Eucalyptus and power lines: California's fastest-growing line-clearance problem

No California clearance rule sets a eucalyptus distance. For mature trunks the answer sits in the objective criteria CPUC Decision 97-10-056 required utilities to file by April 20, 1998. The rest lands later: 44 percent of recorded blue gum failures are root failures, UC IPM asks for December pruning, and a cut stump needs three years of checks.

A blue gum over a 12 kV tap owes the same clearance an oak owes at that voltage. The rest of the file differs, and four questions on it are answered somewhere other than the clearance rule:

• how the tree fails

• when it can be cut

• what happens to the wood

• whether the stump stays cut

California's clearance rules never say eucalyptus. GO 95 Table 1 is written in voltages and distances, and PRC 4293 in voltages and radial feet. A species reaches the number through the utility's own specification, and the arithmetic that turns a published figure into an assumed rate belongs to the cycle.

One California decision does name the species. It is worth knowing exactly what it settled.

The one decision that names the species, and what it made utilities file

In 1997 PG&E petitioned the California Public Utilities Commission to exempt old established trees whose trunks sat more than six inches, but less than eighteen inches, from an overhead line. Decision 97-10-056 records why:

PG&E says that there are numerous examples of such trees throughout its service territory, and it appears that many of them are located in aesthetically sensitive communities such as Carmel, and in parks or rural areas where large species such as redwood, pine, and mature eucalyptus trees are growing in proximity to power lines.

Three species named in a Commission decision, because their mature trunks were what the eighteen-inch rule had turned into a removal problem. The scale PG&E put on it was 54,000 trees out of the 1.7 million subject to the pruning requirements, at roughly $4,000 per removal, or $216 million over a four-year compliance period.

The Commission granted the exemption. It became Rule 35's Exception 4, which is where the current text and its risk allocation live. What matters here is the condition the Commission attached, in Ordering Paragraph 1:

Any respondent which intends to avail itself of the provisions of this exemption shall, within 180 days after the effective date of this order, file with the Commission and serve upon the other respondents objective criteria specifying when tree limbs, either large or directionally-pruned, will be routinely considered if adequate strength and stability to prevent contact with lines.

The order was effective the day it was dated, October 22, 1997, which put the filing deadline at April 20, 1998. In the body of the decision the Commission said what it expected those criteria to look like:

For a vertical tree trunk, for example, such criteria might specify, for a given trunk diameter, the maximum height from the ground for which movement in high winds would not reduce the clearance to less than six inches. Such criteria would likely vary according to the species of tree and the weather in each locality.

So for a mature blue gum trunk standing fourteen inches off a primary distribution conductor, the document that decides whether the span is compliant is the utility's filed criteria. The Commission also kept the last word for itself, reserving the final say on whether the exemption reaches any given location. A pre-inspector who cannot produce the criteria cannot make the call, and the utility carries the risk of getting it wrong.

A failure profile for blue gum already exists

The Western Tree Failure Database, run as the California Tree Failure Report Program, collects failure reports filed by tree care professionals and sorts them by species. Its blue gum page reports 268 failures as of August 12, 2010:

Failure type	Share of 268 reports
Root	44%
Branch	42%
Trunk	14%

The trees in those reports had a mean age of 62 years, a mean diameter at breast height of 43 inches, and a mean height of 81 feet.

Eighty-one feet is the number to hold. A distribution conductor hangs well below that, so a tree of this size root-failing in 44 percent of recorded cases is a reach question before it is a clearance question, and reach is decided first. Pruning the canopy back to specification changes nothing about whether the stem lands in the span.

Decay was reported in 61 percent of the root failures, 60 percent of the trunk failures, and 41 percent of the branch failures. That is the part of the tree a drive-by patrol does not see.

Where the branch failures start

Among the branch failures in that same set, 48 percent failed at the point of attachment and 42 percent failed from 1 to 6 feet away from the point of attachment. The defects most often recorded alongside a blue gum failure were heavy lateral limbs at 28 percent, multiple trunks or codominance at 12 percent, and dense crown at 12 percent.

