Drought Tolerant Plants: Water Conservation Landscape at Idaho Botanical Garden

Demonstration gardens are one of the best places to learn about drought tolerant plants that are appropriate for your region. Such gardens not only help you decide which species you should plant, but also show you what the plants look like at maturity, what they are doing at any given time of year, and how to organize them (or how not to organize them, depending on the quality of the garden) in an aesthetically pleasing way. A couple of years ago, I explored the Water Efficient Garden at the Idaho State Capitol Building. This year I visited the Water Conservation Landscape at Idaho Botanical Garden in Boise, Idaho.

The Water Conservation Landscape is planted on a large L-shaped berm on the edge of Idaho Botanical Garden’s property. It is the first thing that visitors to the garden see, before they reach the parking area and the front gate. It is nearly a decade old, so the majority of the plants are well established and in their prime. Because the garden is so visible, year-round interest is important. This imperative has been achieved thanks to thoughtful plant selection and design.

This demonstration garden came about thanks to a partnership between Idaho Botanical Garden and several other organizations, including the water company, sprinkler supply companies, and a landscape designer. An interpretive sign is installed at one end of the garden describing the benefits of using regionally appropriate plants to create beautiful drought tolerant landscapes. If you ever find yourself in the Boise area, this is a garden well worth your visit. In the meantime, here are a few photos as it appeared in 2017.

February 2017

bluebeard (Caryopteris incana ‘Jason’) – February 2017

Sedum spurium ‘Dragon’s Blood – March 2017

winter heath (Erica x darleyensis ‘Kramer’s Red’) – March 2017

May 2017

avens (Geum x hybrida ‘Totally Tangerine’) – May 2017

July 2017

American cranberrybush (Viburnum opulus var. americanum ‘Wentworth’) – July 2017

Fremont’s evening primrose (Oenothera macrocarpa ssp. fremontii ‘Shimmer’) – July 2017

Fremont’s evening primrose (Oenothera macrocarpa ssp. fremontii ‘Shimmer’) – July 2017

August 2017

cheddar pink (Dianthus gratianopolitanus ‘Firewitch’) – August 2017

smoketree (Cotinus coggyria ‘Royal Purple’) – August 2017

gray lavender cotton (Santolina chamaecyparissus) – September 2017

showy stonecrop (Hylotelephium telephium ‘Matrona’) – September 2017

showy stonecrop (Hylotelephium telephium ‘Matrona’) – September 2017

Adam’s needle (Yucca filamentosa ‘Color Guard’) – October 2017

fragrant sumac (Rhus aromatica ‘Gro-Low’) – October 2017

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Drought Tolerant Plants: Pearly Everlasting

Despite being such a widely distributed and commonly occurring plant, Anaphalis margaritacea is, in many other ways, an uncommon species. Its native range spans North America from coast to coast, reaching up into Canada and down into parts of Mexico. It is found in nearly every state in the United States, and it even occurs throughout northeast Asia. Apart from that, it is cultivated in many other parts of the world and is “weedy” in Europe. Its cosmopolitan nature is due in part to its preference for sunny, dry, well-drained sites, making it a common inhabitant of open fields, roadsides, sandy dunes, rocky slopes, disturbed sites, and waste places.

Its common name, pearly everlasting, refers to its unique inflorescence. Clusters of small, rounded flower heads occur in a corymb. “Pearly” refers to the collection of white bracts, or involucre, that surround each flower head. Inside the bracts are groupings of yellow to brown disc florets. The florets are unisexual, which is unusual for plants in the aster family. Plants either produce all male flowers or all female flowers (although some female plants occasionally produce florets with male parts). Due to the persistent bracts, the inflorescences remain intact even after the plant has produced seed. This quality has made them a popular feature in floral arrangements and explains the other half of the common name, “everlasting.” In fact, even in full bloom, the inflorescences can have a dried look to them.

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Pearly everlasting grows from 1 to 3 feet tall. Flowers are borne on top of straight stems that are adorned with narrow, alternately arranged, lance-shaped leaves. Stems and leaves are gray-green to white. Stems and undersides of leaves are thickly covered in very small hairs. Apart from contributing to its drought tolerance, this woolly covering deters insects and other animals from consuming its foliage. In The Book of Field and Roadside, John Eastman writes, “Insect foliage feeders are not numerous on this plant, owing to its protective downy ‘gloss.’ … The plant’s defensive coat seems to prevent spittlebug feeding on stem and underleaves. The tomentum also discourages ant climbers and nectar robbers.”

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Not all insects are thwarted however, as Anaphalis is a host to the caterpillars of at least two species of painted lady butterflies (Vanessa virginiensis and V. cardui). Its flowers, which occur throughout the summer and into the fall. are visited by a spectrum of butterflies, moths, bees, and flies.

Because the plants produce either male or female flowers, cross-pollination between plants is necessary for seed development. However, plants also reproduce asexually via rhizomes. Extensive patches of pearly everlasting can be formed this way. Over time, sections of the clonal patch can become isolated from the mother plant, allowing the plant to expand its range even in times when pollinators are lacking.

