Tree growth regulators: why a utility treats instead of cutting
A tree growth regulator is a pesticide. Food and Agricultural Code section 11408 makes regulating plant growth an agricultural use, and a right-of-way is not an excluded setting. That brings business licensing, a signed written recommendation, two years of use records, and a monthly county report. It slows regrowth; it does not create clearance.
The federal label for the growth regulator most often used on utility trees carries two lists of species. Table 2 is headed LANDSCAPE APPLICATION. Table 3 is headed RIGHTS-OF-WAY APPLICATIONS. They name many of the same trees and put them in different dosage categories, and the rights-of-way table splits several species at 10 inches diameter at breast height where the landscape table gives one number.
The label goes further than that. Inside its dosing instructions, under Dosage Reduction Considerations, it names line clearance work directly:
Canopy Missing: Look at the canopy of the tree and compare it to a "normal" canopy for that trunk diameter. For example, if a tree is missing large branches from storm damage or utility line clearance pruning it is necessary to estimate the percentage of canopy missing and subtract this percentage from the dosage amount. i.e. subtract 30% from dosage if 30% is missing from the canopy.
The corridor changes the dose. A tree that was V-trimmed last cycle gets less product than the same tree in a park, because there is less crown left to regulate.
That is the working half of the subject. The other half is that the rules governing the treatment itself sit outside General Order 95. A growth regulator is a pesticide, and in California the code that reaches it is the Food and Agricultural Code.
The California definition of agricultural use names growth regulation
Nothing dies, so a crew new to pesticide work tends to file this treatment somewhere else. The statute settles it in its first sentence. Food and Agricultural Code section 11408 reads:
"Agricultural use" means the use of any pesticide or method or device for the control of plant or animal pests, or any other pests, or the use of any pesticide for the regulation of plant growth or defoliation of plants.
Regulating plant growth is agricultural use by definition. The section then excludes five settings, and a utility right-of-way falls outside all five:
Home use.
Use in structural pest control.
Industrial or institutional use.
Control of an animal pest under the written prescription of a veterinarian.
Local districts operating under a cooperative agreement with the State Department of Health Services.
The Department of Pesticide Regulation says the same thing in its enforcement manual. Chapter 4 of Volume 1 of the Pesticide Use Enforcement Program Standards Compendium lists what has to be recorded and reported, and rights-of-way are on the list: "All pesticides used in non-production agricultural use settings (such as cemeteries, golf courses, parks, ditch banks, and rights-of-way)."
The permit regime passes over this active ingredient. DPR's page for paclobutrazol records it as a growth regulator, chemical class triazole, DPR chemical code 2259, with "California Restricted Material: No" and "Potential Groundwater Contaminant: No." The restricted materials list at 3 CCR 6400 omits it, and so do both groundwater protection lists at 3 CCR 6800. That drops two filings, the restricted materials permit and the notice of intent. Everything else in the pest control apparatus still applies.
What the label permits, and the table written for the corridor
Cambistat is registered as EPA Reg. No. 74779-3 to Rainbow Treecare Scientific Advancements of Minnetonka, Minnesota. The active ingredient is paclobutrazol at 22.3 percent, the container holds 2 pounds of active ingredient per gallon, and the signal word is CAUTION. The label's General Information paragraph is the product claim EPA actually accepted:
Cambistat™ is a plant growth regulator that slows the vegetative growth of plants by inhibiting gibberellin biosynthesis. Cambistat™ is designed to gently and predictably slow the growth of trees. A single application provides a long lasting reduction of vegetative growth, effectively extending the trimming cycle of trees and reducing the amount of woody growth that must be removed.
The same section names the sites: "Cambistat™ may be used on utility rights-of-way, residential areas, urban areas, and other non-crop areas."
