Chemical Fruit Thinning and Other Plant Growth Regulator Uses

Apple Fruit Thinning

Written by: 
Duane Greene, University of Massachusetts Amherst
Revised by Renae Moran, Univ. of Maine

Fruit thinning is an essential practice for producing commercial quality fruit and for getting consistent yield from year to year.  It increases the overall value of fruit because the reduction in crop load increases the size of fruit that remain.  For some varieties, there is an improvement in red color, as well. 

 Apple trees are prone to biennial bearing or the condition in which trees produce abundantly one year and poorly the next.  Chemical thinning in one season will increase the amount of bloom in the following season (return or repeat bloom).  Hand thinning is not as effective as chemical thinning for promoting return bloom.

The chemicals and concentrations a grower chooses, the timing of their application, and the environmental factors encountered before, during, and after application all influence the ultimate thinning response. This section of the Guide will discuss the chemicals most frequently used, the circumstances when they are used and the precautions associated with their use. Also discussed will be the timing of the applications and the environmental factors one must be mindful of when applying chemical thinners.

Thinning Chemicals

Blossom thinners

12 mm is equivalent to ½ inch

Some of the first attempts to thin involve using caustic chemicals to prevent pollination, pollen germination or pollen tube growth. Many of these caustic chemicals can cause phytotoxicity to the leaves and russeting of the fruit. If poor pollination occurs during bloom, thinning should be postponed until early fruit set. 

Lime sulfur, lime sulfur plus oil or ammonium thoiosulfate are mild blossom thinners.  Although none of these have label approval for thinning they may be applied legally on apples for other reasons. 

While less frequently used, hormone sprays can thin when applied at bloom. A bloom spray of NAA and NAAm can reduce fruit set, but are generally not applied commercially during bloom because of grower uncertainty about the extent of initial set and the desire to assess initial set before attempting to adjust crop load. Further, the most effective time to apply NAA as a chemical thinner is when fruit diameter is 7 to 12 mm.

Ethephon may also thin when applied at bloom or even several days earlier, at the balloon stage or red stage. The response appears to be quite cultivar and temperature sensitive. The use of ethephon as a blossom thinner has not been widely adopted except in locations where chemical thinning with other compounds is difficult and satisfactory results using other thinners is generally inadequate.

Postbloom thinners

The majority of chemical thinning is done with postbloom thinners. There is a comfort level for growers to delay thinner application until they know the extent of bee activity and pollination.

Naphthaleneacetic acid (NAA). It is probably the most potent of the general-use thinners on the market today, and it is the preferred material for cultivars that are difficult to thin. Its thinning effectiveness is concentration dependent. Lower concentration may cause only modest thinning while higher concentrations may overthin and reduce fruit size or not increase fruit size even though the crop load is substantially reduced. Over application of NAA may also lead to pygmy fruit formation and severe leaf epinasty on some varieties. Therefore, NAA is often combined with another thinner, especially carbaryl, and used at lower and “safer” rates. The thinning action of NAA is sometimes not immediately apparent since fruit abscission following NAA application if often delayed by as much as one or two weeks relative to untreated trees.

Naphthaleneacetamide (NAAm, NAD). NAD is a useful thinner that is frequently applied as a petal-fall spray. It is considered safer to use than NAA and it does not cause as severe leaf epinasty following application that is often experienced with NAA. NAD should be avoided on 'Delicious' since it may result in a high percentage of pygmy fruit that persist to harvest. NAD can be combined with carbaryl in situations were more aggressive thinning is desired.

Carbaryl.  Carbaryl is the most versatile thinner in general use. It is a mild thinner, and since the thinning is not rate responsive, overthinning is rarely observed. It can be used effectively over a wide range of developmental stages from petal fall until fruit grow to 18 mm in diameter. It is very toxic to bees so practically speaking the earliest time of application is at petal fall after the bees have been removed from the orchard. The Sevin XLR Plus formulation may be less of a problem near bloom since the particle size is less like pollen thus it is less likely to be transported by bees back to the hive. One of the most important characteristics of carbaryl is that it can break up fruit clusters. Carbaryl is an insecticide rather than a hormone so it is applied at higher concentrations than other postbloom thinners. 

