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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. Without regular watering throughout their lifespan desert willows may not get very large nor will they develop a shady canopy. As is the case with all desert trees, soil type and drainage have a bearing on the tree's development as well. Desert Willows do not 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. Another option to control ants is to make a square trough around the trunk with 1 1/2 to 2 inch PVC pipe cut lengthwise and glued together at the corners to create a moat. Fill the moat with vegetable oil, which ants will not cross, unlike water. For caterpillars, remove 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 application 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 where the lower angle of winter sunlight won't shade the house.
Foothills palo verde is much slower growing and will take a long time before it shades buildings, assuming it even gets tall enough.
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
 Red Push Pistache - Slow to moderate growth rate to 18 feet tall and 20 feet wide. Attractive fall color (see photograph to right). Drops leaves for 2-3 months. Requires moderate watering to thrive but otherwise a very low maintenance tree. Red push pistache trees are growing along Tyndall Ave. just north of 6th St. Also look for them at the intersection of I-10 frontage roads and Miracle Mile, and growing along the south side of the parking lot on the south side of St. Elizabeth Ann Seton Church at 8650 N. Shannon Rd.
Desert Ironwood - Pictured, left. Long-lived, drought-tolerant trees that can get good-sized but may take twenty years to do so. Plant as a 15 gallon or 24 inch boxed specimen to speed things up.
NEWLY PLANTED TREE WATERING REQUIREMENTS
In general, water newly planted five and fifteen-gallon sized trees everyday for the first week after planting when daytime temperatures exceed 90 degrees (late April to early October). After the first week you may be able to reduce watering to every second or third day. Watch for drooping leaves or leaves that lose color, or worse yet dry leaves. This indicates lack of water.
Keep in mind that watering requirements depend on soil conditions (does water drain rapidly or does the ground stay wet for a longer period), grade of planting site (does water run off away from the tree, or does it soak into the soil near the tree), amount of sunlight and reflective heat (is the tree planted in full sun, or where it is in the shade for a portion of the day), type of tree (mesquite and palo verde are the lowest water users but still need frequent watering initially to develop roots to store sufficient water) and time of year the tree was planted.
TREE PLANTING AND CARE INFORMATION
(detailed information provided when trees are delivered)
Basically, plant trees as soon as possible once they are delivered, and don’t let the soil dry out during the warm months!
- Holes do not have to be dug before trees are delivered; trees can stay in the container for a week or two without a problem but the longer they stay unplanted the more likely they are to dry out.
- Winter is a good time to plant trees in Tucson. The only time to avoid planting trees is during the hottest and driest (May into July) and even then it is okay if the trees receive frequent watering (daily initially). The only tree that may suffer frost damage is willow acacia especially in cold air drainages along desert washes and rivers. It will come out of it with new spring growth unless it is exceptionally cold (28 degrees or lower) in which case you should cover with cloth for the first winter during sub-freezing nights. Frost will cause mesquite and desert willow to lose their leaves which is normal.
- Watering needs vary depending on season, soil conditions and type of tree. While the tree is in the container, water every day when above 80 degrees and every other day or every few days if it is in the container during cooler months. Basically, water enough so that the soil doesn’t dry out while it is still in the pot.
- After planting water the tree every day for the first week especially if the temperature is above 80 degrees. Make sure the water is soaking into the root ball and not just running off. If watering by hand, build a small tree well three feet wide that allows 5-10 gallons of water to soak in. If it is on a drip irrigation system start out with two drip emitters that put out a couple gallons an hour and run for an hour making sure they are close enough to the trunk to adequately soak the root ball. Initially, the water only needs to soak down a couple feet deep.
After the first week or so you can start to water less but watch to see if the leaves droop or they loose their color or, worse yet, dry out. If so, increase the frequency of watering. In winter months much less watering is needed especially when leaves have dropped. Just don’t let the soil get dry but at the same time is shouldn’t be kept constantly wet or soaked. Just make sure it is damp an inch or two below the surface during cooler months when water needs are minimal.
