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I. INTRODUCTION 

This document is the Environmental Protection Agency’s (EPA or the Agency) Interim 

Registration Review Decision for paclobutrazol (case 7002, PC code 125601) and is being issued 

pursuant to 40 CFR sections 155.56 and 155.58. A registration review decision is the Agency's 

determination whether a pesticide meets, or does not meet, the standard for registration in the 

Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). The Agency may issue, when it 

determines it to be appropriate, an Interim Registration Review Decision before completing a 

registration review. Among other things, the Interim Registration Review Decision may: 1) 

require new risk mitigation measures; 2) impose interim risk mitigation measures; 3) identify 

additional data or other information required to complete the review; and 4) include schedules 

for submitting the required data, conducting the new risk assessment, and completing the 

registration review. For further information on paclobutrazol, additional documents can be 

found in EPA’s public docket (EPA-HQ-OPP-2006-0109) at www.regulations.gov. 

FIFRA, as amended by the Food Quality Protection Act (FQPA) of 1996, mandated the 

continuous review of existing pesticides. All pesticides distributed or sold in the United States 

generally must be registered by the EPA based on scientific data showing that they will not cause 

unreasonable risks to human health or to the environment when used as directed on product 

labeling. In 2006, the Agency implemented the registration review program pursuant to FIFRA 

section 3(g) and will review each registered pesticide every 15 years to determine whether it 

continues to meet the FIFRA standard for registration. The registration review program is 

intended to make sure that, as the ability to assess and reduce risk evolves and as policies and 

practices change, all registered pesticides continue to meet the statutory standard of no 

unreasonable adverse effects. Changes in science, public policy, and pesticide use practices will 

occur over time. Through the registration review program, the Agency periodically re-evaluates 

pesticides to make sure that as these changes occur, products in the marketplace can continue to 

be used safely. Information on this program is provided at 

www.epa.gov/oppsrrd1/registration_review/. 

The first pesticide product containing paclobutrazol was registered in 1985. 

Paclobutrazol is a systemic plant growth regulator registered for use to produce dense, compact 

plants. Paclobutrazol is currently registered for use on ornamentals in greenhouses and 

nurseries. It is also registered for use on turf grass (including in parks, athletic fields, golf 

courses, and rights-of-ways). It is applied as a tree injection, as a soil injection/basal tree drench, 

and as a seed treatment for cucurbit, tomato, pepper, cabbage, broccoli, and cauliflower. There 

are no registered residential uses of paclobutrazol. Paclobutrazol was not subject to the 

reregistration program because products containing paclobutrazol were first registered after 

November 1984. Pursuant to 40 CFR section 155.50, EPA formally initiated registration review 

for paclobutrazol (case 7002) in March 2007. The following highlights are significant events 

that have occurred during the registration review of paclobutrazol and can be found in the public 

docket (EPA-HQ-OPP-2006-0109, accessible at www.regulations.gov). 

 March 28, 2007 – The Paclobutrazol Summary Document was published for a 60-day 

public comment period. The Summary Document included the Preliminary Work Plan

(PWP) and was accompanied by the human health scoping document and ecological fate 

and effects problem formulation. Five public comments were received during the 60-day 
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comment period for the paclobutrazol PWP. The public comments received did not result 

in changes to the Agency’s work plan for paclobutrazol. 

 August 29, 2007 – The Paclobutrazol Final Work Plan (FWP) was published. In the 

FWP, the Agency responded to the comments received concerning the PWP, the problem 

formulation, scoping document and other supporting documents in the registration review 

docket. 

 September 19, 2008 – The Generic Data Call-In Notice (GDCI) for paclobutrazol was 

issued. The GDCI required the following studies for the registration review of 

paclobutrazol: 

o Guideline 830.7050—UV/visible absorption 

o Guideline 850.1400—Fish early-life stage toxicity 

o Guideline 850.2300—Avian reproduction test 

o Guideline 850.4100—Terrestrial plant toxicity (seedling emergence) 

o Guideline 850.4150—Terrestrial plant toxicity (vegetative vigor) 

 September 27, 2013 – The Agency placed in the docket the Registration ReviewEcological Risk Assessment and Effects Determination of Paclobutrazol and the

Paclobutrazol: Preliminary Human Health Risk Assessment for a 60-day public comment 

period. During that time, three public comments were received from Syngenta, Crop Life 

America, and an anonymous private citizen. To address uncertainty in the human health 

risk assessment regarding paclobutrazol’s neurotoxicity, Syngenta subsequently 

submitted acute neurotoxicity data to EPA (with the understanding that these data were 

not required under the GDCI for paclobutrazol). New acute neurotoxicity data warranted 

revisions to the Agency’s Paclobutrazol: Preliminary Human Health Risk Assessment. 

Syngenta also submitted new data for freshwater invertebrates, estuarine/marine 

invertebrates, terrestrial plants, and aquatic vascular and non-vascular plants, which also 

warranted minor revisions to the Agency’s original ecological risk conclusions. 

 February 2, 2015 – The Agency published the Response to Public Comments on the 

Paclobutrazol Registration Review Ecological Risk Assessment and Paclobutrazol: 

Response to Registrant Comments from Syngenta on the Preliminary Human Health Risk 

Assessment (September 11,2013;DP Barcode:D394230).

 August 10, 2015 –The Agency published the Paclobutrazol Proposed Interim Decision, 

along with supporting documents, for a 60-day comment period. No comments were 

submitted to the Agency during this comment period. Supporting documents for the 

paclobutrazol proposed interim decision include the following: 

o The addendum to the environmental fate and ecological risk assessment for 

paclobutrazol, titled Revised Chronic Mammalian Risk Conclusions for the 

Paclobutrazol Registration Review Ecological Risk Assessment. The addendum 

refines the original mammalian risk assessment using a revised chronic rat 

endpoint. 
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o The addendum to the environmental fate and ecological risk assessment for 

paclobutrazol, titled Amended Terrestrial Plant Risk Quotients for the 

Paclobutrazol Registration Review Ecological Risk Assessment. The addendum 

revises the terrestrial plant RQs to reflect newly submitted seedling emergence 

data. 

o The Paclobutrazol: Final Human Health Risk Assessment for Registration 

Review, which refines the original human health risk assessment based upon 

submission of neurotoxicity data. 

 December 2015 – The Agency is now publishing the Paclobutrazol Interim Registration 

Review Decision on www.regulations.gov under docket number EPA-HQ-OPP-2006-

0109. 

II. USE AND USAGE 

Paclobutrazol is a systemic plant growth regulator that slows vegetative growth by 

inhibiting cell elongation. It is registered for use to produce dense, compact growth in plants. 

Paclobutrazol is currently registered for use on turf grass (including in parks, athletic fields, gold 

courses and rights of ways), on ornamentals in greenhouses and nurseries, as a tree injection, as a 

soil injection/basal tree drench and as a seed treatment for cucurbit, tomato, pepper, cabbage, 

broccoli and cauliflower. There are no registered residential uses of paclobutrazol. 

The majority of paclobutrazol applications are: golf course turf, landscaping settings and 

nurseries and greenhouses. Paclobutrazol is used on turf to provide suppression of annual 

bluegrass (where creeping bentgrass is desired), provide suppression of dollar spot turf disease, 

and to suppress vertical growth. Thus, use of paclobutrazol in golf course turf maintenance 

provides both aesthetic as well as economic benefits (i.e., reduced mowing and maintenance 

costs). 

Use of paclobutrazol in the ornamental sector provides certain benefits as well. 

Paclobutrazol promotes uniform flowering and improves cutting quality and yield, while neither 

causing a delay in flowering nor reducing flower size. Paclobutrazol is also noted to provide a 

broad-range of control across many species (email communication from a technical registrant, 

June 10, 2015). 

III. SCIENTIFIC ASSESSMENTS 

 A. Human Health Assessment 

The HED scoping document, its supporting documents, and the most updated Agency 

science policies and risk assessment methodologies were used to prepare a quantitative risk 

assessment in support of the registration review of paclobutrazol. A preliminary human health 

risk assessment for registration review was completed in September 2013, entitled, 
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Paclobutrazol Preliminary Human Health Risk Assessment for Registration Review. A revised 

human health risk assessment was completed in January 2015, entitled Paclobutrazol: Final 

Human Health Risk Assessment for Registration Review, which incorporated new neurotoxicity 

data and revised toxicological endpoints. For these and other documents, please see the public 

docket EPA-HQ-OPP-2006-0109, which can be accessed on http://www.regulations.gov/. 

