Like any assembly line there are an array of steps involved in the process of producing trees and quality control measures are applied at each step. "Raw materials" like seeds or mother stock (for clonally produced trees), mulch and soil mixes, containers, fertilizer and watering schedules must be carefully selected with the market ready tree in mind. Horticultural practices, pruning, staking, transplanting (bumping from one container size to the next largest) and irrigation must be individually tailored for each specie grown.

Propagation Stock: Quality trees begin with well-selected and screened propagation stock. Both seed and vegetative (cloning) methods are used in the production of desert trees species. At AZT seed propagation has two immediate quality checks: 1) the seed source (tree(s)) must produce seeds that retain the qualities of the source tree (produce "true to type") and 2) the structure, form, rooting and other horticultural characteristics are highly uniform and consistent with our standards. AZT collects seeds from trees that we maintain in the nursery that have demonstrated a high level of uniformity when grown from seed. Seeds are initially grown in liners and, as they grow, are transplanted into progressively larger containers until they are ready to be moved to our saleable inventory. Because of the high potential genetic variability inherent in seed propagated trees they are evaluated at each stage of production and about 20% are discarded. These inspections are done to eliminate or reduce the likelihood of trees lacking the qualities required by AZT to be introduced into our saleable inventory.

Trees that will serve as "Mother" trees for our clonal propagation are evaluated for several years in our demonstration garden to assess horticultural qualities like form, growth rate, response to pruning, cold hardiness, root growth in proportion to canopy growth and water requirements. Where appropriate, flower color, size, abundance and seasonality are also considered. New trees are added to the "Mother" block as they are identified, offering landscape design professionals an ever-expanding number of varieties to choose from. Like seedlings, clone trees start in the greenhouse, remaining there until rooted and are then hardened off and repeatedly transplanted into increasing larger containers as they grow and mature. While cloning trees addresses the desire for genetic uniformity they are continually evaluated for imperfections in form and structure at each transplanting.

Transplanting or Bumping: Bumping is a term commonly used in the nursery trade to describe the practice of moving plants that have reached their optimal growth in one size container and transplanting them into a larger container. Plants left too long in small containers can become overgrown and develop circling or bound roots. Root binding is a horticultural time bomb that if allowed to develop when trees are young will severely shorten the life of maturing trees by causing crown girdling or by significantly increasing the risk of wind throw. Quality control at this level of production involve taking steps to insure that the root system is vigorous, appropriate to the size and stage of development of the tree, and that roots are well distributed without being overgrown.

Each time trees are bumped the root mass is inspected with an eye for a well distributed root mass and a fine net of lateral roots. Smaller containers (2, 5 & 15 gallon) are painted on the inside with a copper based product called "Spin Out" that chemically prunes root tips that come in contact with the sides of the can. This pruning process prevents circling roots and encourages the development of a more dispersed root system. Prior to bumping, the root mass of all trees is examined and light vertical cuts are made through the outer edge of the root-ball to further encourage lateral root growth and prevent circling. Because this practice is so critical to the development of a strong, healthy and well distributed root system it is followed when bumping all sizes of trees (liners to 54" boxes).

The planting soil mix is adjusted to meet the needs of different species keeping a homogeneous blended soil at all stages of development. This eliminates any interfacing from different soil profiles for a healthy and rapid developing root structure when transplanted into new containers. Early in the propagation process (greenhouse) soil mixes are lighter and soil-less. With most species, as trees are planted into larger containers (5 gal. and 15 gal.) and boxes an industry standard ratio of native and organic material is used and for the few very drought tolerant species pumice is added to increase the rate of water percolation. We do not recommend the addition of organic amendments to the backfill soil when transplanting desert species. Consequently we grow our trees in a soil mix of native soil and mulch that promotes strong root develop in the nursery and that encourages rapid rooting and aid in water penetration when trees are transplanted (Arid Zone Times April 2002, "Tree Planting Practices an Overview").

Pruning, Shaping and Staking: The use of cloning in the propagation process to foster improved uniformity and more consistent and predictable growth of desert species is of little value if trees are not pruned and shaped early and continually as they grow. Regular pruning is done on all trees in all sizes with the goal of enhancing and complimenting the natural form of the species we grow. The process begins when plants leave the greenhouse and continues until the trees are sold. Selective pruning and the use of temporary branches (lateral branches that are left to grow on main branches to promote the development of caliper and taper but that are ultimately pruned off) are essential to produce trees with strong, sound structure. All pruning is done by a select group of AZT’s field staff who have been trained and who demonstrate an understanding and appreciation for proper pruning techniques and desirable tree form. Our field pruning staff are supervised and trained by our professional staff who bring decades of academic and field horticultural experience to the task of growing and maintaining landscape trees.

Staking in the nursery is used primarily to help set and support the shape of trees (particularly multiple trunk specimens). Where possible bamboo stakes are used as they offer only limited support and are flexible enough to allow trunks to move in the wind developing taper and caliper. Bamboo has the added advantage of reducing branch and trunk scaring that is often associated with thick wood stakes and the soft wood of many desert species. When necessary lodge poles are used for staking larger specimens, typically 36" boxes and larger. The lodge pole method used is similar to those seen in many landscape architects planting detail drawings with stakes placed at a distance away from the trunk(s) and using guy wires and hose to provide limited support but that do not scar the trunks or branches. Wires and hose are checked and loosened as needed throughout the trees growth in the nursery to ensure the branches or trunks are not wounded or damaged by the increased branch or trunk diameter. Most trees are shipped with these stakes attached. Once planted into the landscape, stakes should be removed as soon as it is appropriate. We strongly recommend against staking trees indefinitely in the landscape.

As outlined here quality control at AZT begins with the selection of propagation stock and continues until the tree is loaded onto a truck for delivery to the job site. In a sense our newsletter, Arid Zone Times, is an attempt to extend our commitment to quality control by offering readers state-of-the-art botanical and horticultural information that we believe will help them select the appropriate tree(s), used proven planting methods and maintain trees for vigorous growth and long life.