The “Hazardous” Trees of Glen Canyon

[Note: This article has been edited for clarity and to add the video clip of Dennis Kern of SFRPD misstating the number and hazard of the trees.]

The latest information for the number of trees to be removed very soon for the Rec Center Project (the Entrance, tennis courts, playground, restroom, and heating system) is 58 trees. We are told – repeatedly – that these trees are being removed because they’re hazardous.

Only they’re not. Using the  standard rating system from SFRPD’s own hired arborist Hort, only one tree to be removed is hazardous.

Read on for more.

Dennis Kern tells the Park Commission about 60 hazardous trees.

Here’s a link to a video of Dennis Kern at the Parks Commission meeting, saying they had very specific numbers: 60 trees were hazardous, and another 10 were in the way of the project. The SF Weekly newspaper reports: ” However, 52 of the 58 trees set for imminent chopping were identified in 2004 as safety hazards; renovation funds finally gave the department the chance to weed them out, [SF RPD’s  Sarah] Ballard says.”


The signs say 58, but the arborist’s latest report (October 2012) lists only 57 trees by tag number. This is  a minor level of confusion compared to what went before. The estimates went from 10-11 trees mentioned during the community meetings (which already upset many neighbors), to 70 trees end-July when Kern claimed they had “a very specific number”, and now is back down to 57 or 58.

But the more important point: Are they really hazardous?

A few days ago, we received the arborist’s new report [dated October 1, 2012]. It covered the same 627 trees, but now the 147 trees with the project area have actually been given a risk rating. (Before, they only got an eyeball estimate of condition.)  So we had a look at the risk rating for the trees recommended for removal.

If the trees for removal were hazardous, they’d have a high risk rating. Did they? Not so much.


The methodology being used for risk rating yields a scale of 3-12. (The details are below.)  The average tree would fall somewhere in the 5-8 range. San Francisco city uses a rating of 9 as a threshold for action – a tree rated 9-12 needs removal or remediation.

So, of the 58 trees for removal, how many were rated 9 or higher?

Only one.

There weren’t even very many 8s. Most of the trees tagged for removal were rated 5-7 — better than most trees across the city.


The method Hort uses for rating trees is this. It considers three factors:

  • Failure potential (How likely is it that the tree would fall or drop a limb);
  • Size of the part that would fail (the size of the tree or the branch);
  • Value of the target area (if it fell, what would it damage?).

Each is scored on a scale of 1-4, and the scores are added together. This gives a tree a rating of 3 (least problematic) to 12 (most problematic). Only a small young tree far from any road or building or playground would be a 3. The City’s action threshold is 9, and most trees with a 9 or higher rating would be removed.

This methodology is strongly biased against large trees in busy areas. For instance, a big tree near a roadway would get a score of 4 for size, and 4 for “value of target.” This means it is an automatic 9, because the score for “failure potential” cannot be less than 1.


So is Glen Canyon full of failing and dangerous trees? Not according to Hort Science updated evaluation.

They looked at 480 trees elsewhere in the Canyon. Of the 59 trees they recommended for removal among those trees, only 13 had a risk rating of 9 or higher. Another 26 trees had a risk rating of 8 and were in poor condition. Twenty more trees were recommended for removal because they were growing on a steep slope.

And that wonderful tree full of kids  in the picture?  It’s Tree #22, a Bailey’s acacia. It’s tagged for removal with a risk-rating of 7.

This entry was posted in Felling Trees and tagged , , , . Bookmark the permalink.

4 Responses to The “Hazardous” Trees of Glen Canyon

  1. Pingback: Glen Canyon: The Good, The Bad, and the Ugly « San Francisco Forest Alliance

  2. Pingback: Saving the Trees of Glen Canyon Park | Save Mount Sutro Forest

  3. Pingback: Fighting The NAP Nativist Agenda | San Francisco Forest Alliance

  4. Pingback: Opposing ROSE’s Policy 4.2 – Update | San Francisco Forest Alliance

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s