Dry Riparian Communities


dry wash
     A riparian zone is an area along banks of rivers, shoreline, marshes, and lakes where distinctive plants and animals live.  The word riparian is derived from the Latin ripa, meaning river bank.  Desert ecologists broaden the concept of riparian zones to include banks of dry washes in the desert calling them "dry" riparian habitats, as opposed to "wet" riparian habitats.

     In my area of the Sonoran desert, these dry washes occupy less than five percent of the area, but they are the most productive ecosystems in the desert.

Abert's Towhee
      Riparian vegetation provides food for a variety of animals, and provides refuge from heat and drought, especially for animals crossing the barren stretches of the surrounding desert.  The Sonoran desert dry washes support ninety percent of the desert's bird life, including not only birds that live there, such as Abert's Towhee, and Least Bell's Vireo, but also migrating bird populations.

Desert Willow tree

     The washes also support a woodland of  Desert Willow, Ironwood, and Cottonwood trees, as well as animals such as the Gray fox, Harris' Antelope squirrel, and Whitethroat woodrat.

Gray fox

     This concentration of plants and animals, that would otherwise not occur in the area, is the result of the availability of water, even though the wash may carry water for only a few hours or days a year.

     Unfortunately,  ninety percent of Arizona's riparian areas, the richest arid landscapes in the desert,  have been lost in the last century because of urban and rural development, crop production, mining, air and water pollution, and droughts.

New Additions


Snow White tea tree
      I recently bought a few plants for the yard to replace plants that were killed in the freeze we had in December.  These plants survive to 20 F so they should survive here with no problem. 

     I have two Snow White tea tree plants and two Jubilee New Zealand tea tree plants (Leptospermum scoparium).  They are native to New Zealand and southeast Australia. They like dry, low-nutrient soils, so they are perfect for this area.  These are related to, but not the same as, the Melaleuca alternifolia tree which is also commonly called tea tree and from which tea tree oil is produced.

     Leptospermum is a shrub or small tree that can grow eight to ten feet tall and six feet wide.  Mine are small, about one and a half to two feet tall.  They are evergreen, with dense branching and small leaves.   The flowers on the Jubilee tea tree are called scarlet, but they look like a deep pink.

Jubilee tea tree

     The common name for these plants is Manuka from the Maori word manuka.  Manuka honey from their flowers is darker and richer in taste than clover honey and has antibacterial and antifungal properties.  The wood on the plants is tough and hard and is often used for tool handles.

Jubilee tea tree flowers

     Wilipedia says the name tea tree arose because Captain Cook used the leaves from both Melaluca and Leptospermum to make a tea drink.

Cape honeysuckle

     I also planted four Cape Honeysuckle (Tecoma capensis) plants which I have never grown before.  I have read that these plants originated in from the Cape of Good Hope region of South Africa.  It is the only idigenous South African species out of twelve species of the genus Tecoma.  The rest of the genus originates from Arizona to South America.  It is also now cultivated in the tropics and subtropics.  It has flowers that are similar in shape to a honeysuckle, but it is not a honeysuckle and is not fragrant.

     My Cape Honeysuckle has vivid orange flowers, although there are varieties with yellow, scarlet or apricot flowers.  It is evergreen in this area.  Since it is a fast grower,  it can be invasive.  I have read that it can be grown as a barrier hedge, a climbing vine or a ground cover for steep slopes and rocky banks.

The Sonoran Desert

Saguaro cactus
     North America has four deserts:  Great Basin, Mohave, Chihuahuan and Sonoran.  I live in the Sonoran Desert that runs from the state of Sonora, Mexico up through southern Arizona to the Phoenix area, and west to sourtheastern California through Baja California.  The Sonoran Desert covers 100,000 (260,00 sq km).

