Tree Care, Part 2: Training Young Trees

Following the Santa Ana windstorm that began in the early morning hours of December 1st, I started to consider whether trees would have better withstood these unusually strong winds if they had been grown and cared for according to best horticultural practices. This led me to write the first article, Selecting and Planting Trees for Long Term Success, of a series of three on tree care.

Assuming that the best tree was selected, and that it was planted and cared for properly from the start, proper care during the next stage of development can determine whether the tree grows into a beautiful, strong, long-lived specimen, or is subject to disease and damage. In fact, it may be that some of the limb failures during the storm were due to poor pruning practices on young trees.

Once a young tree has become established in its new site, usually taking a few years, good pruning can facilitate the development of a strong and healthy structure. Training a young tree through pruning will be discussed here. Many of these pruning tips apply to trees throughout their lives, but establishing a healthy structure is very important and best accomplished early on.  Much of the information here came from the excellent book by Lee Reich called The Pruning Book. I highly recommend it for its clear and excellent photographs illustrating good pruning cuts and practices.

A young Engelmann oak at the LA County Arboretum with juvenile branches left intact.

Training a young ornamental tree entails selecting the main branching structure through pruning. Since removal of branches slows the growth of a tree, pruning should be minimal and judicious. During the first few years after planting, allow small temporary branches to remain along the main stem of the tree. These juvenile branches feed the main stem and root system, resulting in a larger and stronger tree.

Select leader branch, remove co-dominant branches
After several years, approximately 3-5 depending on the tree and how quickly it is growing, begin to select the leader and scaffolding branches. First, determine which branch will be the leader. This is the central main stem that continues from the trunk. Some trees, like liquidambars, have strong leaders, with little branching along the main stem unless the leader is cut. Other trees, such as mulberries, branch more freely along the main stem and the leader must be selected and maintained through pruning. Eventually the leader may lost as the tree develops a broad, rounded crown. This is fine as long as a good structure develops early on. As a general rule of thumb, do not cut the leader.

Liquidambar with co-dominant stems. In time these could split apart causing the tree to fail.

If the main axis of the tree naturally splits into two equal branches, the tree is said to have a co-dominant leader. Over time the co-dominant branches do not have enough room to increase in width and the tree can split at this juncture. When the tree is young remove one of the co-dominant branches, selecting the stronger and/or straighter of the two. A co-dominant branch can also be reduced in size by pruning some of its secondary branches.

Select scaffolding branches
A strong tree will have secondary or scaffolding limbs along the stem. These branches should be well spaced and smaller than the main leader in size, and they should extend from the main stem with a wide crotch. Furthermore, they should branch out in all directions so the tree is not lopsided but has a fully rounded form. Limbs growing out in all directions feed a symmetrical root system and result in a stronger and more beautiful tree. Over several years, remove limbs that are too closely spaced, growing with a narrow crotch (acute angle to the larger branch), growing too low on the trunk, or unevenly spaced around the tree.

Scaffolding branches were removed because they were too close to- gether, without enough room to develop.

Scaffold branches will develop side branches as well. Pruning these should follow the same rules: good spacing, wide crotches, and removing or reducing branches that compete with the main scaffold branch.

In addition to training a tree to have a leader and strong symmetrical scaffolding branches, several other conditions may need to be corrected throughout the life of the tree. Correct crossed branches as they occur. Rubbing between branches causes injury that can allow disease to enter the tree. Furthermore, as both branches grow they can create an area of weakness that is susceptible to rot. In time low growing branches will need to be removed as these may interfere with sidewalks or roads. Suckers and water sprouts have weak attachment to the tree and should be removed whenever they occur. Branches with narrow crotches should be eliminated since these too result in weak attachment and areas susceptible to rot. And finally, branches that are dead, diseased or dangerous should be removed.

As trees age they will always have some dead branches. This is especially true of our native coast live oak. Occasional pruning of small dead branches may be good for the tree, but remember that dead branches have wildlife value. Birds feed off the insects that live in snags and other dead limbs. Pollinators often nest in these areas as well. Nature is not perfectly neat, so if your trees and your garden are a bit messy, you are creating better habitat.

Two coast live oaks in my yard, the one on the left has this and numerous other narrow crotches. This one should have been corrected earlier before the limbs reached this size. The one on the right has a nice wide crotch with a healthy bark ridge.

This coast live oak has a split trunk at the base. The secondary trunk crosses the main trunk. The two trunks will interfere with each other both at the base and above where they cross.

The problems noted above were corrected and now the tree has a good chance of growing into a strong, healthy oak.

When to prune
In general deciduous trees should be pruned when they are dormant. The absence of leaves makes it easier to select the right branches to create a well-formed tree.

