Wednesday, April 26, 2017

Plants For Dry Shade

One of the more vexing situations for gardeners: those sun-loving perennials and small shrubs that you put in years ago...when the surrounding trees and shrubs were much smaller...may be rather stressed these days, thanks to all the shade that they now have to tolerate. Add to that the modern Golden State requirements for a more water-efficient garden. The new question is: which plants don't require much water...but can thrive in the shade here in Northern California?

Out at UC Davis and at locales throughout the state, the California Center for Urban Horticulture has been conducting plant trials for over ten years, to find out just how little water a wide assortment of perennials, small shrubs and grasses can tolerate and still thrive. They've conducted tests on both sun-loving plants and shade lovers (growing plants beneath a 47% shade cloth). They've trialed plants, including California natives and hybrid varieties, from many sources, including the UC Davis Arboretum All-Star collection.
 Master Gardener Pam Bone (left) rates the Sunny Knock Out rose in the sun garden at the spring 2017 trials held at UC Davis, with UC Cooperative Extension Environmental Horticulture Advisor Karrie Reid.

The plants are judged on a variety of criteria before, during and after being subjected to varying amounts of water over a two-year period. Criteria includes judging overall appearance, health, foliage and bloom.

Here, then is a handy list of....

Low Water Shade Plants 
2013-2015 trials, conducted at UC Davis.
(links contain plant lists for both sun and shade, as well as pictures)

From the 2013-2015 executive summary report: "In these trials, perennial landscape plant species were evaluated for overall performance on a range of reduced irrigation levels in clay loam soil in the hot interior Central Valley of California. All plants were grown in-ground for 2 years. Planting in October 2013 was followed by an establishment period of irrigation at 80%-100% of reference evapotranspiration (ET0) and 25% management allowable depletion through April 2015. Plants were then subjected to 1 of 4 different levels of reduced irrigation at 20%, 40%, 60%, or 80% of ET0 during the dry season through the first week of October 2015. During the deficit irrigation season they were evaluated across treatments for growth, health and vigor, overall appearance, flowering, pest tolerance, and disease resistance. From these assessments, irrigation recommendations are made for their use in the landscape."

For these shade plant test trials in 2013-2015:
80% = 4 irrigations, May through August (once a month)
60% = 3 irrigations, May, July September
40% = 2 irrigations, June, September
20% = irrigated only once (September)

16.6 gallons of water applied at each irrigation, via drip irrigation. Two methods were used: 2 gph button emitters on either side of the plant; or, 1/4" inline drip emitter tubing encircling the plant (4, 1/2-gph emitters, spaced 6" apart).

Correa pulchella ‘Pink Eyre’
"This small Australian shrub cultivar was a consistently high performer on all irrigation levels in our trials. There were no significant differences in growth or overall appearance ratings between treatments with all levels achieving an average overall appearance of 4.0 (very good) or above. What is significant to note is that the lowest irrigation treatment in the shade received no irrigation until almost the end of the trial period on September 27! They reached an average height and width of 36.5” x 57” (93 x 146 cm). Since flowering for this species occurs in the fall through winter, the flowering during the trial period attributable to irrigation was only evident in October, when the fascinating result was that the 80% treatment had 3 plants in bloom, the 60% treatment had 4 plants in bloom, the 40% treatment had 5 plants in bloom, and the 20% had all 6 plants in bloom. The flowering ratings are not included in the quality ratings table, since all the plants had very few blooms open and were mostly in bud, which would have resulted in a universal rating of ‘1’. The pink, bell-shaped flowers were an attractive feature for a long period of time in the fall and winter preceding treatment and would be an asset to the low-water shade landscape."

