Valley oak (Quercus lobata): limb failure and line-strike risk
Valley oak's dominant failure is a live branch. Its 251-report failure profile puts 43 percent of failures in branches, 95 percent of failed branches alive, mean wind at failure 9 mph, and the peak in August. GO 95 Rule 35 and PRC 4293 reach dead, rotten or diseased wood, so both miss it.
On two consecutive nights in October 2017, a valley oak limb came down on a PG&E 12 kV conductor and started a fire. The CPUC's Safety and Enforcement Division investigated both. Both files closed without a vegetation management violation. One tree was rooted approximately 15 feet from the conductors and carried internal decay inside sound-looking wood. The other was rooted approximately 50 feet away and was sound all through.
PG&E's own procedure already rated the species at Very High failure potential. Both trees passed inspection with that rating on file. This page is about why.
The failure profile is mostly a branch profile
Valley oak has a published structural failure profile. L.R. Costello, K.S. Jones and C. Drake wrote it in 2015, commissioned by the Britton Fund of the Western Chapter of the International Society of Arboriculture, from 251 valley oak reports held in the International Tree Failure Database. The ITFD was established in 2003 and its predecessor, the California Tree Failure Database, in 1987. As of 2013 the database held 6,680 failure reports across all species.
That is the difference between this page and the one on gray pine, where the absence of a species profile is itself the finding. Valley oak is the fifth most reported species in the Western Tree Failure Database, at 363 reports as of January 10, 2026. The database cautions that report counts likely reflect the relative abundance of a species in the areas reports come from, rather than how often it fails. The profile remains the best structured record a program has for this tree.
The 251 reports split by the part that failed.
Failure part | Reports | Mean age | Mean height | Mean d.b.h. | Mean wind speed |
Branch | 107 | 163 years | 66 feet | 47 inches | 9 mph |
Trunk | 62 | 153 years | 62 feet | 44 inches | 12 mph |
Root | 82 | 143 years | 60 feet | 41 inches | 15 mph |
All reports | 251 | 150 years | 62 feet | 44 inches | n/a |
Branch failures are 43 percent of the record. The mean tree in it is 150 years old, 62 feet tall and 44 inches through at breast height. A tree that size has outgrown what a cycle trim can reach, and the limb that fails off it travels to the conductor on its own.
The limb is live, large, and lands in August
Ninety-five percent of valley oak branch failure reports are for live branches. Only 5 percent of all branch failures were associated with the failed portion being dead. A valley oak does not have to be dying to drop the limb that takes out a span; in this record it is alive when it goes.
Where the break happens is recorded separately, and the sizes read best alongside the dates.
Break location | Cases | Mean branch diameter | Mean air temperature |
Along the branch | 68 | 20 inches | 74 degrees Fahrenheit |
At the attachment | 37 | 27 inches | 69 degrees Fahrenheit |
August is the peak month at 22 failures. From May to October, 81 branch failures occurred, and from November to April, 15 were reported. The mean wind speed across all branch failures is 9 mph, and the mean temperature is 63 degrees Fahrenheit.
The still-afternoon summer failure, and the research that placed it, belong to the eucalyptus guide. What valley oak adds is a size. The limb arriving in that window is 20 inches through when it fails out along its length, and 27 inches when it fails at the union. That is a limb a patrol wants on paper before it lands.
Heavy lateral limb is the defect in three cases of four
The Fire Effects Information System review of Quercus lobata was written by Janet L. Howard in August 1992. It records the form plainly. Valley oak is the largest North American oak, the crowns of open-grown trees are very broad, and valley oaks have a highly branched growth habit. A roadside or pasture valley oak grows the shape a corridor least wants, which is long horizontal wood carried a long way out from the stem.
The profile puts a number on it. Heavy lateral limbs contributed to 75 percent of branch failures, 72 cases against 23 with none. Two defects that field notes reach for more readily sit far down the list.
Heavy lateral limbs: 75 percent of branch failures.
Dense crown: 14 percent of branch break cases.
Included bark at the union: 5 percent of failures at the attachment.
A note recording included bark on a valley oak has captured the 5 percent factor and skipped the 75 percent one. The pull-test research behind codominant union ratings, and what that research establishes, is set out in the gray pine guide. For valley oak the species profile answers the question directly, and the answer is limb load.
