Selecting, Planting and Training Young Trees for Long-Term Benefits

    Here on the UC Davis campus, we plant trees hoping they will provide many long-term benefits, including casting shade, beautifying the landscape, and providing environmental benefits. Unfortunately, this has not been the outcome for many of our plantings over the past decade. We plant an average of 100 trees/year on campus, and until recently, roughly 30% of these new plantings would fail to thrive. This article outlines the reasons for these young tree failures, and the lessons we've learned along the way. Our hope is that other institutions learn from our experience.

    1. Prepare the site
    Challenge: With over 35,000 students at UC Davis and a dizzying number of construction projects, it can be hard to find a planting location on our campus that is not heavily compacted. Often times the compacted soil (or hardpan) lies 6-18" below the soil surface. Trees planted over a hardpan fail to thrive, because their roots have trouble penetrating the compacted layer, and water does not drain adequately. The result is a poorly rooted tree growing in anaerobic soil - a combination that few shade trees will survive.

    Solution: We've found it necessary to loosen the soil at the planting site (at least 3 times the width of the root ball), mix in nutrient rich soil, and perform a percolation test prior to planting to ensure proper drainage. If the planting hole does not pass the percolation test, a deeper and wider hole must be dug to further break up the compacted soil. In sites with more severe drainage issues, a perforated drain pipe may be installed in the root zone to assist with drainage.

    2. Plant a diversity of tree species appropriate for the climate
    Challenge: The UC Davis campus forest is comprised of over 14,000 trees, the majority of which are not native to our region. Over time, we are learning that these non-native species are often not well-adapted to our local climate, and bring with them a suite of pest infestations. The California drought has accentuated which trees can survive in our climate, and which do not.
    Solution: It has become a priority of our Landscape Architects and Horticulturalists to select tree species that are adapted to our local climate and have a good chance of surviving future drought years. We are experimenting with many new tree species, striving to grow a highly diverse tree canopy, with no single species comprising more than 5% of the campus forest.

    3. Select high quality trees

    Challenge: In the past, we have planted too many trees on our campus that had major root defects originating from the nursery. Trees with girdling roots can grow well for many years before showing signs of stress, but once the girdling roots cut off enough of the trunk's vasculature, the tree begins to show signs of weakness, such as early leaf drop and branch tip dieback. Trees with girdling roots are also poorly anchored in the soil, and can become hazardous as they grow large, heavy canopies. Ultimately, when we plant a tree with girdling roots, we waste time and money, and we never see the benefit of a mature, healthy tree.
    Solution: Every tree that gets planted on the UC Davis campus is now inspected upon shipment. Trees that do not have a healthy root system are returned to the vendor.

    4. Install the tree correctly
    Challenge: Due to lack of time, proper training, and/or vigilance, many trees have been incorrectly planted on our campus over the years. Often times, the tree is planted too deeply in the soil, covering up its trunk flare. This becomes problematic over the long run, and can lead to trunk rot and girdling roots. Even when planted at the proper depth, the roots of container grown trees may fail to establish if they are not loosened up and pruned at the time of planting.

    Solution: When installing a tree, the root collar should be even with the landscape soil surface and some trunk flare should be visible above grounds once the hole is backfilled. Root pruning has become a mandatory component of our protocol prior to planting any tree on campus. Trees can tolerate a significant amount of root pruning at the time of planting, as long as they are given adequate irrigation. We prune roots growing over the root flare, as they will eventually girdle the trunk if left intact. We also prune roots that circle or mat along the sides and bottom of the root ball. Our trees that have received root pruning at the time of planting, followed by proper irrigation, have well-established roots within one year of planting, and we commonly see 3-5' of branch growth per year.

    5. Prioritize Young Tree Care
    Challenge: Young tree care took the back burner during tough budgetary times. Many of our campus trees were left unattended for years after planting. While some trees can weather this type of neglect, other suffer from it. Inadequate water, problematic staking, and poor branching structure are the primary problems we've experiences when we don't tend to our young trees.
                                                                                 This tree was left unattended for several years
                                                                                   after planting while it's stake girdled the trunk.
    Solution: An upfront investment in young tree care goes a long way towards growing healthy, are trees that will thrive to maturity. We have re-prioritized our Young Tree Care Program, whereby each new planting gets a "check up" once a year for the first three years after planting. These quick inspections ensure that the tree is getting adequate water, its roots are establishing, it is developing a good branching structure, and its stakes are properly installed. Small adjustments made early on save us from making major corrections down the road.

    UC Davis's award-winning landscape is an inspiring backdrop for learning, recreation, and community building. Our challenges over the past decade with our young trees have led to triumphs in recent years, thanks to some minor changes in our protocols.

    Cary Avery, CGM is Associate Director at UC Davis and Melanie Gentles is the UC Davis Campus Arborist.

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