Nine out of ten recorded branch failures began within six feet of the union. On a eucalyptus with a heavy lateral running out over the conductor, that puts the thing worth examining at the trunk end of the limb, not at the tip where the clearance measurement gets taken. The two are often on opposite sides of the tree from the road, which is a question about what assessment level the scope of work bought.

A sound limb comes down on a still afternoon

Summer branch drop is the failure that defeats a weather trigger. Richard Harris, writing in Arboriculture & Urban Forestry in 1983, placed it on hot calm summer afternoons and in calm weather following a heavy summer rain that ends a period of increasing soil dryness. On where the limb breaks:

Most commonly, breakage occurs 1 to 4 m from the branch attachment on long limbs that extend to or beyond the tree canopy.

That is roughly 3 to 13 feet from the union, which runs out past the Western Tree Failure Database's 1-to-6-foot band. One of the California cases Harris cites is a eucalyptus: the failure of the top forty feet of a mature Eucalyptus globulus in Los Angeles in 1977, which seriously injured a child and produced an out-of-court settlement of $1,625,000.

The operational consequence is narrow and worth stating plainly. A patrol scheduled off a red flag warning or a wind event will not be in front of the tree on the day this happens. The condition that precedes it is a hot, still afternoon.

The clearance prune is what invites the borer

Two longhorned borers work eucalyptus in California. UC IPM Pest Notes Publication 7425, dated September 2009, dates their arrivals. Phoracantha semipunctata was introduced into Southern California in the 1980s. Phoracantha recurva was found in 1995, in four counties:

• Los Angeles

• Riverside

• Orange

• San Bernardino

Table 2 of that publication lists blue gum among the species more susceptible to P. semipunctata.

The publication's pruning instruction is one sentence:

Remove infested trees and branches immediately, but do all other pruning during December and January when adult beetles aren't present.

It then explains what a heavy prune does to the bark it uncovers:

extensive pruning suddenly exposes previously shaded bark to direct sunlight, which can result in sunburn (sunscald) cankers that are susceptible to borer attack

A line clearance cycle is routed by circuit and worked in the order the route runs. A eucalyptus-heavy circuit reaching its scheduled window in July gets cut in July. The crew standing under it meets neither condition in the publication: the beetles are flying, and the specification calls for the kind of removal that opens the bark.

UC IPM also advises spacing extensive limb removal over several years where the work is not about dead wood or a hazardous tree, which is a schedule no clearance specification is written to accommodate.

This is a program-level conflict and the program resolves it. A general foreman can flag a circuit for a winter window. A crew leader standing at the pole in July cannot.

The wood left behind has a clock on it

The same publication puts a number on how long cut eucalyptus stays attractive to egg-laying beetles:

Larvae require about 70 days to develop in fresh wood or logs during hot summer months and up to 180 days in drier logs.

Only relatively fresh logs sustain the larvae. Its instruction for infested material is to bury, burn where permitted, or chip it, and it notes that removing bark from felled logs shortens the window during which the wood can support beetle development.

Take a removal worked on July 14. The rounds are cut and stacked at the base of the pole for the landowner to take as firewood, which is a common and reasonable arrangement. Seventy days later is September 22, and the pile has functioned as a nursery for one full generation in a stand the utility still has to maintain.

Chipping the same material and hauling it off closes that. What leaves the site and what stays is a decision made at the unit, and on eucalyptus it has a pest consequence that other species do not carry.

The stump is still a tree

Cut a blue gum and the stump sprouts. Hamilton and McHenry, working on Eucalyptus globulus stumps between 1973 and 1978 and publishing in Arboriculture & Urban Forestry in December 1982, located the buds:

Most sprouts originate at the ground surface and none are attached deeper than 4 inches below the ground line.