Keep in mind that even though mesquite and palo verde trees are desert trees they still need frequent watering until their root systems expand and can store more water. Desert willows, willow acacias and red push pistache will need more frequent watering than mesquites and palo verdes.
Research has shown that in most cases, the root development on trees started in containers tends to be closer to the surface. They will likely develop some anchor roots but usually not deeper tap roots unlike trees growing naturally from seed along desert washes that seek out saturated deep soils by developing tap roots.
IMPORTANT TREE PLANTING STEPS:
1) For five gallon-sized plants dig a one foot deep and one foot wide hole and fill with water. If it doesn’t drain in several hours or if you hit caliche (hard, almost cement like material) when digging you may have to dig deeper or plant elsewhere to ensure adequate soil drainage.
2) Carefully cut the bottom off of the container and then cut down one side but leave a little of the plastic attached in the middle of the side to hold the rootball together until you get it in the hole right where you want it. DO NOT pull the tree out or roll the container on its sides to loosen it. Now hold the bottom of the rootball and place the container in the hole. Carefully finish cutting the side of the plastic container and remove it. This approach will ensure minimal disturbance to the root ball. Trees, palo verde in particular, often die if the root ball is damaged. If the root ball was disturbed during planting the tree may lose its leaves. Keep watering daily and the tree will grow new leaves in most cases.
3) It is not necessary to amend the soil with mulch but replace any rocks and chunks of caliche with good topsoil from elsewhere in your yard. If you do use mulch don’t use more than one part mulch to three parts original soil. Put 5-6 inches of mulch or compost on the surface to prevent soil from drying out. You can also put rocks in a basin around the trunk to keep the soil from drying out. Don’t put soil on the top of the root ball or pile it up on the trunk higher than it was when it was in the container. This may rot the bark and kill the tree. Light mulch and rocks allow air to the surface but block the sun and prevent the soil from drying out. Do not compact the soil. Let the water settle it.
4) Collecting rain run-off from your roof or elsewhere in your yard into a one-half foot deep basin four feet or more in diameter around your tree should enhance it growth. Remember though that relying entirely on rainwater harvesting is not sufficient in most cases to ensure tree health especially if you want the trees to attain a significant canopy to shade your house in a relatively short time. While mesquite and blue palo verde trees are low water use they have a small water storing capacity when planted from a five gallon container. They will need frequent watering to thrive until they develop an extensive root system that can store more water. This is even more important for other trees such as desert willow, willow acacia and red push pistache. How long before they develop a large enough root system to only require supplemental watering in droughts (no rain for several weeks or more) depends on soil conditions and reflective heat from buildings and surface materials.
HOW TO PREVENT AND CORRECT WIND DAMAGE TO YOUR TREES
Trees with a lot of new growth in the spring and summer have increased weight and wind resistance. Add moist loose soil and weak trunk and limbs from improper staking and they may not stay standing in monsoon winds.
WHAT TO DO
Don't hesitate to radically prune the tree if it has a lot new growth especially if it is a young mesquite or palo verde as they are the most prone to blow over or have limbs break. Generally, if the tree is more than 5-6 years old it is much less likey to blow over. It may sustain broken individual limbs. Continue to thin trees gradually as they grow.
Avoid just cutting the top branches of the tree. (NEVER top trees unless they are growing into powerlines). This will just result in stunted growth or a misshapen tree. Try to leave some main upright branches by thinning back lateral branches to these main branches.
Once the tree has had its weight and wind resistance significantly reduced you may have to stake the lower part of the trunk to allow roots to repair and strengthen.
Always allow desert trees such as mesquite, desert willow and palo verde to develop at least 2-3 main trunk branches. (Never trim them up to a straight tall trunk which will create later problems with wind damage). Multiple trunks and low branching is their natural and more stable growth form. They will be much less likely to blow over and they will still attain adequate height.
