Action Program for Dormex Application on Peaches

Arlie A. Powell
Horticulturist - Fruits
Auburn University
January, 1999

Background Information

On a worldwide basis, Dormex (hydrogen cyanamide) has been used for a number of years on fruit crops to replace lack of winter chilling, induce uniform bud break and yield increase plus provide several other desirable changes in plant performance. It is being used in countries such as Mexico, Chile, New Zealand, Australia and Israel on several fruit crops including grapes, kiwifruit, apples, pears, peaches and plums.

The first successful registration of Dormex on US fruit crops was on table grapes in 1992 in California and Arizona. California's kiwifruit industry was successful in helping obtain a federal label (Section 3) for use of Dormex on kiwifruit in 1997.

Alabama was successful in obtaining a Section 18 for use of Dormex on peaches in 1992. At least two other Section 18s were filed with EPA but were never approved. An EUP for use on peaches, apples and several other fruits was approved in 1995. A full Section 3 for use of Dormex on peaches and several other crops has been pending with EPA since 1997. Alabama, Georgia and South Carolina have been approved for the use of Dormex on peaches, nectarines and blueberries via state labels (24C) as of January, 1999.

Research and demonstration studies have been conducted over the past ten years in Alabama to evaluate the use of Dormex in alleviating problems associated with lack of winter chilling on peaches.  Results have been very positive and have demonstrated this growth regulator will result in good fruiting and shoot growth of peaches suffering from lack of chilling when applied in an appropriate manner.

It is worth noting that Dormex is the only growth regulator available on a worldwide basis that will effectively replace lack of chilling in peaches.  It is manufactured by SKW - Trostberg Ag of Trostberg, Germany. During the past ten years, DK International of Marietta, Georgia has been the distributor for Dormex across the United States. As of January, 1999, Helena will be distributing dormex to producers in Alabama.

To better understand the importance of adequate chilling for peach varieties, refer to Table 1.  'Ruston Red' (850 chill hour requirement) was the variety studied over a period of four years in South Alabama to more closely examine how each level of chilling deficiency affected overall plant performance.  Similar effects have generally been observed with other varieties although a given level of chilling deficiency (such as 200 chilling hours) may prove more severe on some varieties than others.

Table 1. Observed effects of lack of winter chilling on 'Ruston Red' peach in south Alabama
over a 4-year period, 1988-1992.1

Chilling Deficiency Level 
(Hrs < 45 F)
Tree Performance
Less than 50 100% Crop - Fair to Good Fruit Quality (Generally marketable) Slightly Slow Leafing From 1-Yr Wood But Good Foliage Development.
Less than 100 50 to 100% Crop - Fair Fruit Quality (Cullage usually light to moderate) Slow Leafing From 1-Yr Wood.
Less than 150 25 to 75% Crop - Poor To Fair Fruit Quality (Poor pack out) Slow, Poor Leafing From 1-Yr Wood.
Less than 200 10 to 25% Crop - Poor Fruit Quality (Of little value) Slow, Poor Leafing From 1-Yr Wood, Considerable Leafing From 2-Yr Wood.
Less than 250 Total Crop Loss to 10% Crop - Very Poor Fruit Quality (No value) Very Slow, Poor Leafing From 1-Yr Wood and Some Heavy Leafing From 2-Yr Wood.
Less than 300 Total Crop Loss - Little to no leafing from 1-Yr wood. Leafing is primarily from 2-Yr and older wood. Dieback of 1-yr wood.
1'Ruston Red' requires 850 chilling hours < 45 F to satisfy its rest period.

Results of Studies In Alabama

Studies in Alabama peach orchards (similar results have been found in Georgia and Texas) have allowed us to more clearly establish proper use of this chemical on peaches for optimum performance.  There are several very important points which must be made regarding the use of this material.

     1)   Dormex can and will over thin fruit buds resulting in crop reduction if applied at the wrong time and/or at an excessive concentration.  (Among fruit crops the peach seems to be one of the most sensitive to bud damage and subsequent crop reduction.)  Thus real care must be exercised when applying Dormex to peaches.  Damage to some 1-year-old shoots often occurs if applied at rates above 2%.

     2)   Dormex will effectively replace up to 250 to 290 chilling hours (might approach 300 hours with some varieties), based on our research studies over the past ten years, using 650 to 900 hour varieties.

     3)   We have determined that peach buds begin becoming receptive to Dormex once 45 to 50% of their chilling requirement has accumulated. However, we have not determined exactly how much additional chilling beyond the 50% level is necessary to ensure successful cropping in all varieties. Dormex was not able to overcome a chilling shortage of 350 to 400 hours with 850 and 900 hour varieties in South Alabama studies in 1989. Field studies have shown that once 65 to 68% of chilling (555 to 578 hours) is satisfied for 850 hour varieties, Dormex will easily handle the deficiency and provide good vegetative growth and cropping. Dormex may work on lower chilling varieties when only 55 to 60% of chilling has been satisfied. We estimate that as much as 70% of the chill requirement of high chillers (950+) must be satisfied before Dormex works effectively.