1. Risk Estimates 

Dietary Exposure Assessment 

Paclobutrazol does not have food uses, therefore the dietary risk assessment only 

considered exposure from drinking water. An acute dietary risk assessment (drinking water 

only) was performed that incorporated potential drinking water residues (see Revised Tier I 

Drinking Water Assessment for the Registration Review of Paclobutrazol on Turf (commercial, 

ornamental and golf courses), Ornamental and Shade Trees, and Seed Treatment of Tomato, 

Pepper, and Cucurbit, dated March 13, 2014). The resulting acute and chronic drinking water 

risk estimates were below the Agency’s level of concern (LOC) where estimated exposure for 

the general population was 17% of the acute population adjusted dose (aPAD) and was 54% of 

the aPAD for infants <1 year old, (the most highly exposed population subgroup). Chronic 

dietary exposure was estimated to be 12% of the chronic population adjusted dose (cPAD) for 

the general population and 31% of the cPAD for infants <1 year old, (the most highly exposed 

population subgroup). 

Residential Exposure Assessment 

Paclobutrazol is not registered for homeowner use in residential settings, therefore, a 

residential handler assessment was not conducted. However, a residential post-application 

assessment was performed because paclobutrazol application to turf, in settings such as parks 

and golf courses, is expected to result in residential post-application exposure (via dermal and 

incidental oral routes of exposure). Residential post-application risk estimates did not exceed the 

Agency’s LOC (where the LOC for margin of exposure (MOE) is 100). Dermal MOEs range 

from 170 to 16,000, and oral non-dietary MOEs (from hand-to-mouth activity on treated turf) 

range from 210 to 1,400,000. 

 

The Agency considered indirect residential exposure to children and adults who may 

have contact with paclobutrazol residues assumed to have been deposited via spray drift. The 

Agency determined that the residential post-application exposure risk assessment was protective 

for exposure from spray drift and therefore, it was determined that a spray drift residential risk 

assessment was not needed. 

Aggregate Exposure Assessment 

Aggregate risk estimates take into account both dietary (food and drinking water) and 

residential sources of exposure. Since paclobutrazol does not have food uses, the aggregate 

assessment for paclobutrazol included only drinking water exposure estimates plus residential 

post-application exposure estimates. 

 Acute aggregate risk is equivalent to drinking water exposure only and therefore 

is not of concern. 
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 Short-term aggregate risk is equivalent to drinking water exposure plus short-term 

residential exposure, and is not of concern (MOE=780 for children 1-2 years old, 

MOE=140 for adults, where the LOC for MOE=100). 

 Intermediate term aggregate risk was not estimated because residential postapplication exposure is expected to be short-term in duration only. 

 Chronic aggregate risk is equivalent to chronic dietary risk because residential 

post-application is expected to be short-term in duration. Chronic aggregate risk 

therefore is not of concern. 

Cumulative Effects

Section 408(b)(2)(D)(v) of the Federal Food, Drug, and Cosmetic Act (FFDCA) requires 

the Agency to consider the cumulative effects of exposure to substances that have a common 

mechanism of toxicity. These considerations include the possible cumulative effects of such 

residues on infants and children. 

EPA has not made a common mechanism of toxicity finding for paclobutrazol and any 

other substances, and paclobutrazol does not appear to produce a toxic metabolite produced by 

other substances. Therefore, EPA has not assumed that paclobutrazol has a common mechanism 

of toxicity with other substances and a cumulative effects assessment was not performed. 

Occupational Exposure Assessment 

The occupational assessment estimates risk to handlers (those who mix, load and apply a 

pesticide) and to workers (those who re-enter a treated area to perform tasks) in an occupational 

setting via the dermal and inhalation routes of exposure. 

Occupational handler exposure was assessed for use of paclobutrazol on turf, 

ornamentals, and as tree treatments. Short and intermediate term risk estimates for occupational 

handlers for nearly all scenarios were not of concern to the Agency (MOEs range from 29 to 

19,000,000, with the LOC for MOE=100). The one exception is mixing/loading/applying liquids 

via backpack sprayer for basal tree drench treatments where the MOE=29 with baseline personal 

protective equipment (PPE), and the MOE=58 with full PPE. Additional information 

surrounding these risk estimates can be found below in section 2. Human Health Risk 

Characterization. 

The Agency also assessed occupational risks associated with paclobutrazol use as a seed 

treatment. The Agency assessed risks associated with seed treatment activities, as well as risks 

associated with seed planting activities. Risk estimates for all occupational scenarios were not of 

concern to the Agency, where MOEs range from 16,000 to 13,000,000. 

Occupational post-application inhalation exposure (via volatilization of paclobutrazol and 

resuspension of dusts/particulates) was considered. The Agency expected that handler exposures 

(associated with ornamental, tree, and turf uses) would be higher than those that would result 

from post-application activities. Therefore, since handler exposure can be considered protective 

of post-application inhalation exposures, and handler risks are not of concern, the Agency 
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determined that a quantitative occupational post-application exposure assessment was not 

needed. 

Occupational post-application dermal exposure (from activities such as golf course 

maintenance, hand-harvesting, pruning, weeding, and grafting) was assessed. Based on currently 

registered uses, short-term (1 to 30 days) and intermediate-term (30 days to 6 months) exposures 

are expected. Occupational post-application dermal risk estimates were not of concern where 

MOEs range from 110 to 2,800. 

2. Human Health Risk Characterization 

The human health risk assessment identified potential risks for occupational handlers 

mixing/loading/applying liquid paclobutrazol via backpack sprayer for basal tree drench 

treatments. The Agency notes that paclobutrazol labels do not specify the application method for 

this use and so this scenario was initially assessed assuming application with a backpack sprayer. 

The Agency estimated an MOE = 29 at baseline (i.e., assuming no PPE) and an MOE = 58 with 

PPE (i.e., assuming gloves and double layer clothing). However, based upon information from 

the technical registrants, the Agency learned that backpack sprayers are not used to make basal 

tree drench applications because backpack sprayers do not efficiently deliver the volume of 

material usually needed for basal tree drench applications. Rather, basal tree drench applications 

are typically made with either a hand-held hose connected to a truck-mounted spray tank (email 

communication from a technical registrant, January 23, 2015) or with a hydraulic sprayer with a 

hand held application nozzle (email communication from a technical registrant, January 28, 

2015). Further, in situations where the target tree(s) are inaccessible with a truck-mounted or 

hydraulic sprayer, basal tree drench applications are made with a bucket or a 2.5 gallon jug 

(email communication from a technical registrant, January 30, 2015). 

The Agency determined that application via truck-mounted, or hydraulic sprayer can be 

represented by another occupational handler scenario already assessed by the Agency: 

“mixing/loading/applying liquids with a mechanically pressurized hand wand” for drench tree 

treatments (p. 33 of the human health risk assessment). Similarly, the application via jug 

application can be represented by the “mixing/loading liquids for the tree soil injection” 

exposure scenario already assessed by the Agency (p. 31 of the human health risk assessment). 

When these existing scenarios are adjusted to represent paclobutrazol basal tree drench 

application (i.e., 0.31 lbs. active ingredient per tree for 50 trees in a day), estimated MOEs are 

not of concern, where the MOE for the hand wand scenario is 2,100, and the MOE for the jug 

scenario is 1,100 (see Table 1 below). 
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Table 1. Estimated dermal and inhalation exposure for the basal tree drench use of 

paclobutrazol, from jug application and application via hydraulic sprayer/truck-mounted spray 

tank. 

Activity Dermal dose 

(mg/day) 

Dermal 

MOE 

Inhalation dose 

(mg/day) 

Inhalation 

MOE 

Total 

MOE 

Jug application 0.00924 1100 0.0000491 200,000 1,100 

Application via 

hydraulic sprayer or 

truck-mounted spray 

tank 

0.00447 2,200 0.000239 42,000 2,100 

 The current paclobutrazol labels do not specify the application method or the equipment 

to be used for basal tree drench. To ensure the most realistic and safe application methods are 

being used in the field, the Agency is requiring revised label language to be placed on each 

applicable label with directions for use as a basal tree drench. This language can be found in 

Section IV of this document. 

3. Human Incidents 

For this registration review, an incident search was initially conducted in February 2007. 

