A corollary goal is to provide the nursery industry with a source of new and interesting, economically advantageous and environmentally sound plant revenue. With increasing pressure from state and regional waterquality control boards for zero runoff in the nursery industry , plants requiring fewer inputs will be a welcome addition.A field was prepared to test 240 plants on a UC Davis research farm in USDA Zone 9 . Plants were placed 2 yards apart along rows that were 2 yards apart, with 20 plants per row in each of 12 rows. This allowed the simultaneous testing of six individual plants on each of four different water treatments for each of the 10 species. The rows were covered with 3 to 4 inches of bark mulch, and two 2-gallon-perhour drippers were buried beneath the mulch in the root zone of each plant. The plants were placed according to a randomized complete block pattern in three blocks throughout the field. Each row was furnished with four water lines to deliver one of the water treatments to each plant after they were established. It is important that even drought-tolerant plants be given supplemental water until well established, because the development of an adequate root system is a key component of drought-tolerance . The 10 species were planted in fall 2004, and frost-killed specimens of creeping sage were replaced in spring 2005. Likewise, the plants were also watered during long, rain-free periods in the winter of 2005 to 2006. Experimental irrigation treatments were carried out during the 2006 growing season. The four irrigation levels were based on percentages of reference evapotranspiration as described in Water Use Classification of Landscape Species III . ETo was defined as the amount of water evaporated from a 4- to 7-inch-tall, cool-season grass in open field conditions. WUCOLS classifies landscape plants according to how much water they need compared to cool-season turfgrass,drainage planter pot which is high water-use and needs 80% of ETo to look green and healthy in the summer growing season.
In our trial, we used the following percentages of ETo: 20% , 40% , 60% and 80% . We wanted to assess not only if these plants were truly drought-tolerant, but also if they could survive under garden conditions where they might be combined with higher water-use species or adjacent to a high water-use lawn. The average water-holding capacity of the soil was determined from soil samples collected at field capacity along a transect across the field. Irrigation was measured to replace half of the soil’s water-holding capacity in the root zone of each treatment to a depth of 1.5 feet. Since some of the moisture in the soil is held too tightly to soil particles for plant uptake, plant water stress is usually avoided by providing an irrigation when 50% of field capacity has been depleted. This amounted to 21.2 gallons of water per plant delivered over a period of approximately 5.25 hours. We used ETo values calculated by the California Irrigation Management Information System , which comprises data collection stations in various locations throughout the state that measure precipitation, relative humidity, solar radiation, temperature and wind speed. The California Department of Water Resources provides values daily for ETo online for the public . During the May to October 2006 irrigated growing season, the Davis CIMIS station was accessed daily via the Internet, and the ETo values were placed into a water budget worksheet to calculate the four percentages of accumulated water deficit. From this data, the subsequent need for irrigation in any one of the water-use treatments could be determined. In brief, all the plants received the same amount of water at each irrigation, but how often they received it was determined by their water-use percentage of ETo treatment . This low water-tolerance screening is somewhat unique to the needs of a California introduction program, since most states do not deal with complete drought from May to November each year.After the first summer of regular irrigation followed by wintering over in open field conditions, four species had suffered 50% or greater mortality, leaving six species in sufficient numbers to collect data . The species that did not survive the first year were coast gum plant, California beach aster, seaside daisy and creeping sage. The species that did survive were Apache plume, California lilac ‘Valley Violet’, serpentine columbine, rosy coral bells, eyelash grass and San Diego sedge. The first three species that did not survive are native to warm coastal areas, as reflected by their common names. Although they had grown well in the UC Davis Arboretum for years, the unmitigated summer heat and cooler winter temperatures of our field-trial site proved too inhospitable for them.
The fourth species that did not survive, creeping sage, was bitten back by frost in winter 2005 and did not transplant well into the clay-loam field soil in spring. However, the few creeping sage plants that did survive spread up to 9 feet in two directions across bare paths where the soil did not stay moist. It is native to well-drained slopes and is probably a good choice for restoration in its native range in the coastal and Sierra foothills, but was not deemed a good selection for most Central Valley gardens with space restrictions and heavier soils.One of the six species that survived in the UC Davis open field, Apache plume, did not advance to the next stage of zone garden trials. It is a woody shrub with small, dissected leaves and a profusion of pink staminate flowers that lend it a fuzzy appearance when in bloom. While the September 2006 plant growth index was higher with moderate levels of irrigation than with either low or high levels , this difference was statistically insignificant . This species bloomed heavily over a long period of time, and showed no signs of disease or pest damage. However, Apache plume also had some undesirable characteristics. Large branches tended to flop over, yielding an untidy, open habit as the season progressed, and the abundant seeds selfsowed rather freely in dry paths and mulched beds.The second woody shrub was a UC Davis Arboretum selection of California lilac that has become our banner species, ‘Valley Violet’. This California lilac performed beautifully at any watering level,plant pot with drainage which was unexpected since so many other species of this genus will not tolerate summer water. It should be noted that July 2006 was exceptionally hot, even for Davis , and yet the lilac’s appearance was unaffected even at the lowest level of summer water. Steady increases in relative plant growth index over the season from 1.15 to 1.45 were observed for all irrigation levels, with no significant difference between the treatments . In the spring, this plant bloomed in profusion from the base of its branches to the tips and was unbothered by pests or disease. This California lilac, with its yearlong deep green color and staggering spring floral display, was eagerly accepted by all the demonstration gardens involved in the next phase of the trial.Two of our herbaceous species, serpentine columbine and rosy coral bells, are naturally found in shady woodland locations. Consequently, they all showed a loss in plant growth index at all irrigation levels during the hottest part of the growing season in our exposed site, with values between 0.7 and 0.9 .