     The Sonoran Desert differs from the three other North American deserts in that it has mild winters.  Frosts are few or rare in some parts of the Sonoran Desert.  My area had six nights of  below freezing weather last winter, but we have had only two nights below freezing so far this year.  Another defining characteristic of the Sonoran Desert is the bi-seasonal rainfall pattern.  The area gets gentle rains from December to March from storms originating in the North Pacific.  From July to mid-September, the summer monsoon brings violent thunderstorms and localized deluges.

Organ Pipe cactus
         There are six subdivisions of the Sonoran Desert based on distinctive vegetation.  I live in the hottest and driest subdivision called the Lower Colorado River Valley area (although the Colorado River is 180 miles from where I live).  Summer highs can exceed 120 F (49 C), and surface temperatures can near 180 F (82 C) from the intense solar radiation from cloudless skies.  My area of the Sonoran Desert gets the least amount of rain, from three to eight inches a year.  Last year we had four inches of rain.  The terrain consists mostly of broad, flat valleys with widely-scattered, small mountain ranges of almost barren rock.  There are many areas of loose sand.

Velvet Mesquite tree

     I have read that this desert supports a surprising 2,000 species of plants.  Two species distinguish the Sonoran Desert from other North American deserts:  legume tress and columnar cactus.  Examples of columnar cactus include the Organ Pipe cactus and the Saguaro cactus shown above.  The Sonoran Desert is the only place in the world where the Saguaro cactus grows in the wild.  Legume trees that grow here include the mesquite and acacia.  Trees grow only along the larger dry washes, even though the wash may carry water for only a few hours or days a year.

Creosote bush


White Bursage bush

     The valleys are dominated by low shrubs, primarily Creosote bush and White Bursage.  These are the two most drought-tolerant plants in North America.

Desert Globemallow
Indian Paintbrush
      We do have some spring wildflowers, such as Desert Globemallow and Indian Paintbrush.  Though desert plants must cope with scarce water, it is a misconception that they struggle to survive.  The native species are adapted to and thrive under desert conditions.

What Do We Mean By "Native"?

Palo Verde tree, native to southern Arizona
     I don't know any gardeners that have only native plants in their yards or gardens, although there are gardeners probably striving to grow only natives.  But defining what we mean by native is challenging.

     Many gardeners accept only indigenous plants as native.  Gray's Anatomy of Botany says that to be indigenous there must be scientific evidence that the species has inhabited an area for a great length of time, that it's an integral part of local, evolutionary relationships with other plants and animals.

Saguaro cactus, native to southern Arizona
     But what is a great length of time?  Thousands of years? Hundreds of years?  For example, there were plants that were present in an area of the globe before a glacial period, different plants that were present during a glacial period and different plants after a glacial period.  Can all be considered native?  When European colonists arrived in the Americas hundreds of years ago and introduced species that live on unassisted today, are these species considered native?  If a plant has  become naturalized as a member of the wild flora, needing no human assistance in self-perpetuating, can that plant then be called native?  What of an indigenous plant that migrates to a new area and becomes naturalized without assistance?  Is native a function of time and place?

Lantana, not native to southern Arizona
      Situations like this occur all over the world.  If native status is a function not only of time but also of place, defining place can be a perplexing.  There can be dramatic differences in habitat within a relatively small area.  In my own state of Arizona, within a twenty mile distance, you can go from desert habitat to tundra.  And the plants in the tundra habitat in Arizona are the same as the plants in the tundra of Alaska.

     And consider what we call invasive plants.  They would not exist unless they were better adapted to current conditions than so-called native species.  As Richard Darke says in the book The New American Landscape, " ...most natives are no more than earlier arrivals that established themselves because, at the time, they had a competitive advantage."

gazania, not native to southern Arizona

     Non-native plants fulfill roles that can't be accomplished by natives.  Many food plants grown in North American gardens do not have their origins here.  Many flowers that gardeners grow are not indigenous to their area, but provide beauty, fragrance, color and pleasure, such as the lantana and gazania pictured above that I grow in my yard.. 

     Most gardeners realize that no garden or landscape remains static, that local and global ecologies change.  In the end, I think most gardeners strive for balance among all their plants.