In California the federal registration is half of it. DPR's California Product/Label Database carries CAMBISTAT under California registration number 74779-3-AA, status ACTIVE, with an active date of March 14, 2008. Food and Agricultural Code section 12973 then makes the label binding:
The use of any pesticide shall not conflict with labeling registered pursuant to this chapter which is delivered with the pesticide or with any additional limitations applicable to the conditions of any permit issued by the director or commissioner.
Which returns the question to the two tables. Each species gets a letter, and the letter sets a rate of ready-to-use solution per inch of trunk diameter.
Dosage category | Ready-to-use solution per inch DBH |
A | 75 ml |
B | 100 ml |
C | 125 ml |
D | 138 ml |
E | 150 ml |
F | 200 ml |
Here is what the corridor table does to five species:
Species | Table 2, landscape | Table 3, rights-of-way |
American basswood | A | A under 10 in. DBH, B over |
Norway maple | B | B under 10 in. DBH, C over |
Elm, all others | B | B under 10 in. DBH, C over |
Live oak | B under 10 in. DBH, E over | C under 10 in. DBH, E over |
Eucalyptus | F | F |
In each of the four species that differ, the corridor table splits the entry at 10 inches DBH and puts at least one class a category higher. On a 20-inch elm, category B is 2,000 ml of ready-to-use solution and category C is 2,500 ml. Eucalyptus is already at the top of the scale in both tables, so the corridor version changes nothing for it.
The corridor sets the dose, because the corridor already took the crown
Work a real tree through it. A 24-inch valley oak under a distribution span is category E in the rights-of-way table, which is 150 ml per inch, so 3,600 ml of ready-to-use solution. The label builds that from 300 ml of Cambistat and 3,300 ml of water, and at 250 ml delivered per injection site it comes to 14.4 sites spaced around the root flare.
Now apply the canopy rule. Valley oak carries heavy lateral limbs, and a side trim to clear the conductor may have taken a third of the crown. Subtract 30 percent and the dose is 2,520 ml. The fourteen injection sites become ten.
Estimating that percentage from the ground three years after the cut takes a record of what came off. What the pre-inspection produces and which trim type was prescribed are the two records that make the dosing decision possible, and a program that has used them only for billing will find they are now an input to a label instruction.
The rest of the application method is short and specific. Injections go 2 to 6 inches deep at 50 to 200 psi, oriented to release horizontally, divided evenly around the base, with at least four sites on any tree under 6 inches DBH. Timing gets two options:
For a more manicured look, apply Cambistat™ to trees 30 to 180 days before they are pruned. To allow some regrowth and a more natural look, apply Cambistat™ at the time of pruning.
The label also sets the interval: "Retreat every 3 years or wait until the effects from the previous application subside."
The treatment does not produce clearance
GO 95 Rule 35 owes a distance measured from the conductor, and a soil injection moves no branch. The label is honest about the lag: "Visible results may be seen in as little as 2 months but measurable growth reduction may take as long as a year to occur." The clearance is cut on the day the crew is there, measured at time of trim, and the treatment goes to work on what happens afterward.
How much it does is the number worth having, and there is one published trial on an energized distribution circuit. Haugen and colleagues, in Arboriculture & Urban Forestry 42(2), treated 48 live oaks on an Entergy Louisiana circuit in Baton Rouge.
Trees were pruned to Entergy's 3.048 m clearance standard in late January and early February of 2010, treated at eight timings from 180 days before to 180 days after, and re-pruned three growing seasons later in late January and early February of 2013. Pruning wounds were painted orange at the cut and repainted in 2011 and 2012 so the regrowth could be measured from the original scar.
Over three growing seasons | Mean branch regrowth |
Untreated control live oaks | 285.8 cm |
Paclobutrazol-treated live oaks | 187.6 cm |
Treated trees grew 187.6 cm of new branch, which is six feet two inches, and the reductions across the eight timings ran from 29.7 percent to 43.2 percent. The study found the best window to be 90 days before through 90 days after the prune, with economic gains still present out to 180 days either side.
