13 Simple Steps to Evaluate Trees

There are now dozens of open-source tree maps in communities worldwide. The most successful mapping projects are those that provide citizen scientists with simple guides on how to accurately identify tree species and assess tree health. The more accurate the information on the tree maps, the more accurate the green infrastructure calculations they produce – and the overall valuation of the urban forest. (OpenTreeMap, which most of these open-source projects are built on, does green infrastructure calculations using i-Tree).

But how to ensure that the information entered is accurate? Not everyone is a tree professional, and the whole point of these maps is that any citizens can contribute to mapping the urban canopy. How can a non-arborist get a quick sense of urban tree health in the field and usually be right? To help address that, we’ve put together a 13-step checklist for tree evaluation. This can be used by professionals such as engineers, planners, and landscape architects, or by regular citizens willing. Most of the items are simple, although in a couple of question you may need to stretch yourself (just a little bit!).

Some of the conditions listed here are lethal to the tree, but most are not. Where solutions exist to improve tree health, we’ve listed them – click on the links to the right of each condition to read more about how. (Note that this evaluation tool is geared to deciduous trees in North American cities receiving more than 25”/63cm of rainfall annually.)

Tree Evaluation Checklist

Questions 1, 2, and 3 are the most important for beginning a tree evaluation.

These three items all address the soundness of the tree’s trunk. If the trunk is healthy throughout, then many other factors (outlined below) can, with care and attention, be reversed to help turn otherwise struggling trees into successes. The odds of a turnaround are also increased if the tree has a larger diameter trunk (>20″/51 cm DBH) and if it is 30 years old or more.

1. Trunk wounds
What percentage of the trunk is intact?

a) 100% of the trunk bark is present and firm = Healthy tree
b) ~80-90% of the trunk diameter is missing bark = Unhealthy tree // Can this tree be saved?
c) ~40%-80% of the trunk diameter is missing = Unsafe tree // Can this tree be saved?
d) Spiral of bark missing from the trunk from high up in the tree = Unhealthy tree // Solution: protect the trunk
e) Numerous woodpecker holes on trunk = Unhealthy tree // Solution: protect the trunk

2. Mushrooms
Are mushrooms visible?

a) None visible = Healthy tree
b) Visible on large branches = Unhealthy tree // Solution: (1) rehabilitate roots, and (2) get real soil to the roots
c) Visible on main trunk = Unsafe tree // Can this tree be saved?

3. Co-dominant branches
Are co-dominant branches present?

a) Strong central leader, no co-dominants visible = Healthy tree
b) 1-2 co-dominant branches = Unhealthy tree // Solution: remove co-dominant branches
c) >3 co-dominant branches = Unsafe tree // Can this tree be saved? 

4. Annual branch extension
Every branch contains a small scar that indicates how much it grew the previous year. Measure between terminal bud scars to determine annual growth.

a) >10” (25cm) per year = Healthy tree
b) <2” (5cm) per year = Unhealthy tree // Solution: (1) rehabilitate roots, and (2) get real soil to the roots

5. Annual trunk diameter growth
Using a diameter tape (or the formula diameter = circumference/pi if using a regular tape measure) calculate how much the tree grew (*this assumes you have made the same measurement in the previous year).

a) > 3/4″ (2 cm) per year = Healthy tree
b) <1/4″ (0.63cm) per year = Unhealthy tree // Solution: (1) rehabilitate roots, and (2) get real soil to the roots

6. Trunk flare
Can you see the trunk flare?

a) Visible = Healthy tree
b) Buried = Unhealthy tree // Solution: rehabilitate trunk flare and surface roots

7. Surface roots
How do the roots appear at the surface?

a) Radiate from trunk like spokes on a wheel = Healthy tree
b) Wrap around trunk and each other = Unhealthy tree // Solution: rehabilitate trunk flare and surface roots

8. Soil drainage rate
Using a hand spade, take a small hole about the size of a drinking glass in the soil at the base of the tree, taking care not to cut any healthy roots. Fill this hole with water and allow it to drain through. Refill the hole and time how long it takes to drain completely. Divide the number of inches/cm that drained by time in hours to calculate the infiltration rate.