However, there were no statistically significant differences between the irrigation treatments, leading us to conclude that during the hottest months, protection from the sun was more critical to the success of these species than the availability of water. Interestingly, under the highest watering regimen, two of the six columbines died by the end of July and two more died by the end of August, possibly showing an intolerance of wet soil during the hot season. However, the remaining two columbines were already beginning to recover by September when temperatures began to drop, and all irrigation levels for both of these woodland herbaceous species showed dramatic recovery by the following June. Noteworthy in both species was the prolific flower display, far beyond what was observable with specimens in shady locations in the nearby Arboretum during the same year. While both the coral bells and columbine leaves showed signs of sunburn and necrosis during the summer, their flowering seemed to benefit from the availability of light during the winter and spring months. Both were attractive to bees and syrphid flies, but were unbothered by pests or diseases. So, even though our test site’s exposure was damaging to foliage, their mere survival under these conditions, combined with their spring beauty and attraction of beneficial insects, caused us to advance them to the next phase of the trial with a recommendation for planting sites with at least afternoon shade during the summer. Plants suited to dry shade are sorely lacking in the nursery trade, making serpentine columbine and rosy coral bells good introduction candidates.Also called blue grama grass, eyelash grass is a bunchtype grass with a wide native range in prairies throughout North America. The amount of water it received in our trial made no significant difference in the amount of summer growth, with a September relative plant growth index of 1.9 to 2.0 . Regardless of the amount of water, this species maintained a neat, fountain-form habit desirable for an ornamental grass, and had no pest or disease problems. For these reasons, we advanced eyelash grass to the zone garden trials.San Diego sedge showed an unexpected toughness and drought tolerance for a plant that grows along streams in the wild. It sent up handsome flower spikes that matured to an attractive, buff-colored seed head held above sword-shaped leaves. None of these seeds has been observed to self-sow in the field, making it unlikely to be invasive in dry areas. At all irrigation levels, the plants showed consistent, positive changes in plant growth indices until the end of August, when growth leveled off, presumably in favor of seed production . San Diego sedge plants irrigated at the two lowest water levels did show slightly lower relative growth indices as the season progressed, with those given the second lowest water level inexplicably displaying the lowest relative plant growth index. However, the only statistically significant differences were between the low-medium and high-medium treatments in September . Plants at all irrigation levels became more attractive as the season progressed, and they were pest-free and disease-free. There was no consistent pattern to which watering level the plants preferred, making San Diego sedge a good candidate for a strong structural element in a variety of garden situations.The key to the next stage of this endeavor was the Master Gardener Program, which is coordinated by UC Cooperative Extension. Because these programs are located in most counties throughout the state, they are uniquely situated to grow and collect data on the plants that are advanced from the first phase of the trials. Many counties have demonstration gardens, which make perfect sites for both data collection and exposure to the public. The counties currently participating in the second phase of the native plant trials are Shasta , Placer/Nevada , Alameda , Santa Clara , Mariposa , Fresno and San Diego . The sites include coastal, inland valley and low mountain gardens, but all are within the boundaries of the climate zones recognized as “Mediterranean.” As plants became available beginning in fall 2006 through fall 2007, each site was provided with six plants each of several prospective species advanced from the irrigation trial. Master Gardeners in these areas have planted, tended and collected data on the plants provided. They are taking monthly measurements using the same protocol as the plant growth index used in the field trial. Soil types have been noted, irrigation frequency is being tracked and any unusual weather events noted. In addition, each garden is supplied with data sheets that allow them to rate each plant on a scale of 1 to 5 each month infive categories: foliage, flowering, pest resistance, disease resistance and overall vigor. Table 3 and figure 4 provide examples of the first year’s compiled observations for rosy coral bells.