Benzyladenine (BA). BA is a mild thinner when used by itself, but when combined with carbaryl it is a potent thinner combination that can overthin. The combination of BA with NAA for use on 'Delicious' and 'Fuji' is not recommended since pygmy fruit may form in some circumstances. Unlike other thinners, BA can increase fruit size beyond that attributed to a reduction in crop load.

Protone® (S-Abscisic Acid, ABA). ABA is a naturally occurring compound that plays a critical role in regulation of several physiological processes in a plant, especially those related to stress. If a plant is stressed or if it is sprayed with ABA stomata close, resulting in a significant reduction in photosynthesis. This in turn results in a carbon deficit in the plant. In the case with pome fruit, this happens for a long enough period during the time when developing fruit are competing for photosynthate (7-15 mm) fruit abscission will be initiated. Special attention should be paid to the weather conditions that occur especially the three days following application. If the weather is cloudy and or the temperatures are warm to hot, thinning will be favored because these conditions will increase the carbon deficit within the tree. Protone® is OMRI organic certified.

  • Apples. Label recommendations for the use of Protone® on apples include using 1 to 2 applications from 5-12 mm fruit size. Starting out I would recommend using mid- concentration rates at the 10-12 mm stage.  A good nonionic surfactant is recommended for use with this product. In the past we used Regulaid with ABA with good success. This may be used with MaxCel (not NAA) for added thinning.  Protone® may cause some leaf yellowing and leaf abscission. The severity of this is  cultivar dependent. A small amount of 6-BA (MaxCel) may help reduce leaf yellowing and abscission.
  • Pears. Protone® is cleared for use on pears for thinning. Pears are more sensitive to ABA than are apples. Everything being equal, a greater thinning response may be expected when used on pears. Some leaf abscission and leaf yellowing when used on pear can be seen. Based upon experience with Bartlett pears, 6-BA (MaxCel) was unable to reverse this yellowing and leaf abscission effect when used in conjunction with Protone®.

Ethephon.  Ethephon can be used as a postbloom thinner in situations where other chemical thinners are less effective or have undesirable side effects. Ethephon has a reputation for being an erratic thinner. Part of this can be attributed to a dramatic increase in thinning response with increasing temperature following application. Also, bloom and fruit susceptibility to ethephon varies depending upon the stage of development. Ethepon is an effective blossom thinner but application made just 7 days later appears to be much less effective. Fruit redevelop thinning sensitivity to ethephon at diameters between 16 and 22 mm. Since larger fruit are sensitive to ethephon, unlike most other chemical thinners, ethephon may have a place in a normal chemical thinning program as a "last chance" thinner where other thinners do not work or where a grower made a miscalculation early in the season, and failed to apply a thinner at the normal time.   

Timing Thinner Application

The weather following thinner application is probably the single most important factor influencing thinner efficacy. The weather can not be regulated, and an accurate forecast of the weather may not extend beyond 2 or 3 days. Increasingly, growers in the eastern part of the United States make two or more thinner applications over the thinning period. This is a good strategy since it spreads thinning out over time, and increases the possibility that thinner application will coincide with favorable thinning weather. It also tends to be safer since less aggressive thinning treatments are generally used, and the chance of overthinning is reduced.

Before application. Cool, cloudy, wet periods preceding thinner application generally mean that thinning will be easier. Part of this is attributed to altered epicuticular wax and cuticle development on leaves which predisposes leaves to absorb more thinning chemical.  These conditions during and immediately after bloom may also lead to less vigorous fruit set, characterized by fruit that are not growing vigorously and have few seeds, increased seed abortion, and reduced carbohydrate reserves. Regardless of thinner absorption, these fruit will be easier to thin. Frost injury to spur leaves also will make fruit easier to thin. NAA penetration is greater into frost injured leaves and the markedly distorted spur leaves undoubtedly have a reduced photosynthetic capacity.