There were two incidents identified in the Main and Aggregate Incident Data System (IDS) from 

1999 to 2007. There were no incidents identified in the Poison Control Center database from 

1993 to 2005. There were no incidents in the National Institute of Occupational Safety and 

Health Sentinel Event Notification System for Occupational Risks (NIOSH SENSOR) database 

from 1998 to 2003. An updated incident search was conducted in January 2015. No new 

incidents were found in IDS from January 2010 to January 2015; no new incidents were found in 

NIOSH SENSOR from 1998 to 2011. For additional details, please refer to the Paclobutrazol: 

Final Human Health Risk Assessment for Registration Review in docket EPA-HQ-OPP-2006-

0109, which can be accessed at www.regulations.gov. 

4. Food Tolerances 

Paclobutrazol does not have registered food uses and therefore no food tolerances have 

been established. 

5. Human Health Data Needs 

No human health data needs for paclobutrazol were identified by the Agency during the 

human health risk assessment for registration review. 
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 B. Environmental Assessment 

The paclobutrazol problem formulation document, ecological risk assessment and 

supporting documents, as well as the most updated Agency science policies and risk assessment 

methodologies were used to prepare a quantitative ecological risk assessment in support of the 

registration review of paclobutrazol. For detailed discussions of all aspects of the environmental 

risk assessment, see the Registration Review—Ecological Risk Assessment and Effects 

Determination of Paclobutrazol, the Response to Public Comments on the Paclobutrazol 

Registration Review Ecological Risk Assessment, the Revised Chronic Mammalian Risk 

Conclusions for the Paclobutrazol Registration Review Ecological Risk Assessment, and the 

Amended Terrestrial Plant Risk Quotients for the Paclobutrazol Registration Review Ecological 

Risk Assessment in the public docket (EPA-HQ-OPP-2006-0109) which can be accessed on 

http://www.regulations.gov/. 

For non-listed species, no risks were identified for non-vascular plants and terrestrial 

invertebrates. However, potential risks were identified for birds, reptiles, terrestrial-phase 

amphibians, mammals, terrestrial and aquatic plants, freshwater fish and invertebrates, aquaticphase amphibians, and estuarine/marine fish and invertebrates. 

The Agency also completed a screening-level endangered and threatened (listed) species 

assessment for paclobutrazol and identified potential risks to several taxa (aquatic vascular 

plants, terrestrial plants, freshwater fish/amphibians, estuarine/marine fish, freshwater 

invertebrates, estuarine/marine invertebrates, mammals, and birds). Birds were used as a 

surrogate for terrestrial-phase amphibians and reptiles. The Agency is working with its federal 

counterparts and other stakeholders to implement an interim approach for assessing potential risk 

to listed species and their designated critical habitats. Once the scientific methods necessary to 

complete risk assessments for listed species and their designated critical habitats are finalized, 

the Agency will complete an endangered species assessment for paclobutrazol. As such, risks to 

listed species will not be characterized herein. 

1. Environmental Fate and Exposure 

Paclobutrazol is not very soluble in water, is non-volatile under field conditions, and has 

the potential to bioaccumulate in aquatic organisms. It is stable to hydrolysis and photolysis and 

appears to be persistent in acidic soils, but not so in neutral or basic soils. Paclobutrazol is 

considered persistent and mobile, with half-lives up to 973 days in terrestrial field dissipation 

studies. Available fate data indicate two major environmental degradates of paclobutrazol: the 

ketone analog of paclobutrazol (CGA149907) and 1H-1,2,4-triazol-3-ol (NOA457654). 

2. Risk Conclusions 

The agency assessed ecological risk from the uses of paclobutrazol, including 

paclobutrazol use on: 

 Turf 

 Seed treatment for agricultural crops 

 Trees, trunk injection 

 Trees, soil injection 
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 Trees, basal drench 

 Potted ornamentals, spray 

 Potted ornamentals, drench 

The Agency has made a “no effect” determination for aquatic non-vascular plants, due to 

the absence of any currently listed non-vascular aquatic plants. 

Direct Risk to Terrestrial Organisms 

Birds, Reptiles, and Terrestrial-Phase Amphibians 

 Acute risk quotients (RQs) were not calculated for birds, (used as a surrogate for reptiles 

and terrestrial amphibians) because all toxicity data were non-definitive (i.e., no definitive toxic 

effect was identified in the study). When non-definitive toxicity values were directly compared 

to estimated environmental concentrations (EECs), it was determined that the potential for acute 

risk to non-listed birds, reptiles, and land-phased amphibians was low for all registered uses of 

paclobutrazol. Chronic RQs for birds only slightly exceeded the Agency’s LOC for the soil 

injection/drench use, where the chronic RQs range from 1.1 – 2.3 (chronic LOC=1). 

 

Mammals 

For mammals, acute and chronic dosed-based RQs and chronic dietary-based RQs were 

calculated for all uses of paclobutrazol. 

Acute RQs were below the LOC for non-listed mammals, and only slightly exceeded the 

LOC for listed mammals. For use on turf, acute RQs range from <LOC–0.11. For use on trees, 

the acute RQs range from <LOC–0.34. Consequently, acute risk to non-listed mammals are 

considered unlikely. 

 

 Chronic RQs for mammals exceed the LOC for all uses except for use of paclobutrazol as 

a seed treatment. For use on turf, chronic dose-based RQs exceed the Agency’s LOC where RQs 

range from <LOC–6.41. Chronic dietary-based RQs were lower and range from <LOC–1.37 

(see the document Revised Chronic Mammalian Risk Conclusions for the Paclobutrazol 

Registration Review Ecological Risk Assessment, dated January 15, 2015, for revised chronic 

mammalian RQs). 

 For potted ornamental spray application, chronic RQs only slightly exceeded the LOC 

ranging from <LOC–1.17. 

 For paclobutrazol use as a basal tree drench, tree soil injection, and tree trunk injection, 

chronic RQs associated with trunk injection range from <LOC–3.9, and RQs associated with soil 

injection/basal tree drench, range from 4–20. 

 For paclobutrazol use on potted ornamentals chronic mammalian RQs range from 

<LOC–2.1. Thus, there may be chronic risks to mammals from all uses of paclobutrazol, except 

as a seed treatment. 
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Terrestrial Invertebrates 

At the initiation of the paclobutrazol registration review, data requirements for terrestrial 

invertebrates (pollinator data) were not identified. However, acute oral and contact honeybee 

data were available for paclobutrazol, but only indicated non-definitive endpoints. Based on 

comparison of the non-definitive values to EECs, the Agency believes that risks to terrestrial 

invertebrates are not expected from paclobutrazol. 

 

Terrestrial and Wetland/Riparian Plants 

While RQs associated with the use of paclobutrazol as a seed treatment did not exceed 

the Agency’s LOC, the remaining uses of paclobutrazol indicate RQs above the LOC (LOC for 

both listed and non-listed plants is 1). 

 For use on turf, RQs range from 0.27–227.78 for both non-listed and listed dry and 

semi-aquatic plants. LOC exceedance from spray drift was also noted for the turf use, where 

RQs range from <LOC–13.23. 

For use of paclobutrazol on ornamentals as a spray application, RQs range from <LOC–

96.30, whereas RQs for ornamentals as a drench application range from 1.48–2,151.68 for both 

non-listed and listed dry and semi-aquatic plants. 

For the use of paclobutrazol on trees as a soil injection/basal tree drench, RQs range from 

0.15 to 223.40, indicating potential risk concerns for both listed and non-listed terrestrial plants. 

In general, semi-aquatic terrestrial plants had higher RQs compared to dry terrestrial 

plants. Risks to terrestrial plants are considered likely, but not surprising considering 

paclobutrazol’s use as a plant growth regulator (see the Response to Public Comments on the 

Paclobutrazol Registration Review Ecological Risk Assessment and the Amended Terrestrial 

Plant Risk Quotients for the Paclobutrazol Registration Review Ecological Risk Assessment for 

revised terrestrial plant RQs). 

Direct Risk to Aquatic Organisms 

 The Agency believes that only use on turf and on ornamentals (as a drench/spray) would 

result in potential exposure to aquatic organisms, and so these were the only paclobutrazol uses 

modeled for aquatic exposure. 

Freshwater Fish and Aquatic-Phase Amphibians 

For freshwater fish and freshwater aquatic-phase amphibians, acute RQs for the 

ornamental drench and turf uses did not exceed the Agency’s LOC for either listed or non-listed 

species. Acute risks to freshwater fish and aquatic-phase amphibians are therefore considered 

unlikely. 
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For the ornamental drench use, chronic freshwater fish RQs range from <0.1 to 6.8 for 

ornamental drench. For the turf use, chronic freshwater fish RQs range from 0.43 to 1.4. Both 

uses therefore exceeded the chronic LOC of 1. 