Dianella caerulea ‘King Alfred’
 "Another Australian native cultivar, this was a lovely, lush, grass-like plant with pale violet blue flowers on long stalks which were followed by bright purple berry-like fruits. Flowering was not dense enough to be the major feature of the plant, however, and the stalks, which came up straight beginning in March, had a tendency to lodge toward the southeast by May. We attribute this to our prevailing winds from the northwest in spring. In a more protected area, or even with higher solar radiation (potentially yielding shorter, stouter stalks), this may not be a problem. Any apparent differences in growth between treatments were statistically insignificant; average height and width at the end of the trial was 43” x 61” (111 x 154.5 cm). A moderate mealybug infestation appeared late in the trial period in September. While one plant on each of the highest treatments had some level of the pest, the two lower irrigation treatments had two (20% ET0) and 3 (40%) plants seriously affected. For this reason, the 40% ET0 treatment had significantly lower pest tolerance and overall appearance ratings (Table 14). There seemed to be some field-position related effect, but the difference couldn’t be completely correlated to that. Due to the double stress of the pest pressure and lower irrigation level, the overall appearance of the two lowest levels in October was really unacceptable. Our recommended level of irrigation for this cultivar is 60% ET0, which for us in a moderately heavy soil was a deep soak every 6 weeks, or three times during the summer."

Lomandra ‘Lomlon’
 "We will note here that this plant’s genetics are controversial, and it is currently marketed under both the names ‘Lime Tough’ and ‘Lime Tuff’. We previously evaluated this cultivar in full sun when it was being marketed under the name ‘Bushland Green’, and it received high marks, especially on the lowest irrigation level. The American patent holder wanted to see how it would perform in shade as well. The most notable difference was that the form became less stiffly upright and more relaxed and fountain-form in the shade, while the color was also a somewhat deeper lime green (Figures 13a-b). The plants consistently received high overall ratings scores on all treatments, with the lowest irrigation level once again scoring marginally highest. No significant differences in size between treatments were found. The ability to thrive in sun or shade on any irrigation level makes this Lomandra one of the most adaptable plants to the landscape that we have evaluated."

Ribes viburnifolium ‘Spooner’s Mesa’
 "Having previously evaluated the species in our trials, we were curious to see what differences this cultivar might display. The straight species tends to send out long new stems with leaves scattered somewhat far apart, so the most notable difference of ‘Spooner’s Mesa’ was the shorter internodes, making the average size somewhat smaller, and the overall appearance more dense, uniform, and appealing. The pleasantly herbal fragrance the foliage emits when brushed up against also seemed more pronounced. There were no significant differences in growth between treatments. Quality ratings were unaffected by irrigation level and were consistently very good throughout the summer, making this a great candidate for the low-water shade garden (Figures 11a-b). As with the straight species, this cultivar did not flower during the two years of the trial. The average height and width at the end of the trial was 28” x 70” (71.5 x 178 cm)"


2011-2013 Plant Trials  (link includes plants for both sun and shade)

Plants for Shade:
Berberis aquifolium ‘Compacta’ (Compact Oregon Grape)
 "The 40% treatment (irrigated twice during the summer) yielded consistently the highest quality ratings. Native species can be slow to establish."

Festuca californica (California Fescue)
"When visiting our trials field, Ellen Zagory, public horticulture director for the UCD Arboretum, remarked that our specimens were the best looking she had ever seen. Its first year in the ground it was the victim of some rabbit damage during the winter, but once the holes in the fence were patched up, most plants recovered well. This California native grass really performed beautifully in 50% shade, producing well-formed plants with good flowering and an attractive overall appearance even on the lowest irrigation treatment of 20% (no summer water). The 60 and 80% irrigation treatments (watering every 5-6 weeks and 2 weeks, respectively) unsurprisingly yielded the largest plants, with the 60% rate being favored somewhat throughout the season. However, if the overall appearance ratings are averaged for just the months of irrigation (rather than the entire year), the highest ratings are the 20 and 40% treatments! So, at least for this species, bigger is not necessarily better."