Ground inspection caught the defect in 46 percent of cases
Reporters are asked whether the defect associated with the failure would have been visible from a ground inspection. In 46 percent of cases they judged it would have been. In 35 percent they judged it would not. The evaluation was not reported in the remaining 18 percent.
Decay is the reason. Wood decay contributed to 61 percent of valley oak branch failures, rising to 70 percent for failures at the attachment and 57 percent for failures along the branch. FEIS says the same thing about the species from the other direction: large oaks are usually hollow or rotten in the center, which is why exact age determinations are impossible.
The fruiting body a crew is trained to look for is mostly absent. No sporophore was reported in 93 percent of branch failures associated with decay, with six cases reporting one. A clean-looking valley oak limb says very little about the wood inside it, so a hazard tree call made from the ground on this species rests on about half the evidence. Whether anybody buys a closer look, under a Level 1 or Level 2 assessment, is a scope question settled in the contract.
October 8, 2017: a seven-inch limb at 15 feet
The CPUC's Safety and Enforcement Division published Incident Investigation Report E20171013-01 on April 29, 2019. Between 2230 and 2300 hours on October 8, 2017, a seven-inch diameter valley oak limb fell onto the conductors of PG&E's Bangor 1101 12 kV two-phase circuit at 167 Darby Road in Bangor, Butte County. The pole-to-crossarm attachment failed, a conductor reached the ground, and the ground fault ignited the La Porte Fire. It burned 6,151 acres, destroyed 74 structures, damaged 2, and cost PG&E about $3 million in North Valley Division restoration.
Weather station BNGC1, 2.7 miles south-south-west, recorded 69 degrees Fahrenheit, east-north-east winds of 4 to 6 mph, gusts to 30 mph, and relative humidity of 10 to 12 percent. CAL FIRE observed the tree rooted approximately 15 feet from the 12 kV conductors.
SED's own finding on the limb:
The subject limb showed no obvious outward signs of decay or disease. The internal decay and possible crack likely weakened the base of the subject limb and caused it to fail when exposed to high winds on October 8, 2017.
That is the modal valley oak branch failure written up as an incident: decay inside, a clean outside, a live limb, a quiet night. The incident area is rural under GO 165, which sets biennial patrols and detailed inspections at five-year intervals. PG&E's patrol records for November 2015 and October 2017 documented no abnormal conditions. The report closes on one line: SED did not identify any General Order violations by PG&E.
October 9, 2017: sound wood, fifty feet from the wire
Incident Investigation Report E20171009-03, published April 26, 2019, covers a valley oak limb that failed onto PG&E's West Point 1102 12 kV circuit near 22894 State Highway 26 in West Point, Calaveras County, at approximately 0110 hours on October 9, 2017. The Point Fire burned 130 acres, destroyed 19 structures, damaged three buildings, and interrupted 2,345 customers.
The measurements in that report are the ones to carry:
The subject Valley Oak tree branch was attached at approximately 55 feet above the ground and was approximately 8 inches in diameter. PG&E and PG&E's contractors did not identify the subject Valley Oak tree, rooted approximately 50 feet from the subject conductor, for removal or trimming during VM PIs from October 2012 through July 2017.
Fifty feet from the conductor sits outside every clearance number that applies to that circuit. PRC 4293 requires four feet radially at 12 kV, and GO 95 Table 1 is written in the same currency. The strike came out of the 55 feet of height, and the clearance rules measure the horizontal distance.
CAL FIRE's arborist examined the limb and found it sound, with no evidence of disease or decay at the break point, and the foliage along it healthy. The arborist's conclusion was that the limb was healthy and the break resulted from the wind event. SED found one violation in the whole file: GO 95 Rule 19, for PG&E disposing of the broken crossarm and the damaged section of conductor before the Commission could examine them.
CAL FIRE reached a different answer on the vegetation. It cited PG&E under PRC 4293 and under General Order 64-A. SED recorded both the disagreement and its basis: CAL FIRE's report does not explain its findings of violations of these provisions, and the CPUC replaced GO 64 and 64-A with GO 95 effective July 1, 1942. Two agencies read one tree and filed two answers, and the question of who reaches what stays open on the face of both reports.