Estuarine/Marine Fish 

Paclobutrazol data were not available for estimating acute toxicity to estuarine/marine 

fish. Data from structurally-similar triazoles were used as a surrogate to estimate risk. Based on 

surrogate data, acute RQs range from <LOC – 0.21 for ornamental drench, which was below the 

LOC for non-listed species (0.5), but slightly exceed the LOC for listed species (0.05). Turf use 

scenarios did not result in acute RQs of concern for either listed or non-listed estuarine/marine 

fish. 

Chronic RQs range from <LOC to 83 for the ornamental drench use and range from 5 to 

17 for the turf use, exceeding the Agency’s chronic LOC of 1. 

Freshwater Invertebrates 

Acute RQs for freshwater mollusks did not exceed the Agency’s LOC for either the 

ornamental drench or turf use. For freshwater invertebrates, acute RQs range from <0.01-1.4 for 

ornamental drench. For the turf use, RQs range from 0.09 – 0.28. 

Chronic RQs range from <LOC – 1.04 for the ornamental drench use, and were below the 

LOC for the turf use (see the document Response to Public Comments on the Paclobutrazol 

Registration Review Ecological Risk Assessment, dated October 9, 2014 for revised freshwater 

invertebrate chronic RQs). 

 

Estuarine/Marine Invertebrates

For estuarine/marine invertebrates, toxicity data for paclobutrazol were available, but 

RQs could not be calculated because the endpoints were non-definitive. Based upon a 

comparison of the highest EEC to the most sensitive endpoint for mysid shrimp, the Agency 

determined that acute risk to estuarine/marine invertebrates is unlikely (see the document 

Response to Public Comments on the Paclobutrazol Registration Review Ecological Risk 

Assessment, dated October 9, 2014 for a revised estuarine/marine invertebrate acute risk 

description). 

Chronic RQs however, range from <LOC–9.5 for the ornamental drench use and range 

from <LOC–2.0 for the turf use. 

Aquatic Vascular and Non-vascular Plants 

For aquatic vascular plants, RQs range from <LOC-202 for the ornamental drench use, 

and range from 13 to 41 for the turf use (see the document Response to Public Comments on the 

Paclobutrazol Registration Review Ecological Risk Assessment, dated October 9, 2014 for a 

revised aquatic plant risk description). 
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For aquatic non-vascular plants, RQs were below the Agency’s LOC for both the 

ornamental drench use and the turf use. 

Table 2. Summary of risks for non-listed species for all taxa across all uses of 

paclobutrazol. 

Taxa Endpoint LOC – 

listed/nonlisted 

Turf 

use 

Treetrunk 

injection 

Tree – basal 

drench/soil 

injection 

Ornamental 

spray 

Ornamental 

drench 

Birds chronic 1 1.1–2.3 

Mammals acute 0.1/0.5 <LOC–

0.11 

 <LOC-0.34 

chronic 1 <LOC–

6.41 

<LOC-3.9 4-20 <LOC–1.17 <LOC–2.1 

Terrestrial plant 

– dry 

 1 <LOC3.57  <LOC–2.01 <LOC–1.24 1.48-19.33 

Terrestrial Plant 

-- Wetland 

 1 1.91-

24.96 

 1.54-20.07 <LOC–8.65 14.81-193.34 

Freshwater Fish chronic 1 <LOC–

1.4 

 <LOC–6.8 

Estuarine/marine 

Fish 

acute 0.05/0.5 <LOC–0.21 

chronic 1 5-17 <LOC–83 

Freshwater 

Invertebrates 

acute 0.05/0.5 0.09–

0.28 

 <LOC–1.4 

chronic 1 <LOC–1.04 

Estuarine/marine 

Invertebrates 

chronic 1 <LOC–

2.0 

 <LOC–9.5 

Aquatic Plants 1 13-41 <LOC-202 

3. Ecological Incidents 

A search was conducted for ecological incidents for paclobutrazol on September 23, 

2011, using the Ecological Incident Information System (EIIS), the Aggregate Summary Module 

of OPP’s Incident Database, and the Avian Incident Monitoring System (AIMS). In the EIIS, 

there was one incident reported for paclobutrazol from 1988 to 2011. The incident involved 

three dead squirrels in Florida and was listed as “unlikely” to have been caused by paclobutrazol. 

There were two incidents listed in the Aggregate Summary Module: a minor fish and wildlife 

incident in 2005, and a minor plant incident in 2009. There were no reported incidents in AIMS. 

For additional details, please refer to Registration Review-Ecological Risk Assessment and 

Effects Determination of Paclobutrazol, dated March 26, 2013 in docket EPA-HQ-OPP-2006-

0109, which can be accessed at www.regulations.gov. 

4. Ecological Effects Data Needs 

The Agency identified several data gaps for paclobutrazol in its ecological risk 

assessment. These are: 

 Acute risk to estuarine/marine fish (guideline 850.1075) 

 Chronic risk to estuarine/marine fish (guideline 850.1400) 

 Chronic risk to estuarine/marine invertebrates (guideline 850.1350) 

 Acute oral risk to passerine birds (guideline 850.2100) 

 Translocation of paclobutrazol in plant parts 
15

 Paclobutrazol uptake rate in turf, trees, and ornamentals 

5. Endangered Species Assessment 

In November 2013, the EPA, along with the Services and the USDA, released a summary 

of their joint Interim Approaches for assessing risks to endangered and threatened (listed) species 

from pesticides. The Interim Approaches1

 were developed jointly by the agencies in response to 

the National Academy of Sciences’ (NAS) recommendations and reflect a common approach to 

risk assessment shared by the agencies as a way of addressing scientific differences between the 

EPA and the Services. The NAS report2

 outlines recommendations on specific scientific and 

technical issues related to the development of pesticide risk assessments that EPA and the 

Services must conduct in connection with their obligations under the ESA and FIFRA. 

The joint Interim Approaches were released prior to a stakeholder workshop held on 

November 15, 2013. In addition, the EPA presented the joint Interim Approaches at the 

December 2013 Pesticide Program Dialogue Committee (PPDC) and State-FIFRA Issues 

Research and Evaluation Group (SFIREG) meetings. The agencies also held stakeholder 

workshops - in April and October 2014, and in April 2015 - allowing additional opportunities for 

stakeholders to comment on the Interim Approaches. Additional workshops are planned to 

enhance stakeholder involvement. As part of a phased, iterative process for developing the 

Interim Approaches, the agencies will also consider public comments on the Interim Approaches 

in connection with the development of upcoming registration review decisions. The details of 

the joint Interim Approaches are contained in the white paper Interim Approaches for NationalLevel Pesticide Endangered Species Act (ESA) Assessments Based on the Recommendations of 

the National Academy of Sciences April 2013 Report3

, dated November 1, 2013. 

Given that the agencies are continuing to develop and work toward implementation of the 

Interim Approaches to assess the potential risks of pesticides to listed species and their 

designated critical habitat, the ecological risk assessment supporting this Interim Decision for 

Paclobutrazol does not contain a complete ESA analysis that includes effects determinations for 

specific listed species or designated critical habitat. Although EPA has not yet completed effects 

determinations for specific species or habitats, for this interim decision, EPA’s evaluation 

assumed, for all taxa of non-target wildlife and plants that listed species and designated critical 

habitats may be present in the vicinity of the application of paclobutrazol. This assessment 

allows EPA to focus its future evaluations on the types of species where the potential for effects 

exists once the scientific methods being developed by the agencies have been fully vetted. 

When the Agencies have fully developed and implemented the scientific methodology for 

evaluating risks for listed species and their designated critical habitats, these methods will be 

 

1 Interim Approaches for National-Level Pesticide Endangered Species Act (ESA) Assessments Based on the 

Recommendations of the National Academy of Sciences April 2013 Report. Available at 

http://www2.epa.gov/endangered-species/interim-approaches-pesticide-endangered-species-act-assessments-basednas-report

2 Assessing Risks to Endangered and Threatened Species from Pesticides. National Academy of Sciences. 1983. 

Available from http://www.nap.edu/catalog.php?record_id=18344

3

 Available at http://www.epa.gov/espp/2013/nas.html
16

applied to subsequent analyses for paclobutrazol as part of completing the final registration 

review decision. 