So the honest framing of this tool is not that treating replaces cutting. It is that treating changes what the next cut costs. The same study measured an 84 percent biomass reduction at its best timing and 56 to 68 percent at the others, and after subtracting the nine minutes the application itself took, a 25 to 56 percent reduction in labor.
The label's own claims sit further out than that. Its OPTIONAL MARKETING CLAIMS section, which EPA accepted as claims rather than as measured performance, includes "Reduced [utility] line clearance costs [up to 60%]" and "Up to 40 to 90% reduction in regrowth [for 3 years]." The brackets are in the original. A program writing a business case is better served by 29.7 to 43.2 percent on one species on one circuit, because that is the figure with a method behind it.
Where treating really does replace cutting
There is one case where the alternative on the table is removal, and the customer has said no.
The label points straight at it. Cambistat "will also benefit trees that are too large for their growing site and increase the longevity of trees growing in stressful environments." Its accepted claims then name the situations one at a time:
Difficult to access trees, such as in backyards.
Cycle busters, described on the label as problem trees.
Zone 1 trees, immediately out of the substation.
Critical feeder lines serving hospitals and industry.
Concerned or difficult customers with right-of-way trees.
Read against how the utility decides between removal and pruning and what happens when a property owner refuses, that list is a negotiating position. A tree that is structurally sound but simply too tall for the span is the one case where a treatment genuinely substitutes for a saw, because the reason for removal was growth rather than defect. A tree on the removal list for death, decay or lean stays on it, because the product reaches growth and nothing else.
Who may apply it, and who signs the recommendation
A line clearance tree trimmer is qualified to work near energized conductors. Pesticide application sits outside that qualification, and the two are separate records. Food and Agricultural Code section 11701 sets the first bar:
It is unlawful for a person to advertise, solicit, or operate as a pest control business, unless the person has a valid pest control business license issued by the director.
DPR's Qualified Applicator License is the individual credential behind that business license, and it is issued by category. There are thirteen. Category B is Landscape Maintenance and category C is Right-of-Way, and a corridor program wants C.
An applicant passes the Laws, Regulations, and Basic Principles examination plus at least one category examination at 70 percent or higher. The application fee is $180 for this license and $115 for each examination requested. After the first renewal the holder accumulates at least 20 hours of approved continuing education every two years, four of which must be pesticide laws and regulations.
Then there is the paper that has to exist before anyone injects anything. Food and Agricultural Code section 12003 governs the recommendation:
Agricultural pest control advisers shall put all recommendations concerning any agricultural use in writing. One copy of each such written recommendation shall be signed and dated and shall be furnished to the operator of the property prior to the application.
The contents are set by 12003(a) through (h) and extended by 3 CCR 6556, which adds total units treated, concentration and volume, re-entry and label restrictions, the criteria used for determining the need for the treatment, and a certification that alternatives were considered.
Here the code asks for something a growth regulator lacks. 12003(b) asks for "the identity of each pest to be controlled," and 6556 closes with "the recommendation shall designate the pest by accepted common name." There is no pest.
The statutory answer is the phrase "when applicable" in 12003's lead-in, which carries a growth regulator through a form built around a pest, and the practical answer is whoever takes the filing. The county agricultural commissioner decides what a complete recommendation looks like in that county, and the utility's own specification decides whether one is written at all.
At least one California utility wrote the answer into its specification. Pacific Gas and Electric's Vegetation Management Environmental Best Management Practices, created March 29, 2004 and revised March 1, 2006, was filed with the Commission with the Santa Cruz 115 kV Reinforcement record. BMP 33 requires that "Each application shall be covered by a written 'Pest Control Recommendation.'" BMP 34 names this class of product directly:
A Licensed Pest Control Advisor shall oversee all herbicide and tree growth regulator applications. A qualified applicator shall supervise contractors making herbicide and tree growth regulator applications for VM.
The paperwork the trim never created
A pruning unit produces a record inside the utility's own system. A treatment produces a filing with a county. That change of kind is the one most programs underestimate.