The attractive foliage and unique flowers are reason enough to include this plant in your dry garden. The flowers have been said to look like eye balls, fried eggs, or even, as Eastman writes, “white nests with a central yellow clutch of eggs spilling out.” However you decide to describe it, this is a tough and beautiful plant deserving of a place in the landscape.

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Photos in this post are of Anaphalis margaritacea ‘Neuschnee’ and were taken at Idaho Botanical Garden in Boise, Idaho.

Drought Tolerant Plants: The Yarrows

Few plants are as ubiquitous and widespread as the common yarrow, Achillea millefolium. A suite of strategies have made this plant highly successful in a wide variety of habitats, and it is a paragon in terms of reproduction. Its unique look, simple beauty, and tolerance of tough spots have made it a staple in many gardens; however, its hardiness, profuseness, and bullish behavior have also earned it the title, “weed.” Excess water encourages this plant to spread, but in a dry garden it tends to stay put (or at least remain manageable), which is why it and several of its cousins are often included in or recommended for water efficient landscapes.

Achillea millefolium - common yarrow

Achillea millefolium – common yarrow

Common yarrow is in the aster family (Asteraceae) and is one of around 85 species in the genus Achillea. It is distributed throughout North America, Europe, and Asia. European plants have long been introduced to North America, and hybridization has occurred many times among the two genotypes.

Yarrow begins as a small rosette of very finely dissected leaves that are feathery or fern-like in appearance. These characteristic leaves explain its specific epithet, millefolium, and common names like thousand-leaf. Slightly hairy stems with alternately arranged leaves arise from the rosettes and are capped with a wide, flat-topped cluster of tightly-packed flowers. The flower stalks can be less than one foot to more than three feet tall. The flowers are tiny, numerous, and consist of both ray and disc florets. Flowers are usually white but sometimes pink.

The plants produce several hundred to several thousand seeds each. The seeds are enclosed in tiny achene-like fruits which are spread by wind and gravity. Yarrow also spreads and reproduces rhizomatously. Its roots are shallow but fibrous and abundant, and they easily spread horizontally through the soil. If moisture, sun, and space are available, yarrow will quickly expand its territory. Its extensive root system and highly divided leaves, which help reduce transpiration rates, are partly what gives yarrow the ability to tolerate dry conditions.

john eastman

Illustration of Achillea millifolium by Amelia Hansen from The Book of Field and Roadside by John Eastman, which has an excellent entry about yarrow.

Common yarrow has significant wildlife value. While its pungent leaves are generally avoided by most herbivorous insects, its flowers are rich in nectar and attract bees, butterflies, beetles, flies, and even mosquitoes. Various insects feed on the flowers, and other insects visit yarrow to feed on the insects that are feeding on the plant. Despite its bitterness, the foliage is browsed by a variety of birds, small mammals, and deer. Some birds use the foliage in constructing their nests. Humans have also used yarrow as a medicinal herb for thousands of years to treat a seemingly endless list of ailments.

Yarrow’s popularity as an ornamental plant has resulted in the development of numerous cultivars that have a variety of flower colors including shades of pink, red, purple, yellow, and gold. While Achillea millefolium may be the most widely available species in its genus, there are several other drought-tolerant yarrows that are also commercially available and worth considering for a dry garden.

Achillea filipendulina, fern-leaf yarrow, is native to central and southwest Asia. It forms large, dense clusters of yellow-gold flowers on stalks that reach four feet high. Its leaves are similar in appearance to A. millefolium. Various cultivars are available, most of which have flowers that are varying shades of yellow or gold.

Achillea alpina, Siberian yarrow, only gets about half as tall as A. filipendulina. It occurs in Siberia, parts of Russia, China, Japan, and several other Asian countries. It also occurs in Canada. Unlike most other species in the genus, its leaves have a glossy appearance and are thick and somewhat leathery. Its flowers are white to pale violet. A. alpina is synonymous with A. sibirica, and ‘Love Parade’ is a popular cultivar derived from the subspecies camschatica.

Achillea x lewisii ‘King Edward,’ a hybrid between A. tomentosa (woolly yarrow) and A. clavennae (silvery yarrow), stays below six inches tall and forms a dense mat of soft leaves that have a dull silver-gray-green appearance. Its compact clusters of flowers are pale yellow to cream colored. Cultivars of A. tomentosa are also available.

Achillea ptarmica, a European native with bright white flowers, and A. ageritafolia, a native of Greece and Bulgaria that is low growing with silvery foliage and abundant white flowers can also be found in the horticulture trade along with a handful of others. Whatever your preferences are, there is a yarrow out there for you. Invasiveness and potential for escape into natural areas should always be a concern when selecting plants for your garden, especially when considering a plant as robust and successful as yarrow. That in mind, yarrow should make a great addition to nearly any drought-tolerant, wildlife friendly garden.

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