6. Endocrine Disruptor Screening Program 

As required by FIFRA and FFDCA, EPA reviews numerous studies to assess potential 

adverse outcomes from exposure to chemicals. Collectively, these studies include acute, 

subchronic and chronic toxicity, including assessments of carcinogenicity, neurotoxicity, 

developmental, reproductive, and general or systemic toxicity. These studies include endpoints 

which may be susceptible to endocrine influence, including effects on endocrine target organ 

histopathology, organ weights, estrus cyclicity, sexual maturation, fertility, pregnancy rates, 

reproductive loss, and sex ratios in offspring. For ecological hazard assessments, EPA evaluates 

acute tests and chronic studies that assess growth, developmental and reproductive effects in 

different taxonomic groups. As part of its most recent registration decision for paclobutrazol,

EPA reviewed these data and selected the most sensitive endpoints for relevant risk assessment 

scenarios from the existing hazard database. However, as required by FFDCA section 408(p), 

paclobutrazol is subject to the endocrine screening part of the Endocrine Disruptor Screening 

Program (EDSP). 

EPA has developed the EDSP to determine whether certain substances (including 

pesticide active and other ingredients) may have an effect in humans or wildlife similar to an 

effect produced by a “naturally occurring estrogen, or other such endocrine effects as the 

Administrator may designate.” The EDSP employs a two-tiered approach to making the 

statutorily required determinations. Tier 1 consists of a battery of 11 screening assays to identify 

the potential of a chemical substance to interact with the estrogen, androgen, or thyroid (E, A, or 

T) hormonal systems. Chemicals that go through Tier 1 screening and are found to have the 

potential to interact with E, A, or T hormonal systems will proceed to the next stage of the EDSP 

where EPA will determine which, if any, of the Tier 2 tests are necessary based on the available 

data. Tier 2 testing is designed to identify any adverse endocrine-related effects caused by the 

substance, and establish a dose-response relationship between the dose and the E, A, or T effect. 

Under FFDCA section 408(p), the Agency must screen all pesticide chemicals. Between 

October 2009 and February 2010, EPA issued test orders/data call-ins for the first group of 67 

chemicals, which contains 58 pesticide active ingredients and 9 inert ingredients. A second list of 

chemicals, including paclobutrazol, identified for EDSP screening was published on June 14, 

20134

 and includes some pesticides scheduled for registration review and chemicals found in 

water. For further information on the status of the EDSP, the policies and procedures, the lists of 

chemicals, future lists, the test guidelines and the Tier 1 screening battery, please visit our 

website.5 

 In the interim, EPA is making no human health or environmental safety findings 

associated with the EDSP screening of paclobutrazol. Before completing this registration 

review, the Agency will make an EDSP FFDCA section 408(p) determination. 

 

4

 See http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPPT-2009-0477-0074 for the final second list of 

chemicals. 

5 http://www.epa.gov/endo/
17

IV. RISK CHARACTERIZATION and INTERIM REGISTRATION 

REVIEW DECISION 

A. Ecological Risk Characterization for Non-Listed Species 

EPA has identified potential risks for mammals, birds (also surrogates for reptiles and 

terrestrial-phase amphibians), terrestrial and aquatic vascular plants, estuarine/marine and 

freshwater fish, and estuarine/marine, freshwater invertebrates, and aquatic-phase amphibians. 

The Agency uses reasonable worst-case scenarios and conservative assumptions in its analysis to 

be protective. Environmental exposure and risk is highly dynamic and variable, that is, it can 

vary greatly with numerous other factors. As a result, environmental risk can be difficult to 

represent either as a single number (RQ) or even as a RQ range. To better understand an 

estimated risk, the Agency can characterize estimates by using alternative assumptions and 

inputs to demonstrate and explore how or whether the estimates of risk change. Below is a brief 

discussion of several, not all, of the factors that can affect both the magnitude and/or the 

frequency of risk that may be associated with the use of paclobutrazol. Only direct risk to nonlisted species will be addressed in this section. 

Risk from basal tree drench/soil injection use on targeted trees

Potential risks from the basal tree drench and soil injection uses were identified for 

mammals and birds, but the highest RQs were for non-target terrestrial plants resulting from 

runoff after application by basal tree drench. 

In order to estimate exposure to non-target mammals, birds, and plants, the Agency 

needed to characterize movement of paclobutrazol from the application site (i.e., at the base of 

the tree) to both the edible portion of the tree as well as to the non-target plant. Since the 

Agency lacked data to characterize the uptake and movement of paclobutrazol in the terrestrial 

environment and in plants, it was required to make some assumptions in the ecological risk 

assessment: first, that 100% of the applied paclobutrazol is taken up by the tree, and second, that 

100% of what is taken up by the tree is moved into the edible portion of the tree and therefore 

available for consumption by birds or mammals. 

With regard to exposure to plants, in the ecological risk assessment, the Agency made the 

converse of these assumptions. That is, that 100% of the applied paclobutrazol was moved from 

the application site to the non-target plant. The Agency recognizes however, that it is likely that: 

(i) less than 100% of the paclobutrazol would be taken up by a tree, (ii) less than 100% of what 

is taken up by the tree would be moved into the edible parts of a tree; and, (iii) less than 100% of 

the product would be washed off the application site. Therefore, the Agency estimated RQs 

based on alternative scenarios, including 75%, 50% and 25% uptake, as well as 75%, 50% and 

25% runoff. Estimated RQs for plants decrease, in proportion with the different uptake/runoff 

assumptions. For example, when 100% uptake is assumed, the estimated chronic RQ for birds is 

2.3, but at 50% uptake the estimated chronic RQ is 1.17; and, when 100% runoff is assumed the 

RQ for terrestrial wetland plants is 70.37, but at 50% runoff the estimated RQ is 35.18. The 

Agency did not further characterize exposure and risk to mammals or birds with alternative 

assumptions about the amount of absorbed paclobutrazol that would move into the edible 
18

portions of the tree because of limitations in the risk assessment modeling tools. But it stands to 

reason that less than 100% of the applied amount of paclobutrazol would be available in edible 

portions of the tree. The open literature seems to suggest that most of the applied paclobutrazol 

would be translocated to the trunk of trees (Arron, 1990; Sterrett, 1985), this would lead to 

reduced exposure and lower RQs for mammals and birds. 

 To further characterize the potential frequency or magnitude of a potential exposure from 

the basal tree drench use, the Agency considered the characteristics of the use of paclobutrazol—

in other words, the types of trees that would be treated with paclobutrazol and how often they 

might be treated. According to the Massachusetts Department of Agricultural Resources, use of 

paclobutrazol on trees is typically targeted at species of high aesthetic or monetary value, such as 

trees that may be in high visibility locations (e.g., parks, historic downtowns, residential areas, or 

golf course settings). In addition, paclobutrazol is not typically applied to trees annually, but 

rather on a bi-annual basis or less. Therefore, while it is possible that a number of trees could be 

treated in one area on a single day, the Agency reasons that as a result of: (i) the type of tree that 

typically receives a paclobutrazol application, and (ii) the number of applications typically made 

to a tree, exposure and risk to non-target organisms, particularly non-target terrestrial plants, is 

likely to be both spatially and temporally limited. 

Therefore, when considering risk characterization, the Agency believes that potential risk 

to birds, mammals and non-target plants from soil injection use of paclobutrazol may exist but is 

likely lower than the estimates provided in the risk assessment, and may be both spatially and 

temporally limited. Therefore, the Agency is not pursuing ecological risk mitigation for this use 

as soil injection at this time. Risk mitigation for basal tree drench use is detailed in section IV of 

this document. 

Risk from tree injection

 Potential risks to mammals were identified from the tree injection use. However, the 

Agency believes that risks to mammals from this use are likely below estimated levels 

considering the low RQs and the assumptions used in risk assessment. The Agency assumed that 

all of the injected paclobutrazol would end up in edible plant parts and be available for 

consumption by mammals. The Agency also assumed that 100% of the mammals’ diet would 

consist of foods contaminated with paclobutrazol residues. In consideration of the low acreage 

that receives this use, i.e., it is typically used on select, high-value trees in high-visibility 

neighborhoods (Massachusetts Department of Agricultural Resources, 2012), as well as the 

conservative assumptions utilized in the exposure assessment, the Agency does not think that the 

potential risks outweigh the benefits. The Agency is not pursuing ecological risk mitigation for 

this use pattern at this time. 