Requirement | What it asks for | When |
An operator identification number from the commissioner of each county, for non-production agricultural use expressly including right-of-way | Before purchase and use | |
Date and time, operator of the property, location, site treated, units treated, and the pesticide with its EPA or state registration number and amount | Each application | |
3 CCR 6624(g) | The same records, retained and produced on request | Two years |
A monthly summary of use to the commissioner of the county where the work was performed | By the 10th of the following month |
6622(c) also settles who carries the number. The operator of the property is relieved of the operator identification number "when a person performing pest control for hire purchases and applies these pesticides." A utility whose licensed contractor buys and injects the product is in that sentence. A utility whose own crews buy it is not, and needs a number from every county it works in.
PG&E's specification assigns two of them. BMP 32 puts operator ID numbers and site ID numbers on each facility as the county agricultural commissioner directs, and BMP 36 puts the monthly report on the pest control business license holder rather than on the utility. That allocation is a contract term. The system of record still has to show which one of them filed, because the commissioner's copy and the utility's copy are the only two proofs that the work happened at all.
Which trees the label rules out
Three of the label's own restrictions eliminate a large share of what a California corridor actually contains.
The first is condition. "Do not treat severely stressed trees or trees in rapid decline," and for trees short of that, "Reduce the dosage rate on highly stressed trees by 25% or more." A tree carrying goldspotted oak borer, the invasive shot hole borer complex, Mediterranean oak borer or a bark beetle complex is a candidate for a different decision entirely.
The second is species response. Cottonwood, pines and spruce carry an asterisk on both tables: "These species typically show less growth reduction compared to other species." That footnote covers most of the conifer inventory on a Sierra foothill circuit, including the pines line clearance crews watch for pitch canker on. At the other end, dogwood and Japanese maple "are very sensitive to Cambistat and can be easily over regulated," with field reports suggesting half the category A rate.
The third is site. Two label precautions land squarely on California terrain. The first directs the applicator to avoid basal drench applications "on inclines and other areas where treated soil is likely to be washed away from the base of the tree by rainfall or irrigation."
The second warns that "shrubs and/or herbaceous ornamentals next to treated trees may be affected if their roots extend into the treatment zone." A cut slope under a span leaves injection as the only method. A landscaped yard puts the treatment zone inside somebody's planting bed.
Species response is not uniform even where the label is silent. Sperry and Chaney found in Arboriculture & Urban Forestry 25(1) that flurprimidol, a different growth regulator active ingredient, suppressed shoot curvature toward light in silver maple at every concentration tested, and that neither curvature nor shoot growth was significantly reduced in American sycamore. Directional pruning creates the light gap that pulls regrowth back toward the wire, and whether a growth regulator blunts that pull is a species question rather than a product question.
The soil under the tree is usually not the utility's
A prune happens in the air, inside an easement written for conductors. A soil injection happens 2 to 6 inches down at the root flare, and on a distribution circuit that ground is often a homeowner's front yard.
Nothing in GO 95, the Public Resources Code or the Food and Agricultural Code answers whether an easement drafted for line clearance reaches a subsurface chemical application on the servient estate. What the right-of-way width does and does not grant is a question of the instrument, and instruments differ. The decision belongs to the utility's land rights function and to the property owner, and the practical form it takes is written consent obtained before the applicator arrives.
That consent is also where the non-target language on the label becomes somebody's lawn. Localized stunting of turfgrass adjacent to the treatment site is a listed possibility. A tree the owner asked to keep is the reason the treatment was proposed, and a stunted strip of grass around its base is the reason the conversation happens twice.
What a growth regulator record has to carry
The trim record and the treatment record are two documents, and a program that files the second inside the first will find the county asking for fields that only the second carries. What a closed unit has to show covers the pruning side. This is what sits alongside it.