The two most important environmental factors that influence foliar penetration of a chemical thinner are temperature and drying time. Warm temperatures enhance uptake of NAA by apple leaves. The longer the drying time of a thinning spray the greater the penetration into the leaf. In foliar penetration studies it has been reported that the penetration of NAD increased steadily over time as long as the spray droplet was prevented from drying. During the drying process uptake into the leaf was accelerated, presumably due to the concentration effect caused by the drying. Once the droplet dried, little additional penetration occurred. Therefore, the longer the time before droplet drying, the greater chemical uptake by foliage and fruit.

Temperature following thinner application is the dominant factor influencing the response to a chemical thinner.  Elevated temperatures provides the stress required for thinners to work. Warm temperatures intensify competition among competing sinks at a time when metabolic demand is highest in the tree. If cool weather follows thinner application, thinning results are frequently disappointing. It is often better to wait 2 or 3 days until warm temperatures are forecast to occur after application than to apply a thinner when cool conditions (< 65 °F) prevail immediately after application. However, temperatures above 85 °F can lead to excessive thinning. 

Several days of cloudy weather during the bloom period where incoming solar radiation is reduced to 10% to 15% of full sun can  can intensify fruit abscission.  Applying chemical thinning sprays at the beginning of a cloudy period probably will enhance thinning. Therefore, it may be advisable to delay thinner application under circumstances where trees may be exposed to several days of cloudy, warm weather to avoid overthinning. One or two days of sun following shading partially reversed the abscission-promoting effect of shading.

Orchardists generally try to apply thinners well in advance of rain, but occasionally this cannot be avoided.  A good rule-of-thumb is if a chemical thinner dries on the leaf prior to the onset of rain, one can anticipate getting at least 80% of the thinning effect.

Chemicals for Apple Thinning

Timing Chemical Trade Name Rate per 100 gal
dilute TRV
Bloom Naphthaleneacetic acid-sodium Fruitone-L, PoMaxa, Refine 3.5 WSG, Refine 3.5L 2-4 oz 48
  ATS (ammonium thiosulfate)*   2 to 4 gal.  
  Lime sulfur (calcium polysulfide)* Lime Sulfur Solution

4 to 10 gal (alone)

2 gal )with oil)

Petal fall Naphthaleneacedtamide Amid-Thin W 4-8 oz  48
  Naphthaleneacetic acid-sodium Fruitone-L, PoMaxa, Refine 3.5 WSG, Refine 3.5L 2-4 oz 48
  Carbaryl Sevin XLR Plus, Sevin 4F 0.5 to 1.5 pt 12
  6-Benzyl Adenine (6-BA) Maxcel, RiteWay 48-200 oz 12

6-Benzyl Adenine (6-BA)

Exilis 9.5SC

Exilis Plus

9.6 to 25.6 fl oz

see label

8-13 mm Fruit Size Naphthaleneacetic acid-sodium Fruitone-L, Pomaxa, Refine 3.5 WSG, Refine 3.5L 2-6 oz 48
  Carbaryl Sevin XLR Plus, Sevin 4F 0.5 to 1.5 pt 12
  6-Benzyl Adenine (6-BA) Maxcel, RiteWay 48-200 oz 12
  6-Benzyl Adenine (6-BA) Exilis plus, Exilis 9.5SC 6.4 to 25.6 fl oz 12
  S-Abscisic Acid (ABA) Protone® 6.6 to 33.1 oz 4
15-20 mm Fruit Size Ethephon Ethrel 1 to 1.5 pt 48
  Carbaryl Sevin XLR Plus, Sevin 4F 0.5 to 1.5 pt 12
  S-Abscisic Acid (ABA) Protone® 6.6 to 33.1 oz 4
VT and CT only ACC Accede® (apple and peach only) 23 to 46 fl oz 12