WHAT TO DO IF A TREE BLEW OVER
Don't overreact and cut the tree to the ground. If the tree has partially blown over it may still be salvagable. So in some cases it may be advisable to leave the tree as it is even if it is at an angle and not otherwise blocking a roadway, sidewalks or causing damage to a structure. If it is stable in this position the roots will likely repair, otherwise stake the tree within three feet of the ground. You will likely need to trim off a significant amount of the canopy that is leaning on or near the ground. The remainder of the tree will naturally grow upward and in some cases may acheive its former size very rapidly depending on the size of its root system.
There are trees throughout Tucson that have been allowed to grow in leaning positions and eventually become large strong trees. A good example is two large mesquites along the south side of the street at about 6400-6500 East Grant Rd.
NATIVE VS. NON-NATIVE DESERT ADAPTED TREES
There is ongoing discussion about the benefits of some trees over others: Whether to plant native trees (those that grow naturally in the Sonoran Desert around Tucson) or non-native trees (introduced from other areas of the U.S. or the world) is a highly subjective issue based on personal preference. Trees for Tucson offers many trees, both those that grow here naturally and ones which are non-native, based on various criteria:
- Water Use: If you want to reduce water use there are both low water use native trees (velvet mesquite, blue palo verde) and non-native trees (Chilean mesquite, Texas Ebony) that do well in Tucson.
- Wildlife Use: If you want to attract wildlife then your best bet is to plant a diversity of trees, shrubs and groundcover that provide two necessary components for animals: food (including insects) and cover. Wildlife will utilize food and cover regardless of whether it is a native or non-native plant, so you can use both. The key is to avoid non-native plants that are categorized as highly invasive or noxious. Non-native birds such as pigeons are attracted to palm trees so avoid those unless you and your neighbors like pigeons, lots of them! Taller growing introduced trees such as eucalyptus and pines attract owls and raptors such as red-tailed hawks, Harris’ hawks and Cooper’s hawks that otherwise wouldn’t be nearly as common in the Tucson area since there are few naturally occurring tall trees. Those tall trees that are native (mostly cottonwood) require huge amounts of water and sandy soils, so are naturally limited to a few scattered areas along the Tanque Verde Wash and the Rillito River.
- Shading buildings: The objective of the Trees for Tucson shade tree program is to reduce energy use by shading residences. The most effective way to accomplish that objective is to provide fast-growing shade trees that also meet residents' needs. The most popular and most effective shading tree is the Chilean Mesquite which are often thornless. The lack of spines (most commonly referred to as thorns), fast growth, and attractive shady umbrella shape account for its popularity. While the Chilean mesquites require more maintenance than the native velvet mesquite and are more vulnerable to wind damage, they grow more rapidly and have a denser and greener canopy. They are offered through Trees for Tucson whenever we are able to get them from tree growers. Because of their popularity they are not always readily available.
- Thorns: Many residents prefer to avoid thorny plants because of the annoyance of thorns when trimming them and concerns about the dangers the thorns pose to children and pets. This is one of the main reasons native trees such as velvet mesquite or blue palo verde aren't always an appropriate tree from a homeowner’s perspective.
- Narrow Planting Spaces: Many new home developments have narrow yards. There are no low or moderate water use native trees that match that spacing requirement. Willow acacia, a non-native landscape tree that is increasingly popular in Tucson, tends to have a narrow growth form (about 15 feet wide by 25 feet tall when full grown) that is well-suited for these yards. In addition, with many two story homes now being built a tall tree like the willow acacia can shade those second floors which native trees can’t because they simply don’t grow tall enough.
- Pollen and Allergies: Desert willow is offered by Trees for Tucson as part of its shade tree program for residences. It takes more water than others to develop a dense canopy and even then it may still not be as effective at shading as the Chilean mesquite. However it is a low pollen tree and has attractive pink flowers. It also drops its leaves for about 3-4 months and lets the warming winter sunlight in to offset winter heating costs. Other trees such as mesquite (both native and Chilean) are deciduous or partially deciduous but for a shorter period.
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