo Alto to Mariposa. 

These qualified volunteers make monthly observations, using the same rating system used in the irrigation trials at UC Davis, and take quarterly measurements to evaluate plant performance in their specific soil and weather conditions.  They report their data and observations monthly on an online website. 

Measuring Plants

In addition, their interaction with the public helps determine if the plants have broad appeal, even before they hit the market.  Their final analysis is a big part of the decision to actually move new plants to market, or if the market will be restricted to certain areas.

In San Diego County, there were initially two test gardens; one in the Pt. Loma area of San Diego and one in Fallbrook at the homes of Master Gardeners.  The first round of plants - all California natives - was put into demonstration gardens in the fall of 2006.  They included Rosada coral bells (Heuchera rosada), blue grama grass (Bouteloua gracilis), San Diego sedge (Carex spissa), serpentine columbine (Aquilegia maxima), and a particularly floriferous Arboretum selection of California lilac (Ceanothus maritimus ‘Valley Violet’). 

In subsequent years, the plant palette has been expanded to include a variety of other water-conserving species including those not native to California.  San Diego test sites now include two public gardens, the Water Conservation Garden on the campus of Cuyamaca College in El Cajon, and the San Diego Botanic Garden in Encinitas. To read more about the irrigation and climate zone trials, visit the California Center for Urban Horticulture website at: http://ccuh.ucdavis.edu/projects/arboretum/aas-trials-program.

To Nurseries and into Gardens

The first plants sold under the “UC Davis Arboretum All-Stars” label went to market in a limited launch during the spring of 2009.  The plants are displayed with a large informational poster that explains the advantages of using these sustainable plant choices in the garden. The location of retail nurseries currently selling these plants can be found on the Arboretum’s website: http://arboretum.ucdavis.edu/allstars_retailers.aspx

“Arboretum All-Stars” plants are also available at some nurseries not involved in the program. At these sites the plants are not labeled as “All-Stars” but interested gardeners can use the “All-Stars” list to seek them out. To see all 100 plants recommended for sustainable gardens, and to use an interactive searchable database, visit the UC Davis Arboretum website: http://arboretum.ucdavis.edu/arboretum_all_stars.aspx

The program is continuing to evolve, adjusting to grower needs and market demands, and an eye is being kept on the California Invasive Plant Council’s list of plants to watch for invasive potential.  The booklet of 100 All-Star Plants is revised each year, striving to keep the information current and reflective of the best recommendations we can make for sustainable landscapes.  Through this process we strive to be part of the solution to California’s need for more beautiful, water-conserving, healthy landscapes.

-Karrie Reid is a staff research associate in the Department of Plant Sciences at UC Davis.