Welcome back to the Our City Forest blog. We took a break over here for the past two weeks in honor of Americorps Week to kickstart the Americorps Experience Blog where we highlight the lives, accomplishments and struggles of the people who serve their nation by serving communities. I’ll continue to profile them for the foreseeable future while maintaining our coverage of environmental and tree-oriented issues. Incidentally, the Silicon Valley has seen some pretty significant storms (flawless segue). We’ve been flooded with calls about downed trees, bent trees and broken support stakes. Our inboxes doth overflow with cries for help from concerned residents. With this in mind here’s what you need to know to manage tree risk. 

Hark! The rare advice bird! 

Hark! The rare advice bird! 

1)    Trees are pretty safe, all things considered. 

Your chance of getting injured or killed by falling trees is infinitesimally small. Between the years of 1995-2007, 407 people were killed by falling trees in the United States (the risk is 1 in 10,000,000 according a UK government study). Compared to motor vehicle accidents (33,804 in 2013) or heart disease (611,105 in 2013) the risk of getting struck by a tree is small. You’re more likely to get struck by lightning than killed by a tree. Add to that context of the tree-related deaths, thunderstorms, tornados, hurricanes and wind-shear and you have an even-less likely scenario. People who work in forestry as lumberjacks, arborists and forest managers are more likely to get struck by trees but unless you work in one of those professions, odds are that you’re never going to be in a situation where a falling tree poses a substantial risk to your health, safety or property. 

2)    Trees can’t move. 

The only moving tree you should see is Treena. 

The only moving tree you should see is Treena. 

I know this sounds crazy but trees are sessile and can’t move without outside intervention, ents, dryads, nymphs and other mythical tree beings excluded. If the tree might fall on something that can be moved then move that thing. This applies to objects like benches, picnic tables, elaborate outdoor sculptures or other moveable elements. If the tree is a risk because humans frequently interact with it, put up a fence or signs to keep people away from the tree. It’s easier and more cost-effective to prevent accidents by removing “targets” from a risk area or by keeping accident prone humans away from a risky area rather than, say, removing a mature tree. This dovetails nicely with the next point which is…


3)    An ounce of prevention is worth a million billion trillion chainsaws. 

As it turns out, because trees can’t move they don’t cause their own accidents. Most accidents are caused by improper tree care, bad site selection, improper planting or accidental damage. Weed-whacking or mowing too close to a tree can injure the cambium and cause tree failure.  Burying the root crown in mulch or planting a tree too deeply can kill a tree. Letting grass grow over a tree’s roots can cause problems with root failure too, as can covering the roots with concrete or asphalt. Improper pruning can introduce structural weaknesses or invite fungal colonization.The best way to prevent tree failure is to take care of your tree properly. If you don’t know how to care for a specific tree Google is often a good friend and barring that, seek an expert opinion. We’re not telling you this to shame you into tree-caring compliance; it’s in your best interest as a homeowner to avoid paying for expensive, tree-removal services, fines for killing a large tree, or legal settlements due to tree-fall property damage. 

So now that we’ve got all that out of the way, there are definitely things you need to look out for if you have a tree on your property. It’s important that you know what hazards look like so that you can head off potential accidents. Some of these can be handled by typical people but larger-scale projects should be handled by qualified arborists. 

Decay, like this, is a sure sign that a tree is in danger of failing. (Source:  James Bowe  under  CC ) 

Decay, like this, is a sure sign that a tree is in danger of failing. (Source: James Bowe under CC

1)    Decayed wood

When wood is colonized by fungi it decays. There are few external indicators of decay but there are things to look out for. Mushrooms or other fungi are indicators that the interior of the tree is decayed. “Rolling” cracks, bulges, hollow holes and cavities are also signs of decay. If decay affects 40% of the afflicted part of the tree (root collar, branch, trunk) the tree is at a high risk of failure. If the trunk has a thickness of less than 1 inch of wood for every 6 inches of trunk diameter or has an opening greater than 30% of the stem’s circumference there is also a high risk of failure. 

