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  This Issue:
    Calendar of Events
    Things to Expect & Do
    Trees for Special Areas
    A Date with History
    Annuals in the
    Buzz; A Book Review
    Velvet Mesquite
    Computer Corner
    Ask a Master Gardener
    How Herbicides Work
    The Unappreciated
          Smell of Rain
    East Valley Escape
    Word Wise
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The Master Gardener Journal

E A R T H - F R I E N D L Y   G A R D E N I N G

Trees for Special Sites
Reprinted with permission from "Desert Seasons", Mountain States Wholesale Nursery's Monthly Newsletter to the Trade; September/October 2004

Landscape architects and designers often face many challenges when it comes to choosing the right tree for the right place. So often they must perform this exercise each time they put pen to paper. To simplify the process, perhaps a checklist of environmental conditions could be useful. The list could include tree species by climatic zone, those that can sustain reflected sun or extreme shade, or perhaps poor soil conditions like alkalinity, salinity or inadequate drainage, and let us not forget windy sites. Each tree species must be checked against the list of conditions to determine if they will be satisfactory.

The check list could include specific design criteria, such as those species suitable for use in narrow locations, proximity to pools, height restrictions, use in containers or restricted root zones and seasonal color. Obviously, by inserting these considerations the focus narrows considerably. We thought it might be interesting to look at a few of these items from a design standpoint.

One of the most challenging design features today is ultimate size. With home lot sizes shrinking to postage stamp proportions, trees used 10 years ago might seem a bit large in proportion to these yards. Designers must challenge the plant palette to locate just the right specimen tree that does not become out of balance with the size of the landscape. Some designers have begun using plants once considered large shrubs. These plants are particularly effective as small stature trees especially if they can be found in relatively large containers.

From this perspective the list of suitable species is rather extensive. Of the lesser known plants, Acacia greggii (Catclaw Acacia), Chilopsis linearis (Desert Willow), Cordia boissieri (Texas Olive) and Vauquelinia californica (Arizona Rosewood) are just a few possibilities.

Some professionals might think this approach to design is a disgrace to the species, perhaps bordering on 'plant abuse.' Many arborists feel that a tree is not truly a tree unless it naturally reaches a height of 15 feet, without excessive training. But if you review the plant palette for a climatic zone, one may find shrubs of a large stature that will comfortably mature to the desired size, offering shade, screening and beauty.

Small trees might be convenient for use near overhead obstructions such as power lines or overhanging buildings. Occasionally there will be CC&R restrictions that mandate maximum height limitations to maintain unobstructed scenic views of perhaps a lake or the skyline of a valley. Such circumstances require that the designer take into account mature height and foliage density.

On the other end of the spectrum, small properties sometimes demand petite, non-invasive root systems. Plants are often subjected to such treatment in urban conditions. Talk about your plant abuse, this is the ultimate. Restricted root zones, often coupled with reflected heat and glaring sun from concrete driveways and patios, presents the most challenging design requirements. We must find plants that can withstand such harsh conditions. A short list might include Vitex agnuscastus (Chaste Tree) Caesalpinia cacalaco (Cascalote) and Bauhinia mexicana (Orchid Tree).

When looking for trees for use in limited root areas, think of soil type and drainage. Many plants found in nature survive in overly moist soils, heavy clay and poor water infiltration. Trees native to riparian areas or bottom lands might be the most successful for use in confined, shallow spaces with limited aeration and drainage. Consider trees such as Platanus wrightii (Arizona Sycamore), Platanus mexicana (Mexican Sycamore) and Salix gooddingii (Goodding's Willow) and Salix exigua (Coyote Willow).

Trees for use near swimming pools offer additional concern. It is incredible how often one hears that palm trees planted near a pool have caused enough damage to warrant removal. Most homeowners and some landscape designers do not realize the relative size of a mature palm tree root system, especially species within the genera Washingtonia and Phoenix. The narrow space between concrete block masonry walls and swimming pools is far too small in most situations for these potentially large plants. Concrete surfaces such as pool decks are no match for the incredible force exerted by the massive root systems. Choose trees carefully to match the site restrictions. Additionally, avoid trees with potentially invasive roots such as willows and sycamores near water features and septic tanks. And remember, large leaves and abundant leaf or flower litter can wreak havoc with pool filtration systems too. Some trees to consider near pools are Acacia aneura (Mulga), Acacia stenophylla (Shoestring Acacia), Dalbergia sissoo (Rosewood) or Pittosporum phillyraeoides (Willow Pittosporum).

While in the design mode, consider plants with interesting features, such as attractive bark, exquisite branching systems, or flowering habits. Many landscapers recognize these attributes and take advantage of them in the design process. For example, check out the zigzag branch patterns of Ebenopsis ebano (Texas Ebony) or the Zizyphus obtusifolia (Graythorn). Don't forget fragrance as a design element, with the sweet smell of grape bubblegum from Sophora secundiflora (Texas Mountain Laurel) and the vanilla scent of the Eysenhardtia orthocarpa (Kidneywood). Fall color is a prized design factor, for which we should consider the vivid reds and oranges of Pistacia chinensis (Chinese Pistache), many of the Oak species including Quercus buckleyi and Q. muhlenbergii. And spring or summer floral displays abound in nature with Acacia and Parkinsonia species as prime examples.

There are so many possibilities, just remember to keep everything in perspective. Be open minded, giving consideration to every plant on the palette. Weigh the list of desirable features with the design constraints, gradually reducing the list to match suitability. By finding the right associations, you are sure to find the perfect specialty tree.

