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|  Urban Forestry Program
Descriptions of Commonly Distributed Trees
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 Desert Willow - Technically not a willow tree but branches hang down somewhat giving it a willow-like appearance. Grows to 20 ft. tall by 20 ft. wide with optimal watering and soil conditions. Desert willows grow naturally in the Tucson region along desert washes. They are considered a moderate water use tree compared to mesquite or palo verde which are considered low water users. Supplemental watering required for desert willows throughout their life span is needed for them to florish and provide significant shade. Absent good, regular watering especially in the dry early summer and fall seasons desert willows will not develop a large shade tree in most cases. As is the case with all desert trees, soil type and drainage have a bearing on the tree's development as well.
They don't get as large as mesquite trees nor do they make as dense of shade cover. However, they are thornless, have pink flowers through the spring-summer-fall, produce low amounts of pollen and drop their leaves over the winter allowing sunlight in to reduce winter heating expenses.
Flower color varies from very light pink to burgundy. Long thin seed pods hang on the tree for a long time. The amount of seeds varies from tree to tree. Some cultivars or selections grown by different nurseries have fewer seeds and don't produce as much litter. Some trees tend to have more robust (wider) leaves than others. More frequent watering and fertilizing can increase leaf size as well. Trees for Tucson attempts to select desert willows for the homeowner shade tree program which have broader leaves and medium pink flowers. Leaf cutter ants, and to a much lesser degree, caterpillars, are the only two insects that cause problems for desert willows. Treat ants with Amdro or other insecticide othewise they will completely defoliate young trees. Remove caterpillars by hand or let them develop into butterflies. (Of course they may well have eaten many leaves by that time but the leaves will grow back.)
 Mesquite - There are several types of mesquite trees growing in the Tucson area. The most common is often referred to as Chilean Mesquite. This is the type of mesquite offered through the Trees for Tucson shade tree program because of its superior shading quality as driving through any part of Tucson will attest. If you prefer a native velvet mesquite please note so on the application. They are often in short supply and/or not the best quality but we will be provide them when good specimens are available. Chilean Mesquites tend to make the best shade trees with a wide dense canopy (fast growth to 25 ft. tall by 25 ft. wide). However, if they are overwatered and/or improperly staked and pruned they may grow too fast, become top-heavy and blow over. They may or may not have thorns but if they do, they tend to be small compared to the Argentine Mesquite which has very long thorns. The true Argentinian mesquite is less vulnerable to wind damage. It is generally the fastest growing mesquite but has the largest thorns. It's semi-deciduous; losing its leaves for a few weeks at most. Winter heat gain through sunlight is very limited.
Both of these types of mesquite generally have dark bark versus the more shaggy brown bark of the Velvet Mesquite which grows naturally in the Sonoran Desert and is also planted as a landscape tree. Although their shade is not as dense as the Chilean Mesquite and they don't grow as large, they require less maintenance since they grow slower and don't tend to outgrow their root system and suffer wind damage. Lastly, there are Texas Honey Mesquites which have drooping branches and long thorns though few in number. It is not as readily available from nurseries as other mesquites but is a fast growing good shade tree. It is truly deciduous in that it drops all its leaves in winter so you get the benefit of winter sunlight. It does suffer from branches splitting and breaking if overwatering results in too much heavy new growth. All mesquite trees produce seed pods to varying degrees. Shade tree applicants can note on the tree apllication any specific type of mesquite they would prefer.
Blue Palo Verde - A moderate to fast growing (with regular watering) desert looking tree that grows naturally in areas with good soils in the Sonoran Desert. Very low water required once it is well-established but regular watering will increase its shading and energy saving effectiveness.
Probably the second most common of the large desert type shade tree in the Tucson urban area after the Chilean (same as hybrid, thornless) mesquite. Beautiful display of yellow flowers in spring. It has small but sharp thorns. Requires some selective pruning but much less than mesquites. Less likely to suffer wind damage unless overwatered such as is the case with mesquites. Of the complaints Trees for Tucson has received about allergies Blue Palo Verde accounts for at least three-fourths of them. Not recommended for anyone with allergies or asthma. Also considerable flower and seed pod litter although leaf litter doesn't amount to much. Dense branching still provides shade in winter when sunlight heating is desirable. Best to plant on north portion of the west or east sides of residences when the angle of winter sunlight won't have the trees shading the house.
Foothills palo verde is much slower growing and will not effectively shade building.
Sonoran palo Verde can be an excellent shade tree but needs adequate space for its very wide canopy. Unfortunately, it starts out in a five gallon container looking more like a shrub than a tree but grows rapidly with regular watering. Native to northern Sonoran and further south. Has long thorns, abundant yellow flowers in the spring and develops an attractive smooth pastel green bark after a couple years. Occassionally available in five gallon containers.
Hybrid Palo Verde - sold by various names such as Sonoran Emerald or Desert Museum Palo Verde. Fast growth rate, no thorns, attractive yellow flowers though not as numerous as on blue palo verde trees. Highly sought after in the nursery trade and consequently not available in five gallon containers but occassionally in fifteen gallon size for a premium price. Its longevitiy still a question mark since it has only been widely used in the last 5-10 years at most.
 Willow Acacia - A fast growing evergreen tree from Australia that does very well in Tucson. Technically not a willow tree but branches hang down like one. Tends to grow taller than wide; up to 25-30 ft. tall and 20 ft. wide. Willow acacias have narrow grey-green leaves and stays green year round but still have leaf litter especially in drier parts of the year. Leaves and outer branches of young trees may be damaged by freezing temperatures (28-32 degrees) but will grow new leaves in the spring. It is best to cover young plants if possible especially if colder than 30 degrees. Larger established trees may drop leaves from these cold temperatures but will get new growth in spring. Relatively low maintenance but branches will split if overwatering generates too much heavy new growth. Over-watering combined with poor-draining soils can result in root rot. Willow acacia is used in many new developments. Cream-colored flowers not showy but somewhat fragrant as they become more numerous. Better suited for narrow planting spaces as compared to mesquites.
Where to see Willow Acacia:
East side: Southeast corner of Broadway and Kolb, planted in 2004 in parking lot between Starbucks and Ross
Northwest side: Southwest corner of Ina and Oracle in front of Einstein's Bagels along Ina
South side: Two are growing in the median directly in front of the public library at 202 W. Valencia
Central: Along west edge of Campbell Ave. next to sidewalk between 1st and 2nd St. just south of Speedway
 Additional trees are available by special request if you just write them in on the application. These include: Blue Palo Verde, Desert Ironwood (pictured to the left), Texas Honey Mesquite
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