Selecting Quality Trees
Professional landscape architects and landscape designers frequently ask for recommendations on how to identify and select the best trees for planting in their designs. This is not as simple as it might sound. In designed landscape settings most trees must serve two masters: one aesthetic and one horticultural/arboricultural. No one has a more complete, 3-dimensional vision of the finished landscape than the designer, so aesthetics tends to triumph over horticulture. There is a greater discussion than can be handled here about the multiple factors that should be considered when placing trees within the landscape. Our objective here is to offer an overview of qualities to look for and, conversely, easily avoided defects. The focus here is to review several key topics including tree canopy, branching, pruning, trunks, roots, and root system structure. Appreciate that these are broad descriptions of qualities and defects, not a prescription. Trees are living things and variation is the norm, not the exception.
Leaf and Branch Canopy: To evaluate the tree canopy requires an understanding of the tree’s growth habits from young to mature tree. With most trees, they are essentially the same, but there are exceptions. Red Push Pistache, Seedless Mesquites and more traditional trees species like Bottle Trees, look quite different as young trees (24” box, #25 containers) than more mature specimens. Most tree species fall into one of two categories Excurrent, or pyramidal canopies (e.g., Bottle Tree), and Decurrent, rounded or broadly oval canopies. Virtually all desert tree species fall into the latter (mesquites, red push, Palo Verdes).
That said, in general, look for an overall canopy form that you desire in the mature tree. To start with a wide canopied tree and hope to wrestle it into the desired narrow, upright shape through several years of pruning is a costly and counterproductive enterprise. Overall appearance of the canopy should be symmetrical, top to bottom and side to side. Trees destined for planting in atypical locations (against buildings, narrow spaces, eccentric specimens used as focal points or at monuments) may deviate from this criterion. Trees with mildly asymmetric canopies can, with modest training and pruning, be corrected. Substantially asymmetric canopies will require time and expense to repair and should be avoided.
Branching and Pruning: Tree’s general branching habits and patterns, when spared human interference, are under significant genetic control and, in natural settings, proceeds without intervention. In “the wild,” aberrations like crossing branches, co-dominance, included bark, while uncommon, are defects that are resolved by branch failure, breakage and an array of anomalous growth characteristics and are rarely fatal to the tree. Built landscapes have far lower tolerances for such defects as they can lead to conditions that are detrimental to the long-term health and survival of the tree and can be hazardous to property and people. In the nursery, inspecting the branching pattern, even in fairly young trees (15-gallon and #25 containers), reflect the basic genetics of the variety being grown and, more importantly, the grower’s attention to form, structure and pruning during the production process. An easy way to approach this is to envision the tree in 3 to 5 years and approximate the size of the trunk and/or branches as they increase in diameter to twice or three times their present size. Will there be enough room between branches/trunks to accommodate the expected growth? If there is not, will extensive corrective pruning be required and is the branches/trunks’ structure unsustainable?
Branch distribution and attachments are also critical. Narrow branch angles, (branches growing too close to each other) should be a cause for concern. As branches mature, they increase in length (linear growth) and diameter (radial growth). Closely spaced branches or branches with narrow branch angles gradually pinch together as they thicken and form areas of included bark. Included bark weakens branch attachment and contributes to branch cracking and failure.
Additionally, examine the size (diameter), number, and degree of healing (closure) of all pruning cuts. The vast majority of nursery grown trees should be free of large (greater than 2” diameter) pruning cuts. Well grown trees are pruned on a nearly year round schedule with most pruning cuts smaller than 1” in diameter (see 80/20 pruning rule). All pruning wounds should have clean margins, be free of tears and appear cut with sharp tools. Such wounds heal the quickest and indicate that the pruning was done properly. Large and/or numerous cuts indicate that trees have been left unpruned for long periods of time and then severely pruned. This is both a poor production method and inappropriate pruning practices. Trunks should also be free of damage that results from poor staking. Tree should never be topper (having the main branch leaders severely cut back to artificially reduce their apparent).
No more than 20% of the tree foliage should be removed at any one time with 80% of this pruning concentrated on the new growth in the outer third of the canopy.
Roots: Nothing has a greater impact on the longevity of nursery grown trees then the structure and distribution of the root system. This is by far the least appreciated, most critical, and the most easily overlooked indicator of well grown trees. The overwhelming majority of landscape tree failures, in southwestern landscapes, are the results of compromised root systems. Trees with bound (circling) roots, numerous kinked roots, or poorly distributed root systems rarely, if ever, correct themselves once planted into the landscape. It is critical that root development be an integral part of any wholesale tree grower’s production scheme.
Perhaps the most devastating defect is a stem girdling root; a root that wraps around a portion of the trunk. Such roots restrict upward water and nutrient flow while also compromising trunk strength and integrity. Too often this root defect begins at the propagation stage in the young tree’s development and remains with the tree for the balance of its shortened life. Root deficiencies of this type contribute to tree failure and death by wind throw, constriction, or with trunks breaking at or near the soil level. These flaws are often only detected years after installation, after a catastrophic event, or when the tree dies.
Make sure the root flare, the outwardly curving base of a tree where it joins the roots, is slightly above the soil surface. This is not always easy to see in smaller trees, but it is essential in determining if the tree had been planted too deep in the container.
It is rarely possible to remove trees from their containers in the nursery while inspecting them for selection. In cases where this is possible, inspectors should ask to see at least a sample of the trees being offered for sale. Some effective above ground criteria include; tree caliper should be appropriate to the size of the growing container (25 gal not larger 1-1.5 inch, 45 gal not larger 2-2.5 inch, 48” box 3-5 inch), large (1 to 2” diameter) roots growing out of the bottom of the container, or appearance of circling root anywhere near the surface of the soil, whether close to the trunk or near the edges of the container. If a tree “appears” too large for the container it is growing in, it is, and most likely has a compromised root system.
Containers: Wholesale tree growers are replacing the traditional wooden boxes with plastic tubs. The bulk of this replacement has occurred with the substitution of #25 and #45 containers for 24” and 36” boxes, respectively. These containers have economic and horticultural qualities that have helped propel their expanding popularity. Some nurseries, like AZT, are transitioning inventory production from boxes to plastic until current boxed inventory runs out. Until boxed inventories are exhausted, trees will be offered for sale in both types of containers. The same key tree quality production characteristics apply, equally, to both container types.
There is no question that selecting quality trees is both art and science. The art is borne of practice and repetition. Consider building a photo archive of the nursery trees you have selected over the years and make notes of the criteria you used or desire. The science piece of the puzzle is clearly the responsibility of the grower. Growing quality trees is no accident. It is only accomplished through an appreciation and integration of production systems that pay close and regular attention to critical growth and structure criteria.