Photos: Acacia greggii, and Cordia boissieri, Candice Sherrill; Bauhinia mexicana, Jo Cook; Ebenopsis ebano and Pistacia chinesis, Mountain States Wholesale Nursery.

BEST SITES FOR... Seasonal Wet Soil Windy Areas Reflected Heat Shady Spots Poor Soils Salt Tolerant Narrow Spaces Swimming Pools Seasonal Color Small Height Containers Ltd H2O
Acacia aneura (Mulga)     X     X X X   20'   X
Acacia berlandieri (Guajillo)                 X 10'-15' X X
Acacia farnesiana syn. smallii (Sweet Acacia) X   X X X X     X      
Acacia greggii (Catclaw Acacia)     X   X       X 15'   X
Acacia occidentalis (Sonoran Catclaw)   X X     X     X 25'   X
Acacia rigidula (Blackbrush Acacia)     X       X   X 10'-15' X  
Acacia salicina (Willow Acacia)     X         X        
Acacia saligna (Orange Wattle)     X   X       X 15'-35' X  
Acacia schaffneri (Twisted Acacia)     X   X         20'   X
Acacia stenophylla (Shoestring Acacia)     X     X X X        
Acacia willardiana (Palo Blanco)     X   X   X         X
Bauhinia mexicana (Orchid Tree)     X       X   X 10'-18' X  
Caesalpinia cacalaco (Cascalote)     X       X   X 15' X  
Celtis laevigata v. reticulate (Canyon Hackberry) X   X X X X            
Celtis occidentalis (Hackberry) X X                    
Cercis canadensis v. mexicana (Redbud)       X     X   X 15'-25' X  
Cercocarpus ledifolius (Curl Leaf Mtn. Mahogany)   X     X   X     25' X  
Chilopsis linearis (Desert Willow) X X X   X X     X 25'-30' X  
Cordia boissieri (Texas Olive)     X       X   X 10'-25' X  
Dalbergia sissoo (Rosewood) X   X     X   X        
Ebenopsis ebano (Texas Ebony)     X   X              
Eucalyptus microtheca (Coolibah) X X X   X X            
Eucalyptus salubris (Gimlet)     X   Clay              
Eucalyptus sargentii (Salt River Mallet)     X   X X            
Eysenhardtia orthocarpa (Kidneywood)   X     X   X   X 10' X X
Fraxinus greggii (Littleleaf Ash) X X   X X   X X   12' X  
Fraxinus velutina (Arizona Ash)         X X            
Havardia mexicana (Mexican Ebony)   X X                 X
Havardia pallens (Tenaza)     X X               X
Juglans major (Arizona Walnut) X X                    
Leucaena retusa (Golden Leadball Tree)             X   X 15'-25' X  
Olneya tesota (Ironwood)     X   X       X     X
Parkinsonia florida (Blue Palo Verde)     X   X X     X     X
Parkinsonia micophylla (Foothill Palo Verde)     X   X X     X     X
Parkinsonia praecox (Sonoran Palo Verde)     X   X X     X      
Parkinsonia hybrid ŒDesert Museum'     X   X X     X      
Pistacia chinesis (Chinese Pistache)   X             X      
Pistacia x ŒRed Push' (Red Push Pistache)   X             X      
Pittosporum phillyraeoides (Willow Pittosporum)   X X       X X   20'-25'    
Platanus mexicana (Mexican Sycamore) X       X       X      
Platanus wrightii (Arizona Sycamore) X       X       X      
Prosopis Phoenix Thornless Mesquite X   X   X X            
Prosopis glandulosa (Texas Honey Mesquite) X X X   X X            
Prosopis juliflora (Arizona Honey Mesquite) X X X   X X           X
Prosopis pubescens (Screwbean Mesquite, Tornillo)   X X   X X           X
Quercus buckleyi (Texas Red Oak)           X     X      
Quercus fusiformis (Escarpment Oak)   X X   X X            
Quercus muhlenbergii (Chinquapin Oak)     X   X       X      
Quercus polymorpha (Monterrey Oak)         X       X      
Rhus virens v. choriophylla (Evergreen Sumac)     X       X   X 15'-18'    
Salix exigua (Coyote Willow)         X X            
Salix gooddingii (Goodding's Willow) X       Clay         20'-30'    
Salix taxifolia (Yewleaf Willow) X X     X X X       X  
Sambucus nigra ssp. cerulea (Elderberry) X       X       X 15'-30'    
Sophora secundiflora (Texas Mountain laurel)     X X X X X X X 15'-20' X  
Ungnadia speciosa (Mexican Buckeye)         X       X 12'-25' X  
Vauquelinia sp. (Rosewood)     X   X   X   X 10'-20' X X
Vitex agnus-castus (Monk's Pepper Tree)     X   X X     X 15'-25' X  
Zizyphus obtusifolia (Graythorn)   X X   X X X          

Maricopa County Master Gardener Volunteer Information
Last Updated November 21, 2004
Author: Lucy K. Bradley, Extension Agent Urban Horticulture, University of Arizona Cooperative Extension, Maricopa County
© 1997 The University of Arizona, College of Agriculture and Life Sciences, Cooperative Extension in Maricopa County
Comments to 4341 E. Broadway Road, Phoenix, AZ 85040,
Voice: (602) 470-8086 ext. 301, Fax (602) 470-8092