Risk from turf uses

 

For paclobutrazol use on turf, potential risks were identified for mammals, fish, and 

aquatic invertebrates, but mainly for non-target terrestrial plants. 

Similar to the analysis for the basal tree drench use, risk calculations for non-target 

terrestrial animals and non-target terrestrial plants relied, in part, on the assumption that 100% of 
19

the applied paclobutrazol would be available in the contaminated food source and that 100% 

would be available to non-target plants from runoff. As discussed above, this assumption is 

based upon a lack of chemical-specific data, and lower estimates of exposure and risk would 

result from alternative assumptions of uptake and runoff (i.e., 75%, 50% and 25%). 

To characterize potential exposure, the Agency considered the typical use of 

paclobutrazol on turf. In a submission provided by one of the technical registrants (email 

communication, December 12, 2014), confidential annual market data confirmed the Agency’s 

belief that most turf use of paclobutrazol is in golf course settings. While golf course 

management programs likely vary considerably from establishment to establishment, it is widely 

acknowledged that golf course settings are highly managed areas that often employ any number 

of best management practices (BMPs) to develop and maintain a desired setting. Golf course 

establishments can employ BMPs for a number of reasons, such as to reduce the need for 

irrigation water, reduce the use of chemicals, as well as to control or reduce surface run-off 

(Virginia Golf Course Superintendents Association, 2012). Consequently, many golf course 

maintenance BMPs can also directly or indirectly reduce pesticide exposure to non-target 

terrestrial and aquatic plants. According to the Connecticut Department of Environmental 

Protection (2006), specific BMPs used in golf course management that would result in control, or 

reduced surface runoff include: 

 the use of retention ponds to capture runoff and stormwater; 

 draining plans that include grass swales or diversions that direct runoff from the 

landscape to collection pond(s); 

 the establishment of grassy swales, vegetated filter strips, and constructed 

wetlands integrated into the course landscape to filter runoff; 

 construction of ponds in a “train” to treat site runoff, with the first pond catching 

the first flush with sediment and pollutants, the second providing additional 

filtering, and the third as a withdrawal pond for irrigation of the course; 

 lining of holding ponds that are expected to capture runoff with elevated chemical 

concentrations in order to prevent seepage of pollutants into groundwater; 

 proper irrigation and application of chemicals to reduce leaching and runoff (i.e., 

not applying chemicals before a possible storm event via use of weather forecasts) 

 use of low volume sprinklers; 

 employ soil cultivation techniques (e.g., spiking, slicing, and core aerification) to 

improve water infiltration and minimize runoff; 

 the use of riparian vegetated buffers along the banks of golf course wetlands, 

watercourses, and other water bodies to function as sediment filters that reduce 

pollutants in runoff; 

 controlling pond overflow; 

 locating pesticide and fertilizer mixing/loading areas away from watercourses and 

diverting runoff from these facilities into appropriate treatment areas; 

 avoiding spraying pesticides when the soil is heavily saturated to prevent runoff 

Considering the characterization of estimated exposure and risk, and the prevalence of 

BMPs widely used in the golf course industry, the Agency believes that risk to non-target 

organisms, particularly, to aquatic organisms and plants, is likely to exist but is lower than that 
20

reflected in the risk assessment. However, the Agency notes that other turf uses of 

paclobutrazol, such as on turf in rights-of-way settings, do not represent the same level of 

landscape management and therefore could likely represent scenarios of higher non-target 

exposure. 

 

Risk from spray drift 

 Potential risks from spray drift were also identified for the paclobutrazol use on turf. The 

Agency believes that spray drift risks are likely less than estimated. The majority of 

paclobutrazol use on turf is on golf course turf--specifically tees, greens, fairways, and roughs. 

Spray drift may be less likely to leave a golf course due to the presence of tree barriers, hills, and 

other landscaping artifacts. Also, golf courses often use certain drift reduction technology to 

reduce the volume and occurrence of spray drift, these include: enlarging spray droplet size, 

using higher viscosity formulations, adjusting spray pressure, and using low-volatility carriers 

(Balogh and Walker, 1992). In addition, certain BMPs and cultural practices are implemented 

by golf courses specifically to reduce the occurrence of drift, these include the use of spot 

treatments, “no spray” zones, use of covered booms, and avoidance of application during periods 

of high wind (Audubon International, 2015). In consideration of these factors, the Agency is not 

proposing risk mitigation specifically to address spray drift risks, although other risk mitigation 

listed herein may help indirectly address spray drift risks. 

Risk from ornamental uses 

 

 For use of paclobutrazol on ornamentals, potential risks were identified for non-target 

mammals, non-target terrestrial and aquatic plants, and non-target aquatic species. The highest 

RQs were for terrestrial and aquatic plants resulting from runoff. In characterizing potential 

risks associated with the ornamental uses of paclobutrazol, the Agency looked at exposure as it 

relates to assumptions, use, as well as BMPs that may be used in the ornamental production 

industry. 

First, similar to its exposure analysis for the turf use and basal tree drench use of 

paclobutrazol, the lack of chemical-specific runoff data required the Agency to assume that 

100% of the paclobutrazol applied to ornamentals would runoff and become available to nontarget plants and aquatic organisms. Use of alternative runoff assumptions would indicate RQs 

lower than what was identified in the risk assessment. 

With respect to the use of paclobutrazol on ornamentals, the Agency’s analysis estimated 

risk based upon the use likely to result in the greatest environmental exposure (i.e., highest 

application rate or highest number of applications). For potted ornamental use of paclobutrazol, 

this is a use rate of 200 ppm solution concentration for ornamental drench (equivalent to 6.08 lbs. 

ai/A). However, paclobutrazol labels are crafted in a manner so as to identify a specific 

application rate for each separate ornamental species, or for small groups of ornamental species. 

The 200 ppm solution concentration is only indicated for a limited number of ornamental species 

(specifically amaryllis and freesia). These species reflect a very small portion of the floriculture 

market (less than 5%, according to USDA’s 2014 Floriculture Survey). While the Agency does 

not have typical application information for the many and varied ornamental species found on 

paclobutrazol labels, it notes that use rates for all other ornamental species listed on 
21

paclobutrazol labels are less than 100 ppm. Anecdotal information from one technical registrant 

(email communication, January 5, 2015) indicated that in ornamental production, paclobutrazol 

is typically applied as a spray to bedding plants at a concentration of 30-50 ppm (equal to 0.03-

0.05 lbs ai/A) and typically applied as a drench to ornamental containers at a concentration of 1-

4 ppm (equal to 0.12 lb ai/A). 

 

Finally, the Agency notes that similar to the golf course industry, the ornamental 

production industry also employs BMPs to reduce the need for irrigation, reduce the need for 

chemical application, and to control and reduce the volume of runoff. According to Majsztrik et 

al. (2011), examples of BMPs for floriculture production include: 

 use of calcined clays as substrate amendments that help absorb excess 

water/nutrients/chemicals; 

 use of drip irrigation, micro-irrigation and precision applications to reduce the volume 

of runoff; 

 use of containment ponds to hold surface water runoff; 

 reuse of water from recaptured surface water for irrigation; and 

 use of vegetated buffers for capturing sediment and nutrients/chemicals in runoff 

water; these are often used in association with containment ponds. 

Information from the open literature provides some indication of the ornamental 

production industry’s efforts to use BMPs that may, among other things, reduce or control 

surface water runoff. Two articles (Fain et al., 2000, and Garber et al., 2002) found that, of 

container nurseries surveyed in Alabama, Georgia, and Florida, 75%, 45% and 50%, 

respectively, have practices in place to capture runoff water. A survey of 85 nurseries in 

California in 2004 and again in 2006 (Salvatore et. al., 2010) found that the percent of 

ornamental production facilities following BMPs was 57% and 65% respectively. While 

adoption of BMPs by the ornamental production industry is often voluntary, adoption in certain 

states is required to comply with state and local regulation of surface water runoff. For example, 

California, Florida, Maryland, Oregon, and Texas have adopted regulations mandating runoff 

capture or control by irrigated agricultural operations (Oki and White, 2011). 