a) 1/2” to 3” (1.2cm to 7.5cm) per hour = Healthy soil for trees
b) <1/8” (.3cm) per hour = Unhealthy soil for trees // Solution: (1) rehabilitate roots, and (2) get real soil to the roots

9. Density of canopy leaves
Can you see through the canopy?

a) Cannot see through = Healthy tree
b) Can see through = Unhealthy tree // Solution: (1) rehabilitate canopy, and (2) get real soil to the roots

10. Canopy perimeter shape
What is the shape of the canopy?

a) Continuous round or oval = Healthy tree
b) Large holes, almost like bites = Unhealthy tree // Solution: (1) rehabilitate canopy, and (2) get real soil to the roots

11. Leaf color and appearance
How do the leaves appear?

a) Dark green, large = Healthy tree
b) Yellow and small = Unhealthy tree // Solution: (1) rehabilitate canopy, and (2) get real soil to the roots
c) >50% leaf scorch = Unhealthy tree // Solution: (1) rehabilitate canopy, and (2) get real soil to the roots

12. Trunk and trunk base branches
Are there branches sprouting from the base or lower trunk?

a) No small branches visible on main trunk or base of tree = Healthy tree
b) Small branches sprouting from the trunk that are relatively new = Unhealthy tree // Solution: rehabilitate trunk flare and surface roots
c) Small branches sprouting from the base of the trunk, looks almost like a shrub = Very unhealthy tree // Solution: rehabilitate trunk flare and surface roots

13. Mulch
If mulch is present, how does it appear?

a) Mulch forms a dish with thin layer (<2”/5cm) over main roots = Healthy application
b) Mulch forms a thick mound and touches trunk = Unhealthy application // Solution: rehabilitate trunk flare and surface roots

 

Solutions

After walking through the 13 steps above, you may realize that there are quite a few trees (probably too many) under your responsibility that are unhealthy or perhaps unsafe. What’s a non-arborist to do? Do nothing and hope for the best?

We want to keep the trees alive – and keep all the benefits they provide. Here are tips for how to salvage trees.

Protect the trunk

Oversized, flexible corrugated pipe sleeves can be placed on the tree’s trunk to protect it. The pipe should be at least 3 feet to four feet (.9m to 1.2m) high and at least 3 to 4 inches (7.5cm to 10cm) larger than the largest portion of the tree diameter. Alternatively, you can place a low (<24”/61cm) iron fence at the edge of the tree pit/trench as far from the tree trunk as possible.

Rehabilitate roots

If your tree’s biggest problems have to do with annual branch extension and diameter growth, there is a relatively simple fix. These two suppressed growth indicators show that the tree’s root system has gone anoxic, probably due to trying to survive in compacted soil while also being short of soil volume.

You can rehabilitate the tree’s roots without losing paved surface areas. The best time to do root rehabilitation is when sidewalks have lifted, cracked and are a tripping hazard, or when curbs are broken and replacement is imperative or when trees are in their dormant season. Pavement demolition is the time to use an air-spade to remove old, compacted fill from around the existing roots and replace it with loam soil under a suspended pavement system (such as Silva Cells). Remove all girdling roots, but do not cut roots from the original root system (spokes on a wheel or tap root).

Get real soil to the tree’s roots

You can safely excavate up to four feet (1.2m) below the paving cross-section with an air-spade exposing all the major roots; do not dig or cut roots that are not girdling roots. If the tree is in a small tree pit or a boulevard with a narrow soil strip, that soil should be air spaded out and replaced.

If there are yards, lawns, or parks adjacent to the tree, create 8 to 10 foot (2.4m to 3m) wide root chases at the tree trunk with an air spade under the sidewalks. Connect these root chases to the adjoining soil. If adjacent soil volumes are not available, increase the total rooting area using suspended pavement to increase the available real soil to 1,000 cubic feet (28 cubic meters) per tree. In the case of a 6 foot (1.8m) wide sidewalk, air-spade out the old fill across the full width and down 4 feet (1.2m) without cutting the root system or any infrastructure, for a length of 42 feet (12.8m).