* not registered for chemical thinning in New England

Specific apple variety thinning recommendations


Variety 30-80%
full bloom
Petal fall 5-6 mm
(1 week after bloom)
8-14 mm fruit size
(2-3 weeks after bloom)


  1 to 2 pt carbaryl 1 to 2 pt carbaryl

Delicious (spur type)

2 gal ATS 2 oz NAA plus 1 pt carbaryl 64 oz 6-BA plus 1 pt carbaryl plus 1 qt ultrafine spray oil

Delicious (non-spur type)

  1 pt carbaryl 48 oz 6-BA + 1 pt Sevin OR 2 oz Fruitone + 1 pt Sevin
Empire   2 oz NAA + 1 pt carbaryl 64 oz 6-BA + 1 pt carbaryl OR 3 oz NAA + 1 pt carbaryl
Fuji 2 gal ATS 64 oz 6-BA + 1 pt carbaryl 64 oz. 6-BA + 1 pt carbaryl
Gala 2 gal ATS 3 oz NAA + 1 pt carbaryl 64 oz. 6-BA + 1 pt carbaryl
Gingergold     2 oz NAA + 1 pt carbaryl
Golden Delicious (no Provide)   3 oz NAA + 1 pt carbaryl 64 oz. 6-BA + 1 pt carbaryl OR 6 oz NAA + 1 pt carbaryl
Golden Delicious (with Provide)   3 oz NAA + 1 pt carbaryl 48 oz. 6-BA + 1 pt carbaryl OR 4 oz NAA + 1 pt carbaryl
Granny Smith     3 oz NAA + 1 pt carbaryl
Honeycrisp 2 gal ATS 4 oz NAA + 1 pt carbaryl 3 oz NAA + 1 pt carbaryl
Jonagold     3 oz NAA + 1 pt carbaryl
Lady Apple   3 oz NAA + 1 pt carbaryl 4 oz NAA + 1 pt carbaryl
Liberty   3 oz NAA + 1 pt carbaryl 64 oz. 6-BA + 1 pt carbaryl OR 3 oz NAA + 1 pt carbaryl
Macoun 2 gal ATS 3 oz NAA + 1 pt carbaryl 64 oz. 6-BA + 1 pt carbaryl OR 4 oz NAA + 1 pt carbaryl
McIntosh (non-spur type)     2 oz NAA + 1 pt carbaryl OR 36 oz 6-BA + 1 pt carbaryl
McIntosh (spur-type)     3 oz NAA + 1 pt carbaryl OR 48 oz 6-BA + 1 pt carbaryl
Mutsu (Crispin)     2 oz NAA + 1 pt carbaryl
Northern Spy   3 oz NAA + 1 pt carbaryl 2 oz NAA + 1 pt carbaryl
Paulared   3 oz NAA + 1 pt carbaryl 3 oz NAA + 1 pt carbaryl

Also see: Variety Thinning Recommendations for mature trees - 8 to 12mm fruit size at Cornell Cooperative Extension, Eastern New York Commercial Horticulture


Branching young trees


Branching young trees

Timing product concentration rate comments
BEFORE buds break in spring on 1-year old wood Maxcel, Promalin; exilis plus, exilis 9.5 SC 5,000 ppm see label If dormant buds are present on one-year-old wood only. DO NOT apply after bud break. See product labels for more details and instructions.
AFTER bud break in spring on 1-year old wood Maxcel, Promalin; exilis plus, exilis 9.5 SC 400 to 500 ppm see label If buds have broken, and the leaf tissue is showing on one-year-old wood. Apply by spraying with a back-pack sprayer. Works best when temperatures are warm and there is enough tissue to absorb the PGR. Note: Never add surfactant to a solution of MaxCel as it is already included in the formulated product. Promalin should be combined with a NIS following the label instructions.
2-year old wood, coinciding with bud break Maxcel 1,500 ppm 9.2 fl oz per gal water Make a notch with a hacksaw blade (narrow, fine-toothed saw, usually used for cutting metal) just above the existing bud scar on the leader. Then, on nonbearing trees, immediately spray the cut with a 1500 PPM solution.