Neomarica caerulea (Walking Iris)
"The walking iris is a little grown plant, but with potential for dry shade gardens since its foliage is tall and striking throughout the year. The only notable difference in size between treatments was a slight advantage of the 80% over the 20% during the last two months of summer. The significance of the difference faded , however, with the first fall rain. As with the Festuca, the highest overall appearance ratings did not go to the largest plants but to the smallest on the 20% treatment. This is understandable when you take into consideration that taller more succulent leaves are prone to bending over in the breeze and creasing. The 80% treatment did have the highest flowering rating, but the flowers, though beautiful, are small, very fleeting, and not the main feature of this plant.
Both Ventura and Orange County recommended the iris for their area as a tall striking plant for dry shade."

Sollya heterophylla (Australian bluebell creeper)
"The Australian bluebell creeper turned out to be one of the favorites in our irrigation trials with its year-round fresh green foliage and dainty blue flowers in summer. There were no differences in growth attributable to irrigation levels, and the quality ratings were very close."

More plants for dry shade from the 2011-2013 trials: Ceratostigma plumbaginoides

Iris ‘Canyon Snow’
Dianella tasmanica
Cordyline ‘Festival Grass’
Cordyline ‘Purple'
Abelia ‘Sunshine Daydream’
Ligustrum sinense ‘Sunshine’


From the UC Davis Arboretum All-Stars Collection (
Very Low Water Use Plants for Shade 

(only two irrigations per summer)

Aristolochia californica - California pipevine
California native plant; leaves provide food for pipevine swallowtail butterfly larvae; versatile plant that can be used as a climbing vine or a groundcover.

Calycanthus occidentalis - western spice bush
California native plant; maroon-red flowers attract pollinating beetles; leaves have a sharp, clean fragrance and turn yellow in the autumn, adding seasonal color to the garden.

Cyclamen hederifolium - ivy leaf cyclamen
Scented rose-pink or white flowers bloom in late summer and early fall before the leaves emerge; ornamental silver-marked foliage sparkles in dry shady gardens; tolerates a wide variety of soil types and can also grow well in containers.

Cyrtomium falcatum - Japanese holly fern
Evergreen fern with dramatic, dark-green glossy fronds that resemble holly leaves; provides a lush look in dark shady areas of the garden; can tolerate high-mineral irrigation water.

Daphne odora ‘Aureomarginata’ - winter daphne
Shiny variegated leaves are attractive all year; requires little maintenance; intensely fragrant flowers perfume cool winter air.

Helleborus argutifolius
- Corsican hellebore
Long-lasting, pale-green flowers brighten the winter garden; needs little maintenance and tolerates dry shade; stiff, gray-green foliage adds sculptural interest to the garden year round.

Heuchera ‘Lillian’s Pink - Lillian’s pink coral bells
California native plant; bright pink flowers attract bees and hummingbirds; excellent groundcover for small shady areas or borders.

Heuchera ‘Rosada’ - rosada coral bells
California native plant; one of the best flowering perennials for dry shade; introduced to the nursery trade by the UC Davis Arboretum.

Heuchera maxima - island alumroot
California native plant; a good informal groundcover for dry shade; tolerates heavy clay soils; frilly green leaves look good all year.

Neomarica caerulea - walking iris
Accent plant with arching, sword-like leaves; produces clusters of gorgeous, intricately-patterned, violet-blue flowers; blooms repeatedly in partial shade during the hottest part of the summer.

Ribes viburnifolium
- evergreen currant
California native plant; good shade-tolerant groundcover under native oaks and in other dry, shady areas; shiny and fragrant foliage looks attractive all year; attracts hummingbirds and beneficial insects. 

1 comment:

  1. Very timely article! I was up today in San Andreas visiting the Zimmerman's Hilltop Market, stalking the hanging flower planters. As I was trying to decide what to get, I figured out that most needed 6+ hours of sun a day... and the spots where I had planned to put them, are, well - in shade now more than they used to be. This research will be a good help going forward!