The existing air spaded tree’s roots will grow rapidly and quickly colonize this newly available soil. As a result, the tree’s crown rate growth in width and height will be extremely rapid (Stahl). To ensure that root plate development keeps up with top growth, your city arborist will need to evaluate annually for the first 3 years, and possibly prescribe crown reduction during this rapid growth phase, especially in areas where hurricanes or tornadoes are common.

If annual precipitation is greater than 25” (63.5cm), expect the following growth rates of crown width for up to seven to fifteen years following root rehabilitation (Stahl, Smiley):

USDA Hardiness Zone Projected crown diameter growth rate (annual)
4 and 5 4’ to 6’ (1.2m to 1.8m) per year
6 and 7 5’ to 7’ (1.5 to 2.1m) per year
8, 9, and 10 >7’ (2.1m) per year.

Rehabilitate the trunk flare and surface roots

Rehabilitating the trunk flare and surface roots is an even simpler fix than root rehabilitation (above).

Use an air spade to remove excess soil from the base of the trunk and expose girdling roots. Remove any girdling roots. You will know when you are at the correct final soil elevation for the trees once the trunk flare is completely exposed, the girdling roots are removed, and the original “spoke wheel” roots are visible at the surface.

Rehabilitate the canopy via the roots

Canopy density, perimeter shape, and color are accurate general indicators of poor root health, so they should clear up once root rehabilitation is complete.

Remove co-dominant branches

Remove co-dominants as soon as possible – the smaller the tree is when the branches are removed, the better. Co-dominant branches larger than 8 inches (20cm) in diameter should be removed one at a time, with further co-dominant branch removal happening only after the wound of the previous removal has callused or closed. This may take multiple years (hence the smaller the tree the better).

Epicormic growth

Small branches sprouting from the trunk and/or small branches sprouting from the base of the trunk indicate a trunk that is buried in soil or in a mulch volcano (and probably the presence of girdling roots, too).

Use an air spade to remove excess soil from the base of the trunk and expose girdling roots. Remove any girdling roots. You will know when you are at the correct final soil elevation for the trees once the trunk flare is completely exposed, the girdling roots are removed, and the original “spoke wheel” roots are visible at the surface.

Can these trees be saved?

These conditions are consistently deadly for trees:

  • Large swaths of missing bark (>40%) indicate that the tree’s circulatory system is severely disrupted and unlikely to be restored.
  • Mushrooms visible on the main trunk indicate cavities and/or heart rot.

In both cases, monitor the tree and remove it before it becomes a significant hazard.

Two or more large (>12”/30.5cm) co-dominant branches are a ticking time bomb for a large tree. There are a few steps you can try depending on the specifics of the tree:

  • remove all co-dominant branches as soon as possible, but at a minimum before they are 12” (30.5cm) in diameter, using Shigo’s target pruning method;
  • a spiral of bark, like a barber pole, is missing from the trunk from high up in the tree indicates a lightning strike, which may or may not be deadly for the tree. If there are large amounts of dead wood and mushrooms visible on the trunk, then the strike was deadly and the tree is unsalvageable.

Conclusion

Research indicates that community stewardship can be hugely influential to tree survivorship. And while there are some cases where an arborist should be consulted, in many cases a simple checklist like the one here can effectively guide anyone to be able to identify healthy, unhealthy, and unsafe trees in over 80% of field cases. This is not comprehensive, and there always exceptions, but this simple 13 step process of evaluation will be reliable in most urban situations.

Use it – and please send your feedback to rethinktrees@deeproot.com so that we can use our collective knowledge to improve the checklist as we go.

L. Peter MacDonagh, FASLA, is the Director of Science + Design for The Kestrel Design Group.

SounderBruceCC BY-SA 2.0

One comment

Leave a Reply

Your email address will not be published. Required fields are marked *