B. Risk Mitigation Measures 

When considering: (i) assumptions used in risk estimates, (ii) risk characterization, (iii) 

the use of paclobutrazol, and (iv) other factors such as the existence of BMPs for certain 

industries, the Agency concludes that potential risk may result from runoff associated with 

paclobutrazol use on trees (basal drench), on turf, and on ornamentals. The Agency believes that 

potential risks, particularly to non-target terrestrial and aquatic plants may be lowered by 

reducing runoff. Based upon this, the Agency engaged the technical registrants of paclobutrazol 

to develop revised label language for the three subject uses in order to reduce runoff, as well as 

to provide clarity and consistency across paclobutrazol labels. All of the corresponding label 

changes referred to below can be found in section IV.C and Appendix B of this document. 

Label changes for use on trees—basal tree drench 
22

The current paclobutrazol labels are unclear with respect to the application method for 

the basal tree drench use. Label language for the basal tree drench use will be revised with the 

aim of: (i) reducing potential runoff, and (ii) specifying the application equipment for this use. 

Label changes for use on turf 

As discussed previously, the majority of paclobutrazol use on turf occurs on golf courses. 

In addition to golf courses, paclobutrazol is also labeled for use on other managed turf sites such 

as park turf, athletic field turf, and institutional turf. However, paclobutrazol labels also specify 

use on turf in rights-of-way settings which are typically not highly managed. The Agency 

believes that use of paclobutrazol on managed turf sites can be distinguished from the rights-ofway use because at managed turf sites, runoff may be a variable that may be controlled via BMPs 

or cultural practices. In addition, unlike unmanaged turf sites, potential risks associated with 

paclobutrazol use at managed turf sites may be offset by the benefits associated with these uses, 

namely the economic, aesthetic, or recreational value of golf courses, parks, or athletic fields. 

Therefore, in discussions with EPA, the technical registrants of paclobutrazol (Syngenta Crop 

Protection, Zhejiang Tide Cropscience, Fine Agrochemicals, and MacDermid Agricultural 

Solutions) have agreed to remove from product labels, the use of paclobutrazol on turf in rightsof-way settings. Label language for the managed turf use will be revised to specify an irrigation 

requirement as well as a requirement to prohibit applications that would be subject to runoff 

from a rain event. 

Label changes for use on ornamentals 

For ornamental spray/drench use, the Agency believes that there is potential risk to nontarget organisms from runoff in outdoor ornamental settings. Similar to measures being required 

for the managed turf use, label language for the ornamental use pattern is required that will 

specify an irrigation requirement as well as a requirement to prohibit applications that would 

subject to runoff from a rain event. 

Other Label Changes 

In addition to label precautions for managing runoff, certain label changes will be made 

to improve the clarity of paclobutrazol labels with respect to the following: 

Elimination of “greater-than” (>) signs from application rate information. Certain 

paclobutrazol labels are unclear with respect to drench/spray applications to ornamentals, 

because the application rate is expressed in terms of “greater-than” a certain value. Product 

labels must be as clear and specific as possible, and application rates expressed as “greater-than” 

do not provide sufficient information for safe use of a pesticide product. The Agency and the 

registrants of paclobutrazol products agreed to revise all affected labels to remove all “greaterthan” (>) signs and specify an upper bound application rate. The following paclobutrazol EPA 

registrations are affected by this issue: 100-996, 228-679, 400-515, 62097-11, 62097-25, 79676-

79, 80697-3, and 82437-5. 

Ensuring information, within a label, is consistent. The Agency notes that certain 

paclobutrazol labels have inconsistencies in the ornamental use section between table text and 
23

the table headings, and need to be corrected. For example, some labels have application rate 

language in one section of the label as “Application of this product will provide height control to 

most bedding plants at a concentration between 5 and 90 ppm”, yet text further below on the 

same label indicates application rates outside of this range. The Agency and registrants of 

paclobutrazol products agreed to revise all affected labels to ensure consistency within the 

labeling. The following paclobutrazol EPA registrations are affected by this issue: 79676-79, 

400-515, 82437-5, 228-679, and 100-996. 

Clarifying labels with respect to mist and fogging applications. Some paclobutrazol 

labels need to be clarified with respect to application by “mist” irrigation and “fogging.” The 

Agency confirmed with the technical registrants of paclobutrazol that fogging is not an existing 

application method for paclobutrazol and therefore, the Agency proposed that references to 

application via “fogging” or “fog system” be deleted from the labels. To prevent confusion, if 

“mist irrigation” is included on a label, the Agency and the registrants of paclobutrazol products 

agreed to delete any corresponding label language on the same label stating that “misting 

techniques will not produce desired results.” The following paclobutrazol EPA registrations are 

affected by this issue: 400-515, 62097-11, 62097-25, and 100-996. 

C. Interim Registration Review Decision 

EPA has completed quantitative human health and ecological risk assessments, including 

an initial endangered species analysis, for paclobutrazol. 

The Agency determined that dietary (food + drinking water) risks to the general 

population or any population subgroup are not of concern. The Agency also determined that 

residential and occupational risks are not of concern. 

When initially scoping the registration review for paclobutrazol, EPA did not identify the 

need for any additional studies to evaluate potential effects on pollinators. However, since the 

issuance of the June 2014 Guidance for Assessing Pesticide Risks to Bees6, EPA has begun to 

require these data where applicable. EPA is currently determining whether additional pollinator 

data are needed for active ingredients in registration review whose dockets opened prior to June 

2014. For those active ingredients where EPA determines that additional pollinator exposure and 

effects data are necessary to help make a final registration review decision, EPA intends to issue 

a DCI to obtain these data. In the near future, EPA will provide further information and 

guidance on this effort. The pollinator studies that could be required are included in Table 3 

below. The Agency will require data it believes are needed to help inform the pollinator risk 

assessment.

Table 3. Possible Pollinator Data Requirements for Paclobutrazol

Guideline # Study 

850.3020* Acute contact toxicity study with adult honey bees (this study has been 

submitted to the Agency but has not yet been reviewed) 

 

6 http://www2.epa.gov/sites/production/files/2014-

06/documents/pollinator_risk_assessment_guidance_06_19_14.pdf
24

850.3030* Honey bee toxicity of residues on foliage 

850.3040** Field testing for pollinators 

Non-Guideline* 

(OECD 213) 

Honey bee adult acute oral toxicity 

Non-Guideline* 

(OECD 237) 

Honey bee larvae acute oral toxicity 

Non-Guideline* Honey bee adult chronic oral toxicity 

Non-Guideline* Honey bee larvae chronic oral toxicity 

Non-Guideline** Residues in pollen and nectar/field residue analysis 

Non-Guideline** 

(OECD 75) Semi-field testing for pollinators (tunnel or colony feeding studies) 

*Tier 1 (Laboratory-based studies)

**Tier 2 and 3 (Semi-field and full field colony-level studies) The need for a higher tier test for pollinators will be determined 

based upon lower-tiered tests and/or other lines of data and the need for a refined pollinator risk assessment. 

In accordance with 40 CFR Sections 155.56 and 155.58, the Agency is issuing this 

Paclobutrazol Interim Registration Review Decision. Except for the EDSP, ESA and pollinator 

components of this case, the Agency has made the following Interim Registration Review 

Decision for paclobutrazol: 

 The Agency has not identified any additional data requirements to further inform either 

its human health effects or exposure database. 

 No tolerance amendments are necessary for paclobutrazol. 

 Based upon a lack of data, the Agency has not assessed potential risk from paclobutrazol 

to terrestrial invertebrates. In the future, the Agency will determine whether pollinator 

data are required for paclobutrazol. 

 The Agency is requiring the following data as data needs for registration review: 

o Acute risk to estuarine/marine fish (guideline 850.1075) 

o Chronic risk to estuarine/marine fish (guideline 850.1400) 

o Chronic risk to estuarine/marine invertebrates (guideline 850.1350) 

o Acute oral risk to passerine birds (guideline 850.2100) 

o Translocation of paclobutrazol in plant parts 

o Paclobutrazol uptake rate in turf, trees, and ornamentals 

 The Agency has identified potential risk to several taxa from paclobutrazol, and has 

required certain changes to paclobutrazol registrations and their labeling to reduce 

potential risk from use on trees (as basal drench), use on turf, and use on ornamentals. 

These label changes are as follows: 

Addition of the following label statement for use on trees as basal drench: 

“Prior to application, make a 2” deep furrow around the base of the tree near the point 

of contact between the soil and the tree trunk. Apply the required dose [x ml for the 
25

application rate and tree size DBH]. Carefully pour the diluted mixture of [product 

name] into the furrow with a graduated container/jug or with a handheld hose connected 

to a truck-mounted tank/hydraulic sprayer. To avoid possible product runoff after 

applying, refill the furrow with untreated soil.”

