Olneya tesota ‘AZT’, Desert Ironwood
Good things come to those who wait. It’s true that the Ironwood (Olneya tesota) is not the fastest growing species in the arid landscape pallet. Few trees better illustrate the unique combination of beauty and austerity that is the Sonoran desert. This tree is slow growing, has beautiful spring flowers, graceful, curved trunks and require special care and consideration when used in landscape setting.
Ironwoods take their name from the dense, dark and heavy wood the tree produces. The wood is prized for carving and as firewood. It is native to the southwestern US and northwestern Mexico. In native settings it is typically found on rocky well draining slopes and plains at elevations below 2500 feet. Foliage is gray green and semi-evergreen with leaves dropped in response to prolonged drought or freezing temperatures. Trees are hardy to 20 degrees but can sustain considerable foliage and small twig damage at 25 degrees. Immature trees can have fairly open canopies and generate limited amounts of shade. As trees mature, particularly in landscape settings, canopies become quite dense, generate ample shade and can limit growth and flowering of nearby understory plantings. As a rule, understory planting should be avoided as they can contribute to over-irrigation of Ironwoods. Mature trunks and branches are pale gray/white while immature wood is pale green and all branches are armed with many, sharp, curved thorns.
The natural growth habit of Ironwood is multiple trunks with branches extending to the ground forming a dome shaped canopy. Natural forms are excellent in plantings that transition landscapes back to undisturbed desert or when used as a security planting. To create a more cultivated appearance trees will require selective, regular pruning when younger. On existing specimens, such pruning should be done gradually over time as trees are vulnerable to sun injury when heavily pruned. Nursery grown Ironwoods typically require little if any pruning as they have been regularly shaped during the growing process. Trees produce clusters of small pink to purple flowers from April to June. Mature trees can grow to 30' by 40' but typically are 15' to 20' tall and as wide.
Ironwoods are extremely well adapted to the hottest environments in the Sonoran Desert. Sparing supplemental irrigations are essential for establishing transplanted Ironwoods in the landscape. Once established, irrigation schedules need to be carefully monitored in order to promote root development and growth while avoiding under or overwatering. Trees will not thrive in lawns or in heavy soils when mixed with understory landscape plantings that require regular winter irrigation or frequent summer irrigation. Like Palo Verdes no single factor contributes to the death of Ironwoods more than does over irrigation. When placing Ironwoods in established landscapes be mindful of casual water sources (rainfall, irrigation overspray, drainage from pedestrian areas) as these can unknowingly contribute to over irrigation that can prove fatal. In the landscape, Ironwoods are commonly used as a focal point specimen, at entry areas, in small groupings (to amplify spring flower displays) and to integrate landscaped areas back to the native desert.
Olneya tesota ‘AZT™’, ‘AZT™’ Desert Ironwood.
The characteristics that set Olneya tesota ‘AZT™’, ‘AZT™’ Desert Ironwood apart from seed selections are a faster growth rate, V-shaped, upright form and larger leaflets. This selection retains the graceful, chalk-gray trunks and branching that has made Desert Ironwood an iconic desert landscape tree. Olneya tesota ‘AZT™’ has exhibited greater cold hardiness than native population of this species and have survived temperatures of 17 degrees F. in AZT’s Demonstration Garden.
Olneya tesota ‘AZT’, Desert Ironwood PDF
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