That is why grinding works and why cutting a sprout off the side of a stump does not. Their ax-frill herbicide treatments averaged 5 to 6 minutes per stump, which they put at 10 to 12 stumps per hour per person.

The timing is tighter than the treatment. The blue gum entry in The Weed Workers' Handbook, published by The Watershed Project and the California Invasive Plant Council, reports the practitioner window:

Herbicide must be applied within 5 minutes, and preferably within 1 minute after cutting, while the cambium can still transport the herbicide into the roots.

One minute. The saw comes out of the cut and the treatment goes on before the crew moves to the next stem. That has two consequences for how the work gets scheduled. The first is that a follow-up visit to treat stumps cut last week is a different and less effective operation. The second is that the person applying the herbicide needs a credential the faller does not: herbicide application for hire sits under the Department of Pesticide Regulation's licensing, separate from the contractor license.

Then the handbook sets the follow-up period: check for resprouts for at least 3 years or more. A removal unit closes on the day the chips are hauled. The tree it removed has three growing seasons left in it, and nothing in the way a removal is recorded and closed creates the revisit on its own.

The litter refills the pole clearing between visits

PRC 4292 work is a clearing with a completion date on it. 14 CCR 1254 sets what comes out at ground level inside that space: "flammable materials, including but not limited to, ground litter, duff and dead or desiccated vegetation that will allow fire to spread." The ten feet is a cylinder rather than a circle, and the ground-level standard is the layer eucalyptus works against.

The US Fire Administration's technical report on the October 1991 East Bay Hills fire, USFA-TR-060, describes the supply:

The eucalyptus has a tremendous production of both leaf and bark litter, which is not readily consumed or broken down in the normal decomposition process and leads to the presence of high volumes of fuel.

It puts fuel accumulations under some East Bay eucalyptus plantations at between 30 and 40 tons per acre. The Forest Service's Fire Effects Information System describes what that litter does once lit: "deciduous bark streamers and lichen epiphytes tend to carry fire into the canopy and to disseminate fire ahead of the main front."

A subject pole cleared in June under a eucalyptus canopy has bark and leaf litter inside the ten feet again by fire season's end. The clearing record shows one date and one condition. The condition is current on that date.

One cold week rewrites the inventory

Eucalyptus responds to freezing weather by dying in numbers, and the fuel it leaves does not fall over. FEIS records the 1972 case:

In December, 1972, the San Francisco Bay Area experienced a severe cold snap, resulting in extensive frost damage to Tasmanian bluegum trees. Frost-killed leaves and twigs increased Tasmanian bluegum litter ten-fold.

USFA-TR-060 records the second one. An unusual period of freezing weather in December 1990 "killed massive quantities of the lighter brush and eucalyptus." The report ties it back to the first, noting that large numbers of eucalyptus had been killed the same way in 1972.

For a vegetation program this is a condition change that no growth model predicts and no cycle catches. A stand recorded as healthy in November can be standing dead fuel in January. That matters to two rules at once. Rule 35 reaches a dead, rotten or diseased tree on actual knowledge rather than on the cycle.

Under 14 CCR 1254, dead, diseased or dying trees inside the pole cylinder come out in their entirety above eight feet, whatever their distance from a conductor. Neither obligation waits for the next scheduled pass, and a hard freeze in a eucalyptus corridor is a reason to go look.

What the eucalyptus record has to name

A record built for this species carries four things a general vegetation entry does not. The first is the species rather than the genus. A row reading eucalyptus leaves the tree anywhere in a genus whose members differ in height, form and susceptibility, and that is the distinction the record keeps losing. The other three follow from the work above.

An entry that survives a later read looks like this. Pole 4412, Cazadero 1103 circuit, 12 kV, Eucalyptus globulus, 38-inch DBH, 76 feet tall, 22 feet from the pole face and leaning into the span. Removal prescribed on the strength of a 76-foot height against a 34-foot conductor attachment and root-collar excavation showing decay on the uphill side.