Addition of the following label statement for use on turf: 

“After [product name] application, water-in within 24 hours to limit surface movement, 

but not to the point of runoff. To prevent product runoff, time applications to allow for 

watering-in and maximum absorption into treated turf prior to a rain event.” 

“Do not apply [product name] to turf within rights-of-ways.”

 

Addition of the following label statements for use on ornamentals: 

“In outdoor commercial ornamental and nursery uses, follow foliar applications of 

[product name] by irrigation within 24 hours to remove product from foliage and limit 

surface movement. If overhead irrigation is not available, time applications to allow 

[product name] to dry on the treated surface prior to a rain event.” 

“To limit unwanted surface runoff in outdoor ornamental uses, do not apply when growth 

media is saturated.”

Certain other changes to paclobutrazol labels are required to improve clarity and 

consistency: 

o Elimination of “greater-than” (>) signs from application rate information and addition 

of upper rate maximums. 

o Revision of paclobutrazol labels to ensure text, within a label, regarding application 

directions is consistent. 

o Clarification of paclobutrazol labels with respect to application as a mist or fog. 

In this Interim Registration Review Decision, EPA is making no human health or 

environmental safety findings associated with the EDSP screening of paclobutrazol, risk to 

pollinators, nor is it making a complete endangered species finding. The Agency’s final 

registration review decision for paclobutrazol will be issued once an ESA Section 7 consultation 

with the Services has taken place, an EDSP FFDCA section 408(p) determination has been 

made, and the pollinator risk assessment has been completed. 

V. NEXT STEPS AND TIMELINE 

A. Implementation of Mitigation Measures and Issuance of Required Data 

Amended labels that include the risk mitigation measures required in this interim decision 

must be submitted to the Agency for approval within 60 days following issuance of this Interim 

Registration Review Decision. A summary of the required mitigation measures are described in 

Appendix A, and changes to labels are described in Appendix B. In the near future the Agency 

will issue a DCI for the data requirements listed in section IV. C. 

B. Final Registration Review Decision 
26

 EPA will make a final registration review decision for paclobutrazol after ESA Section 7 

consultation with the Services has taken place, the pollinator risk assessment has been 

completed, and an EDSP FFDCA section 408(p) determination has been made. 

References 

Arron, G. (1990). Translocation of Uniconazole After Trunk Injection of Silver Maple Saplings. 

Journal of Plant Growth Regulation, 9, 141-146. 

Audubon International. (2015). Environmental Management Practices for Golf Courses. 

Retrieved June 17, 2015, from http://www.auduboninternational.org/factsheets

Connecticut Department of Environmental Protection. (2006). Best Management Practices for 

Golf Course Water Use. Retrieved February 5, 2015, from 

www.ct.gov/deep/lib/deep/water_inland/diversions/golfcoursewaterusebmp.pdf

Balogh, J. C. & Walker, W. J. (1992). Golf Course Management & Construction: Environmental 

Issues. Chelsea, MI: United States Golf Association. 

Fain, G. B., Gilliam, C. H., Tilt, K. M., Olive, J. W., and Wallace, B. (2000). Survey of Best 

Management Practices in Container Production Nurseries. Journal of Environmental 

Horticulture, 18(3), 142-144. 

Garber, M. P., Ruter, J. M., Midcap, J. T., and Bondari, K. (2002). Survey of Container Nursery 

Irrigation Practices in Georgia. HortTechnology, vol. 12, no. 4, 727-731. 

Majsztrik, J. C., Ristvey, A. G., and Lea-Cox, J. D. (2011). Water and Nutrient Management in 

the Production of Container-Grown Ornamentals. Horticultural Reviews, 38, 253-297. 

Massachusetts Department of Agricultural Resources. (2012). Paclobutrazol: Review Conducted 

by MDAR and MassDEP for Use in Sensitive Areas of Rights-of-Way in Massachusetts. 

Retrieved February 4, 2015, from 

www.mass.gov/eea/docs/agr/pesticides/rightofway/docs/paclobutrazol-review-jan-2012.pdf 

Oki, L. R. & White, S. A. (2011). Ecological Approaches Used in Nurseries to Treat Water. 

University of California UCNFA News, 15, 1-6. 

Personal Communications from Technical Registrants of Paclobutrazol Used in EPA’s Risk 

Management Decision. December 2014-June 2015. Available in docket EPA-HQ-OPP-2006-

0109 at www.regulations.gov. 

 

Salvatore, S. M., Newman, J., Mochizuki, M., Zurawski, D., Merhaut, D. J., and Faber, B. 

(2010). Nurseries Surveyed in Southern California Adopt Best Practices for Water Quality. 

California Agriculture, vol. 64, no. 1, 26-30. 
27

National Agricultural Statistics Service, United States Department of Agriculture. (2014). 

Floriculture Crops 2013 Summary. Retrieved June 11, 2015, from 

http://www.nass.usda.gov/Statistics_by_State/New_Jersey/Publications/Floriculture_Statistics/Fl

orCrop-06-19-2014.pdf 

Sterrett, J. P. (1985). Paclobutrazol: A Promising Growth Inhibitor for Injection into Woody 

Plants. Journal of the American Society for Horticultural Science, 110(1), 4-8. 

Virginia Golf Course Superintendents Association. (2012). Environmental Best Management 

Practices for Virginia’s Golf Courses. Retrieved June 11, 2015, from 

http://www.vgcsa.org/sites/images/373/VirginiaBMP.pdf 

 
28

Appendix A: Summary of Required Risk Mitigation for Paclobutrazol

Affected

Population(s)

Source of

Exposure

Route of Exposure Duration of

Exposure

Risk(s) of Concern Mitigation

Humans

(occupational

handlers)

Direct contact Dermal absorption Short‐term Short‐term Label advisory

specifying

application

equipment to be

used for basal tree

drench

Terrestrial

plants, aquatic

plants

Runoff Systemic  

direct contact,

water

Short‐term

Long‐term

Short‐term

Long‐term

Removal of turf use

within rights‐of‐

ways; requirement

for creation of 2

inch furrow for

basal tree drench

applications; label

advisories for

managing runoff

Mammals Dietary

Water

Ingestion Long‐term Long‐term Deletion of turf use

within rights‐of‐

ways; label

advisories for

managing runoff

Birds, reptiles,

and terrestrial‐

phase

amphibians

Dietary

Water

Ingestion Long‐term Long‐term Deletion of turf use

within rights‐of‐

ways; label

advisories for

managing runoff

Aquatic fish,

invertebrates,

and amphibians  

Runoff Water Short‐term

Long‐term

Short‐term

Long‐term

Deletion of turf use

within rights‐of‐

ways; label

advisories for

managing runoff
Appendix B: Paclobutrazol Label Table 

Summary of Required Labeling Changes for Paclobutrazol Uses 

Description Amended Labeling Language for Paclobutrazol End Use Products 

End Use Products

Language for basal tree drench application 

“Prior to application, make a 2” deep furrow around the base of the tree near the point of contact between the and the tree trunk. Apply the required dose [x ml for the application rate and tree size DBH]. Carefully pour thdiluted mixture of [product name] into the furrow with a graduated container/jug or with a handheld hose connto a truck-mounted tank/hydraulic sprayer. To avoid possible product runoff after applying, refill the furrow wuntreated soil.”

Language for turf applications 

“After [product name] application, water-in within 24 hours to limit surface movement, but not to the point of rTo prevent product runoff, time applications to allow for watering-in and maximum absorption into treated turf to a rain event.” “Do not apply [product name] to turf within rights-of-ways.”

Language for application to outdoor ornamentals 

“In outdoor commercial ornamental and nursery uses, follow foliar applications of [product name] by irrigatiowithin 24 hours to remove product from foliage and limit surface movement. If overhead irrigation is not availatime applications to allow [product name] to dry on the treated surface prior to a rain event.” “To limit unwanted surface runoff in outdoor ornamental uses, do not apply when growth media is saturated.”

Removal of references to “fogging” and “fog system” Delete references to “fogging” and “fog system” from label. Clarify use of mist irrigation Delete “misting techniques will not produce desired results” from label. Clarification of ornamental use rates 

Maintain consistency between table headings and table text for applications to ornamentals. 
Clarification of ornamental use rates 

Delete “>” signs from the label. Rates must be stated as either a numerical digit or a numerical range with a cle

upper bound rate.