The California Oak Foundation recommends minimal pruning of oaks. Pruning of evergreen oaks, including our coast live oak, is best done in late summer. Read Pruning Oak Trees in Southern California by the UC Cooperative Extension before pruning or having your oaks pruned by someone else.

Two cuts in a magnolia that have healed over well. The cuts were made above the branch collar so the tree can heal,  leaving a donut-shaped wound that closes over time.

How to cut
Now that you know what and when to cut, let us consider how to prune a branch. Again, Lee Reich’s book, The Pruning Book, has excellent pictures. There are also many websites with pictures of good pruning cuts (see below).

All pruning should be done with clean, sharp and appropriate tools. Bypass hand pruners work well for small branches. I love my Felco #2 pruner! Lopping shears are useful for branches larger than one-half inch. Again, my favorite is the Felco #23. Finally a small folding handsaw is often useful for larger branches, and once again Felco comes through with the F-600.

Equipped with the right tools, the cut must be made properly. Branches extending from a larger branch or the main trunk of a tree have a bulge called the branch collar. The branch collar is more prominent on some trees than others, but a proper pruning cut never damages this area. A flush cut that damages or removes the collar  results in a wound that is difficult to compartmentalize, therefore leaving the tree especially susceptible to disease.

This wound did not heal over completely, leaving the tree open to decay. It is now an area of weakness that is subject to failure.

Since we are discussing training young trees, you should not be making large cuts. If, however, you are removing a fairly large, heavy branch, the three cut method is preferred (or better yet, hire a qualified arborist). The Virginia Cooperative Extension has a clear description of this and other pruning tips: A Guide to Successful Pruning, Pruning Deciduous Trees.

Research has determined that good cuts heal best without being coated, a practice that was popular in the past.  

Conclusion
Clearly, many trees get less than optimal care when young. Some develop into strong, healthy specimens, others look okay until they are stressed. The Santa Ana winds were unusually violent, shearing off tops, ripping off large limbs and uprooting enormous trees, some that appeared to be structurally sound and healthy, and others with obvious defects. Unusual weather events are out of our control, but planting the right trees and taking good care of them is not.

In the next article I will cover pruning mature trees and common pruning errors – and atrocities. Although it is recommended that large, mature trees be pruned by qualified arborists, knowing what a properly pruned tree looks like is an important step to having one.

Unable to heal completely around the wound this tree is susceptible to decay.

These limbs were pruned too far out, leaving stubs that again interfere with proper healing.

Completely healed, closed to decay.

                         

Resources
There is a lot of misinformation out there. In general, I use websites of universities, cooperative extensions, governmental agencies, and horticultural or arboreal organizations or institutions. Be sure to use reliable references.

Good General Info
Lee Reich, The Pruning Book, 2nd Edition. 2010. CT: Taunton Press.

Univ. of California Cooperative Extension and California Master Gardener

• How do I keep my tree healthy and vigorous? (link and resources)
• Pruning (pdf) , Master Gardener Handbook
• Training Young Trees for Structure and Form (pamphlet)
• Planting and Care of Landscape Trees, Part 1 (video with Pam Bone)
• Planting and Care of Landscape Trees, Part 2 (video with Pam Bone)
• Classes and events

Colorado State University Extension, CMG GardenNotes #613. Pruning Cuts
International Society of Arboriculture (ISA), Tree care information
Iowa State University, Forestry Extension, Pruning Young Landscape Trees
National Arborists, Tree Pruning
Oklahoma State University, Training Young Shade and Ornamental Trees
US Department of Agriculture, Forest Service, How to Prune Trees
Urban Tree Foundation, Training and pruning trees for strength, clearance, and aesthetics
Virginia Cooperative Extension, A Guide to Successful Pruning

Oak Trees

• California Oaks Foundation, Care of California’s Native Oaks
• LA County Online: Sudden oak death and care of native oaks
• UC Cooperative Extension, Ventura Co., Pruning Oak Trees in Southern California (excellent!)

Conifers
UC Cooperative Extension, Central Coast & South Region, Pruning Conifers

You Tube on Pruning Crape Myrtle (this is why I suggest sticking with official organizations for info):

How to prune a crape myrtle: (stick with Univ. extensions, and other governmental agency info)
http://www.youtube.com/watch?NR=1&feature=endscreen&v=3hZJRWkRH5M
http://www.youtube.com/watch?v=xzzNaId-XjE&feature=related

How not to prune a crape myrtle
http://www.youtube.com/watch?v=FFibCwxnS14&feature=related
http://www.youtube.com/watch?v=L2fs-STJzU4&feature=fvwrel