For more information on branching youg trees, see the UMass Fact Sheet F-140 Branching Young Apple Trees with Plant Growth Regulators

Minor PGR uses on apples

Always look at the label before making application of any PGR’s as there are further use directions and warnings on the label which may not be included here.

ReTain® (Valent USA) - For increasing fruit set of apple, cherry, European pear. APPLE: Apply one pouch of ReTain per acre, as a single application from pink stage to full bloom. Applications made prior to pink stage or after full bloom will significantly reduce efficacy of the treatment. Do not apply after petal fall. CHERRY: Apply one to two pouches of ReTain per acre during bloom. Retain may be applied as a single application of up to two pouches, or as sequential applications of one pouch per application. Applications between popcorn stage (balloon stage) to first bloom are more effective than earlier or later applications. Do not apply after petal fall. EUROPEAN PEAR: Apply one pouch of ReTain per acre, as a single application from white bud stage to full bloom. Applications made prior to white bud stage or after full bloom will significantly reduce efficacy of the treatment. Do not apply after petal fall. Can also be applied to European pear at 10 mm fruit size at a rate of one pouch of ReTain at 10 mm fruit size to increase fruit set.

Promalin® (Valent USA) and Perlan® (Fine Americas) to increase fruit set in APPLE following frost by stimulating the development of parthenocarpic fruit. Make a single application at a rate of 1-2 pints in 50-200 gallons of water per acre prior to or within 24 hours following a frost or freeze event, when the majority of the crop is between early bloom and full bloom. Do not apply to frozen foliage, blossoms or developing fruit, allow trees to completely thaw prior to application. Do not use a surfactant. Do not apply more than 2 times for this use.

Promalin® or  Maxcel ® (Valent USA) and Perlan® or Exilis 9.5SC (Fine Americas), latex paint application FOR INCREASING BRANCHING AND FEATHERING OF NON-BEARING APPLE, PEAR AND CHERRY TREES. Such an application improves tree structure by improving branch angles and increasing bud break and shoot growth in nursery stock and young trees. At the location where branching is desired, apply a uniform application in latex paint mix at a rate of 5,000-7,500 ppm (0.8-1.2 fl. oz./pint of latex paint). The latex paint mix should be applied using a brush or sponge to achieve thorough coverage of the bark surface. Application should only be made to one year old wood in the spring once terminal buds begin to swell but before shoots emerge. Applications made following shoot emergence may result in injury to the young shoots. DO NOT apply latex paint mix after bud break. Doing so may cause injury to shoot tips and reduce the effectiveness of the application for shoot growth. One-year pre-harvest interval. See F-140 Branching Young Apple Trees with Plant Growth Regulators

Protone® is registered as a defoliant to stimulate leaf drop in the fall. It may be used on nursery trees or on trees in the orchard carrying green leaves into the fall. A good nonionic surfactant should be used with this and at a rate between 250 and 1000 ppm. (16.5 - 66.1 ounces per acre.) Application on trees in the orchard is meant to speed leaf abscission and speed the development of dormancy. (May be particularly handy on non-cropping younger trees still actively growing later into the fall.)

Pre-harvest drop control of apples

Written by: 
Revised by Renae Moran, Univ. of Maine

There are three options for using plant growth regulators for pre-harvest drop control. One is NAA, sold as several different products, another is AVG, sold as ReTain (Valent USA Agricultural Products), and the third is 1-methylcylopropene sold as HarvistaTM (Agrofresh). The three differ in mode of action and timing of application. NAA is typically applied just once before the onset of fruit drop and lasts for about one week before its effect is gone. ReTain is typically applied 2-3 weeks before anticipated harvest, has long lasting drop control if applied under favorable conditions, and repeat applications can be made to extend the period of time apples will remain on the tree. Harvista can be applied 3 to 21 days before harvest and requires specialized equipment (contact Agrofresh).  Both ReTain and Harvista will slow fruit ripening.  For an excellent summary/discussion of uisng apple PGR's for drop control, see this Penn State University article: Apple PGRs - Prevention of Preharvest Drop in Apple Orchards.