Felled and treated January 14, 2027, cut-stump glyphosate applied within one minute of the felling cut by a licensed applicator named in the entry. All rounds and bark chipped and hauled. Resprout inspection scheduled for the spring of 2028, 2029 and 2030.

Four of those elements would be unusual on an oak removal: the pruning month, the one-minute treatment window, the disposal instruction, and the three-year resprout schedule. Each of them exists because of something in a published source, and each of them is the difference between a eucalyptus unit that closes and one that comes back.

Where this leaves the reader

The clearance distance on a eucalyptus is the same distance as on anything else at that voltage. Everything the species adds sits outside the clearance rule:

• the utility's filed criteria, for a mature trunk inside the ordinary clearance

• a failure profile weighted toward root failure

• a pruning month set by an insect

• a stump treatment measured in minutes

• litter that reloads the pole clearing on its own schedule

Where the rule is silent, somebody still decides. On the mature-trunk exemption the Commission said who. The utility bears the risk of the determination, and the Commission rules afterward on whether the exemption held. On the pruning month and the disposal method no California rule speaks at all, and the call belongs to the program that writes the specification.

Further reading

• Fast-growing species and where the rules let growth into the number: Why a record that says eucalyptus instead of Eucalyptus globulus stops being useful.

• Growth rate and the trim cycle: The arithmetic that turns a published clearance figure into an assumed rate per year.

• PRC 4292 pole firebreaks: The ten-foot cylinder that eucalyptus litter refills between clearings.

Frequently asked questions

Does GO 95 or PRC 4293 set a different clearance for eucalyptus?

Neither one names a species. Both are written in distances, voltages and conditions. A species reaches the number two ways. The first is the utility's own specification, which is where any published clearance figure larger than the code minimum comes from. The second is narrower and applies only to mature trunks and major limbs. CPUC Decision 97-10-056 required any utility relying on the Rule 35 mature-tree exemption to file objective criteria with the Commission, and the decision said those criteria would likely vary according to the species of tree and the weather in each locality. For a mature blue gum trunk sitting inside the ordinary clearance, the document that answers the question is the utility's filed criteria.

Is a eucalyptus removal authorized because the species is invasive?

The California Invasive Plant Council rates Eucalyptus globulus Limited on its Inventory, which the Council describes as species that are invasive but whose ecological impacts are minor on a statewide level or for which there was not enough information to justify a higher score. The Inventory is an ecological rating. No removal authority travels with it. Authority to take a eucalyptus down in the corridor comes from the clearance requirement, from a hazard tree finding, or from the property owner.

When should eucalyptus be pruned to keep the longhorned borer out?

UC IPM Pest Notes 7425 splits it. Infested trees and branches come out immediately. For everything else the publication says to do all other pruning during December and January when adult beetles aren't present. It also warns that extensive pruning suddenly exposes previously shaded bark to direct sunlight, producing sunscald cankers that are themselves susceptible to borer attack. A routed clearance cycle that reaches a circuit in July matches neither instruction, and that conflict is the program's to resolve rather than the crew's.

What has to happen to the wood cut off a eucalyptus?

UC IPM 7425 reports that Phoracantha larvae require about 70 days to develop in fresh wood or logs during hot summer months and up to 180 days in drier logs, and that only relatively fresh logs sustain them. Its instruction for infested material is to bury, burn where permitted, or chip it. Removing bark from felled logs shortens the window the wood can support beetle development. A stacked pile of green eucalyptus rounds left at the base of the pole through August is a breeding site with a date on it.

How long does a cut eucalyptus stump have to be watched?

The Weed Workers' Handbook entry for blue gum says to check for resprouts for at least 3 years or more. The reason the follow-up runs that long is where the buds sit. Hamilton and McHenry, working on Eucalyptus globulus stumps between 1973 and 1978, reported that most sprouts originate at the ground surface and none are attached deeper than 4 inches below the ground line. Grinding or cutting below a high shoot reaches those. A shoot coming from the ground line or the roots has to be cut and treated where it stands.