Fungi like this can indicate a problem within the tree. (Source:  Newbiggin Hall Scouts  under a  CC license ) 

Fungi like this can indicate a problem within the tree. (Source: Newbiggin Hall Scouts under a CC license

If you suspect that your tree is decaying you should consult an arborist to conduct an internal investigation of the tree, to determine the extent of the decay. It’s also perfectly possible for a tree to recover from and contain decay (unless it’s a palm tree). Decay isn’t all bad. Small hollows form homes for lizards, birds and mammals without causing additional harm to the tree. Trees (but not palms) can also sequester decay behind inner compartments, allowing them to heal and grow in response to the lost wood. Trees have had millions of years to develop responses to decay so unless the decay encompasses most of the tree or a particularly precarious limb you shouldn’t worry. 

Do these look safe? No? That's because they aren't. ( Source )

Do these look safe? No? That's because they aren't. (Source)

2)    Cracks

When the load of the tree exceeds the trunk or branch’s load-bearing capacity cracks can develop. Most cracks develop along improperly-healed wounds, at weak branch unions, at co-dominant stems or at flush-cut pruning scars. Vertical cracks along the wood grain are by far the most common. If a vertical crack goes through the entire tree the tree is at a high risk of failure and needs attention from a certified arborist. Some cracks heal over forming “ram’s horn” or in-rolled cracks. These are often associated with serious decay. If you have an in-rolled crack, gently strike the crack with a mallet to listen for hollowness in the tree. If the tree sounds hollow you should consult a tree-care expert to see if it is at risk of failing. Horizontal cracks across the wood grain are rare and only seen when the tree is about to fail. If you see one on your tree that section is in danger of failing in the very near future. 

Girdling roots are a serious problem that can  cause the death of a tree. ( source ) 

Girdling roots are a serious problem that can  cause the death of a tree. (source

3)    Root problems

Most root problems are hard to detect but there are some things you can look for. If you see roots wrapping around the base of the tree this can be a problem. Girdling roots can choke a tree as both the roots and tree expand with age. If roots girdle an entire tree the tree will eventually fail. This is especially true during wind storms. Trees with girdling roots will crack under wind at the point where the roots touch the tree.  Other root problems are more difficult to detect and require through investigation by trained professionals. 

Note the vertical crack between the branch and trunk. Bark is growing between the branch and stem. ( source )

Note the vertical crack between the branch and trunk. Bark is growing between the branch and stem. (source)

4)    Weak branch unions 

When two branches grow too close together bark can form between them, inside the tree. This called included bark. Bark does not adhere strongly to wood. This is by design. Old bark is supposed to shed as new bark is formed beneath it. If bark is included, the loosely-adhering bark weakens the junction between the branch and trunk or between co-dominant branches. Under high winds or storm conditions the weak junction can break, leaving a crack or tear a wound down the side of the tree.  This can especially bad if more than two branches emerge from the same location. 

( source )

The other type of weak branch union is caused by injury, environmental stress or topping of trees. When a branch breaks off or if a tree is topped the tree responds to the injury with growth near the damaged area. The tree tries to replace the canopy that was lost in the injury forming “epicormic branches”. These branches grow quickly and shallowly. Some can grow as much as 20 feet in a year, outpacing the growth of the rest of the tree. These rapidly-growing, loosely-attached branches are hazardous and prone to falling and breaking. If multiple epicormic branches grow vertically from a mature branch the entire branch is at risk for failure. This, among other reasons is why arborists discourage the topping of trees. 


This canker was caused by a vehicle collision. 

This canker was caused by a vehicle collision. 

5)    Cankers

Cankers are spots on the tree where the cambium and bark are dead. As the tree grows, the canker will not add any annual growth. This weakens the tree and predisposes it to failure as less wood is available to support the weight of the growing tree. This can be caused by a variety of factors including fungal infection, lightning strikes, insects, weed-whackers, lawn mowers or simple vandalism. If a canker covers more than 40% of a tree the tree is at a high risk of failure. Coupled with decay, cankers are especially dangerous.  

This is by no means a comprehensive list of hazards but they are important to monitoring the health and safety of trees on your property. If you suspect that you have a problematic tree, do not hesitate to call an arborist. While unlikely, trees can and do fail. It’s up to us as stewards of the urban forest to ensure that trees are healthy, well-maintained and allowed to cohabitate with us as safely as possible. We hope that the information provided here will help people be better caretakers of trees and more aware of how they can prevent tree-related accidents. 

If you liked this post and want to support Our City Forest our effort to provide advice, trees, and shrubs to the Silicon Valley you can find our donation page here. If you'd like to volunteer with us you can check the volunteer page here. As always, Our CIty Forest appreciates the continued support of the communities of Santa Clara County and the Silicon Valley. Thank you. 

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