Field on the treatment record | What the county or the label asks it to show |
Tree ID and location | Pole or span, plus the address, because 6624(b)(3) asks for the location of the property treated |
Species and DBH | The two inputs to the Table 3 category and the per-inch rate |
Category and full-label dose | The letter, the ml per inch, and the total before reduction |
Canopy missing | The percentage subtracted, and the trim that removed it |
Dose applied, and method | Final volume, soil injection or basal drench, number of sites |
Product and registration numbers | Both the EPA number and the California number, which is what 6624(b)(6) and 6627(b)(5) ask for |
Date and time | 6624(b)(1), and the day the three-year retreat clock starts |
Recommendation | Adviser, signature, date, and the copy furnished before the application under FAC 12003 |
Applicator | License or certificate number, and the business license the work was performed under |
County filing | Which county, and the month the summary went in under 6627 |
Consent | The property owner's written permission for a soil application |
Eleven fields, of which two are signatures and three are dates. A pruning unit needs none of them.
The last one worth stating plainly is the retreat clock. The label says retreat every three years, the Baton Rouge trees were measured at exactly three growing seasons, and a treated tree at the end of that window has put on most of the growth an untreated tree would have. The cycle the program runs on stays where it was. What changes is how much wood is waiting when the crew comes back, and whether anybody wrote down that the tree was treated at all.
Further reading
GO 95 Rule 35 and the minimum clearances: The distance owed at the wire, which a growth regulator does not change.
Growth rate and the trim cycle: How regrowth between visits sets the interval a treatment is trying to stretch.
Tree removal versus pruning: how the utility decides: The decision a growth regulator is sometimes offered as an alternative to.
Frequently asked questions
Is a tree growth regulator a pesticide in California?
Yes. Food and Agricultural Code section 11408 defines agricultural use as the use of any pesticide or method or device for the control of plant or animal pests, or any other pests, or the use of any pesticide for the regulation of plant growth or defoliation of plants. The section excludes home use, structural pest control, industrial or institutional use, animal pest control under a veterinarian's written prescription, and certain public health districts. A utility right-of-way is none of those, so a growth regulator applied to a right-of-way tree is an agricultural use and carries the full record and reporting obligations.
Does a growth regulator application have to be reported to the county?
Yes. 3 CCR 6624 requires records of each application, including date and time, the operator of the property, the location, the site treated, the units treated, and the pesticide with its EPA or state registration number and amount used. Those records are retained two years. 3 CCR 6627 then requires a monthly summary report to the commissioner of the county where the work was performed, by the 10th day of the following month. DPR's own enforcement compendium lists rights-of-way among the non-production agricultural settings whose pesticide use must be reported.
Does treating a tree satisfy GO 95 Rule 35?
No. Rule 35 owes a clearance measured at the conductor, and a growth regulator moves no branch. The Cambistat label states that visible results may be seen in as little as 2 months but measurable growth reduction may take as long as a year to occur. The clearance still has to be cut, and the treatment changes how fast the tree closes the gap again.
How much regrowth does paclobutrazol actually prevent?
In the one published trial on a utility distribution circuit, less than half. Haugen and colleagues treated 48 live oaks on an Entergy Louisiana circuit in Baton Rouge and re-pruned them after three growing seasons. Untreated trees had a mean branch regrowth of 285.8 cm over three years. Treated trees had a mean of 187.6 cm. The reductions ranged from 29.7 percent to 43.2 percent depending on when the application fell relative to the prune.
Who is allowed to apply a tree growth regulator on a right-of-way?
Not the line clearance crew by default. Food and Agricultural Code section 11701 makes it unlawful to advertise, solicit, or operate as a pest control business without a valid pest control business license issued by the director. DPR's Qualified Applicator License carries a category C for Right-of-Way, obtained by passing the Laws, Regulations, and Basic Principles examination plus the category examination at 70 percent or higher. A line clearance tree trimmer's qualification is a separate thing and does not reach pesticide application.