Application details for NAA (Fruitone-L, Refine 3.5 WSG, Refine 3.5L)

  • rates of 10 ppm are usually effective, however, up to 20 ppm can be used
  • good coverage at dilute application is preferred
  • apply just as apples starts to loosen; apply too early and drop control will be limited; apply too late, and drop will have already started
  • drop control can last for 7-10 days; sometimes two applications (at 5-7 days apart) can extend that to 14 days
  • NAA application can result in accelerated fruit maturity and fruit softening

For more information: Getting the most out of that "old" stop drop: NAA (

Application details for ReTain

!Note that the ReTain label gives very specific instructions depending on intended use -- read it!

General use guidelines for ReTain include:

  • apply 21 to 7 days before anticipated harvest (7 day PHI)
  • use one pouch (or less) per application; two applications/two pouches are allowed
  • on Honeycrisp and Gala, consider using reduced rates -- early application of ReTain can inhibit color development on these varieties
  • use an organosilicone surfactant at a concentration of 0.05 to 0.1% (6.4 to 12.8 oz surfactant per 100 gallons water) with ReTain, do not apply when temperature exceeds 85 degrees F.
  • dilute applications are recommended, up to 2 X concentration
  • avoid application when rain is expected within 12 hours
  • do not apply to stressed trees

For more information: Double Applications of ReTain® to provide longer fruit drop control compared to the standard single application ReTain 

Application details for HarvistaTM

  • applied at a rate of 48 to 242 fl. oz.
  • Do not add any water to the container 
  • Do not allow product to contact any copper
  • drop control can last for 7-10 days

Return Bloom Enhancement

Varieties with a strong tendency for biennial bearing may have insufficient bloom in the 'off' year and consequent low yield.  Ethephon and NAA products can be applied at lose doses in summer for a modest increase in return bloom.  Apply these in the 'on' year in four weekly (or every two weeks) applications beginning six to eight weeks after petal fall.  These compounds can hasten ripening in summer apples. 


Chemical Product Rate / 100 gal. TRV Rate / Acre REI (hours) PHI (days

Fruitone L


Refine 3.5 L

3 fl oz 2 to 8 fl oz 48 2
  Refine 3.5 WSC 1 to 2 oz see label 48 2
Ethephon Ethrel 0.5 fl oz 0.5 to 3 pts 48 7



Vegetative growth control

Vegetative growth control can be achieved by application(s) of Apogee or Kudos 27.5 WDG, both containing the active ingredient prohexadione-calcium. In addition to providing growth control, early application can suppress fire blight of shoots. 

Application details for Apogee or Kudos 27.5 WDG on apples

  • Apply beginning at late bloom to early petal fall; earlier applications seem to give better results (Ed. note: in 2019, Kudos was approved for use beginning at pink bud stage @ 6 oz. per acre. This application in particular may help suppress fire blight and result in improved growth control, although follow-up applications may still be necessary. Only the Kudos formulation of prohexadione-calcium has the specific pink timing recommendation. The Apogee label still says application beginning at 1-3 inches of shoot growth.)
  • Apply at rates of 3 to 12 oz. per 100 gallons of dilute spray water depending on the amount of growth control desired and number of anticipated applications
  • Repeat applications at 1 to 4 week intervals may be necessary for season-long growth control
  • Do not apply more than 48 oz per acre within any 21-day interval, and there is a maximum of 99 oz per acre per season 
  • Always use a water conditioner such as ammonium sulfate, Choice, or Quest.
  • Do not tank mix with sprays containing calcium
  • Chemical thinning efficacy may be decreased when using prohexadione-calcium
  • Direct spray to top of trees if only vigor control is desired in the top of the tree
  • Not recommended for use on Empire, may cause russetting