The rule is written around dead wood
Rule 35 is explicit about which trees the hazard provision reaches:
When a supply or communication company has actual knowledge, obtained either through normal operating practices or notification to the company, that dead, rotten or diseased trees or dead, rotten or diseased portions of otherwise healthy trees overhang or lean toward and may fall into a span of supply or communication lines, said trees or portions thereof should be removed.
The obligation attaches to dead, rotten or diseased wood, and only once the company has actual knowledge of it. PRC 4293 uses the same vocabulary in its last paragraph:
Dead trees, old decadent or rotten trees, trees weakened by decay or disease and trees or portions thereof that are leaning toward the line which may contact the line from the side or may fall on the line shall be felled, cut, or trimmed so as to remove such hazard.
Both texts describe a condition the valley oak failure record puts in the minority. Ninety-five percent of failed branches were alive. Ninety-four percent of trunk failure trees were in relatively good health, and so were 94 percent of root failure trees. The wood is often decayed inside while the tree reads as healthy from the road.
PG&E's internal procedure carries the same shape, with the conjunction that decides it. The La Porte report quotes Section 2.6 of the Distribution Routine Patrol Procedure, TD-7102P-01:
(T)rees or portions of trees that are dead, show signs of disease, decay or ground or root disturbance, AND may fall into or otherwise impact primary or secondary conductors, THEN PRESCRIBE work to make tree Facility Safe per Facility Protect and work Difficulty Classification Procedure.
The same report records that PG&E's Vegetation Management Hazard Tree Rating and Scoring Procedure, TD-7102P-07, indicates a valley oak as a tree with a Very High failure potential. The species rating and the prescription trigger sit in two different documents joined by an AND. A healthy valley oak satisfies the second half and fails the first, which leaves the Very High rating on the shelf. Both trees that burned in October 2017 sat in exactly that gap.
Exception 4 is the valley oak rule
The provision that actually governs a big old oak beside a distribution line is Exception 4 to Rule 35, added October 22, 1997 by Decision 97-10-056:
Mature trees whose trunks and major limbs are located more than six inches, but less than the clearance required by Table 1, Cases 13E and 14E, from primary distribution conductors are exempt from the minimum clearance requirement under this rule. The trunks and limbs to which this exemption applies shall only be those of sufficient strength and rigidity to prevent the trunk or limb from encroaching upon the six-inch minimum clearance under reasonably foreseeable local wind and weather conditions. The utility shall bear the risk of determining whether this exemption applies, and the Commission shall have final authority to determine whether the exemption applies in any specific instance, and to order that corrective action be taken in accordance with this rule, if it determines that the exemption does not apply.
This is how a mature valley oak stays where it is instead of being cut back to the clearance table. The exemption turns on one judgment: whether the limb has sufficient strength and rigidity under reasonably foreseeable local wind and weather conditions.
For this species the reasonably foreseeable condition is a calm day. Branch failures average 9 mph. Forty-nine percent of trunk failures happened under 5 mph, against 24 percent above 25 mph. FEIS adds one more foreseeable condition at the other end of the calendar: valley oak are not adapted to snow, and the heavy, wet snowfalls that sometimes occur within their range cause considerable limb breakage. The night the La Porte Fire started, the nearest station read 4 to 6 mph with gusts to 30.
The rule is unambiguous about who owns the call and who reviews it. The utility bears the risk of determining whether the exemption applies. The Commission has final authority to overturn that determination, after the limb has landed. A contractor recording an Exception 4 tree is recording the basis for somebody else's risk decision, which is a reason to write down what the limb measured rather than what it looked like.
The root failure is the one the corridor causes
Root failure is 82 of the 251 reports. It is the failure a corridor can create on purpose and then wait years to see.
Roots cut or severed: 40 percent of root failure cases.
Roots broken: 26 percent.
Roots lifted: 13 percent.
Roots restricted and broken: 11 percent.
Trenching, pole holes, pad grading and access road work all sit inside that first line. Fill is in the file too, reported at some level in 38 percent of root failures. The trigger comes later, and it is water: 83 percent of root failures happened in saturated soil and 72 percent during rainfall, while 26 percent involved wind above 24 mph. Decay played a role in 81 percent.
The operational consequence is a date rather than a defect. A valley oak whose roots were cut during construction in a dry summer reads as a normal tree at the next patrol and the one after that, and the bill arrives in a wet January. That is a system of record problem before it is an arboriculture problem, because the excavation date in the file is the only thing that connects the two.
What a valley oak record has to carry
The profile says which fields would have predicted the failures in it. Four of them are numbers a pre-inspector can take from the ground in under a minute.
Field | What it decides | Why this species |
Limb diameter at the attachment, and at the point of concern | Whether the limb sits in the 20-to-27-inch failure band | Mean failed diameter is 20 inches along the branch, 27 inches at the union |
Height of the attachment above ground | The strike envelope, independent of clearance | The Point Fire limb was attached at approximately 55 feet |
Trunk-to-conductor distance, measured | Whether clearance rules can reach this tree at all | The Point Fire tree stood approximately 50 feet out, and struck anyway |
Live or dead, recorded for the limb rather than the tree | Whether the Rule 35 trigger is even available | 95 percent of failed branches were live |
Whether any defect is visible, and from where | What the ground look is worth here | Visible in 46 percent of cases, invisible in 35 percent |
Excavation, fill or grade change in the root zone, with a date | The root failure that arrives in a later wet winter | Roots cut or severed in 40 percent of root failures |
Included bark and a visible sporophore belong in the note when they appear, at 5 percent and 7 percent of the relevant cases respectively. What the pre-inspection produces on a valley oak should be a set of measurements, because the utility carrying the Exception 4 risk will be defending a judgment, and a judgment defended with dimensions survives a review that a judgment defended with adjectives fails.
The two October 2017 files are the reason to bother. In one, the decay was real and invisible, and the file closed clean. In the other, the wood was sound, the limb was 50 feet from the wire, and the file closed clean again. A program that means to catch the next one gets there by deciding, on the record, which mature valley oaks it is prepared to leave, and by writing down the measurements behind each decision. Rule 35's trigger had its chance on both of these trees.
Further reading
Gray pine and codominant stems: The same page written for a species that has no published failure profile, and what a program does in that case.
What makes a tree a hazard tree: The defect list and the rating window a heavy lateral limb has to be read against.
GO 95 Rule 35 clearances: Table 1, Cases 13 and 14, and what the radial number does and does not cover.
Frequently asked questions
Does any California rule set a different clearance for valley oak?
No rule names the species. GO 95 Table 1 is written in voltages and distances, and PRC 4293 in voltages and radial feet. A species reaches either number only through the utility's own specification. PG&E's Vegetation Management Hazard Tree Rating and Scoring Procedure does name it: the CPUC's La Porte Fire report records that the procedure "indicates a Valley Oak as a tree with a 'Very High' failure potential." That rating produced no prescription on either tree that started a fire in October 2017.
Is a healthy valley oak limb over a conductor a Rule 35 finding?
On the face of the rule, no. Rule 35 reaches trees when a company has actual knowledge that "dead, rotten or diseased trees or dead, rotten or diseased portions of otherwise healthy trees overhang or lean toward and may fall into a span." In the La Porte Fire investigation the Safety and Enforcement Division applied that language directly and found no violation, because PG&E had no actual knowledge of internal decay that had no outward sign. In the valley oak failure profile, 95 percent of failed branches were live and only 5 percent of branch failures involved a dead failed portion.
Where does the strike risk sit if the tree is outside the clearance radius?
It sits in the height of the attachment. The valley oak that started the Point Fire on October 9, 2017 was rooted approximately 50 feet from the subject conductor. The limb that failed was attached approximately 55 feet above the ground and was approximately 8 inches in diameter. PRC 4293 requires four feet of radial clearance at 12 kV, and that tree stayed well outside four feet of anything.
Should a crew look for a conk before calling a valley oak limb?
A fruiting body is present in a small minority of these failures. The profile reports that no sporophores were found in 93 percent of branch failures associated with decay, with only six cases reporting one. Decay itself was reported in 61 percent of branch failures. On this species, a clean stem tells a crew very little.
What did the two October 2017 valley oak investigations actually conclude?
The La Porte Fire report, dated April 29, 2019, states that "SED did not identify any General Order violations by PG&E." The Point Fire report, dated April 26, 2019, found one violation, of GO 95 Rule 19, for failing to retain a broken crossarm and a damaged section of conductor as evidence. Both files closed without a vegetation management violation. CAL FIRE separately cited PG&E under PRC 4293 and under General Order 64-A on the Point Fire, and SED noted that the CPUC replaced GO 64 and 64-A with GO 95 effective July 1, 1942.