     4)   An important factor growers must understand is that under certain conditions, peach trees can begin forcing their fruit buds well before their chilling requirement is satisfied.  Actually, studies show fruit buds begin forcing when 60 to 70% of their chilling requirement is met (depending on variety) and trees are then subjected to about 10 to 14 days or more of mild to warm temperatures before another period of chilling resumes.  The important point to understand is that Dormex may be safely applied even when as much as 95% of a varieties' chilling requirement has been satisfied so long as fruit buds are still dormant.  Otherwise buds which are showing visible growth will usually be excessively thinned.

     5)   Studies have clearly established that rates of 0.5 to 2% Dormex are the most effective for use on peaches.  However a 2% rate sometimes results in excessive bud thinning and light crops.  Therefore, growers should use 0.5 to 1.5% rates (volume/volume) with 1.0% considered the optimum rate.  A 0.5% rate is the safest to use when some bud activity may have begun although not readily visible.  This low rate will cause trees suffering from lack of chilling to set crops just as well as higher rates but fruit shape is usually not as good.  To use a 1.0% rate requires 1 gallon of Dormex (50% a.i.) plus 1 qt. of non-ionic surfactant (1/4% rate) added to 98.75 gallons of water to make 100 gallons of spray.

     6)   Depending upon the timing of the application, Dormex can cause peaches to bloom as much as 1 to 2 weeks earlier than normal.  This might be desirable in some instances but would generally be a disadvantage for most growers.  All of the factors that cause earlier blooming are not fully understood.  In most cases sprays applied in January may result in bloom dates 7 to 14 days earlier than normal.  Early February sprays may cause blooming a few days earlier while late February sprays do not generally advance bloom dates. It appears when Dormex is applied after only 50 to 60% of the chilling level is satisfied and a considerable amount of the winter period remains, there is the greatest tendency for advancing flowering.

There is some evidence to indicate trees sprayed with Dormex that were pruned in late summer - early fall may bloom earlier than trees pruned in late winter.  It seems Dormex results in less earliness of blooming when unpruned trees are sprayed to an 8-ft. height (upper areas not sprayed) and pruning is completed several days to one or more weeks after application.  It is suggested that growers who plan to use Dormex wait to prune (a few days) until after the application is made. However, whether pruning was completed before or after application, Dormex works quite well.

     7)   Enough studies have been conducted to show that under severe lack of chilling conditions where Dormex may prove inadequate to ensure cropping on very high chillers, it will still stimulate leaf bud break such that trees will resume normal shoot development without loss of limbs.

Proper and Safe Timing of Dormex Applications

Please use the table below to determine when the "safe window" for application begins for each chilling class of varieties (such as 750, 850 etc.).  For example, when the chilling level (using Modified 45 model) reaches 490 hours for your location, the 750 chill hour class of varieties may then be sprayed with Dormex to ensure good leafing and cropping. However, as long as cool to cold temperatures are forecast to continue (beyond this initial application window), waiting to apply Dormex at a higher level of chilling will help reduce the possibility of advancing the date of flowering. Waiting too late to spray can result in excessive bud thinning. The special commentary dated 20 January 99 on this home page explains how to monitor bud development to assist with timing of sprays.

Table 2. Winter Chilling Information For Use As A Guide In Applying Dormex To Peaches In Alabama
Chilling Class 
(Hrs@ 45 F or less)
Optimum Chilling Range For Application 
As % of Chill Requirement
Chilling Hours Received
62 to 85
280 to 385
62 to 85
340 to 470
65 to 85
425 to 555
65 to 85
490 to 640
65 to 85
550 to 725
70 to 90
665 to 855

Keys on Safe and Effective Application of Dormex

A. Apply Dormex at a rate of 0.5 to 1.5% (volume/volume) plus a non-ionic surfactant at 0.25%.  The preferred rate of 1.0% would consist of 1.0 gal of Dormex (50% a.i.) plus 1.0 quart of non-ionic surfactant and 98.75 gals of water to prepare 100 gals of spray solution.

B. The spray should be applied when 65 to 85% (or other appropriate level as given in Table 2 above) of the chilling requirement has been met and while fruit buds are still dormant (no visible activity).

C. Dormex should be applied to dormant trees as a complete tree spray to near the point of drip.  Excessive wetting is not desirable or necessary.  Depending upon tree size 50 to 100 gals of spray should be adequate per acre.  (It isn't necessary to spray upper parts of trees that will be removed during pruning.) For mature, larger trees at least 75 gals of spray per acre is required for optimum coverage of buds. Dormex is not systemic and doesn't translocate throughout the plant. Therefore it is very important to provide a coarse type spray (lower pressure) that provides effective coverage of all fruiting wood. Do not apply spray as a fine mist under high pressure because of wind drift problems that could cause damage to adjacent plants.

D. For optimum effectiveness and no phytotoxicity Dormex should be applied under sunny conditions such that sprays will dry in15 to 30 minutes.  Do not apply under misty/or foggy conditions or if such conditions or rain is forecast within 12 hours after application.  (To be safe spray when no rain is forecast for at least one day after application.)  Applying under these conditions may cause excessive thinning of fruit  buds.

Apply when current temperatures are between 50 to 75 degree F. Do not apply when current temperatures are below 40 degrees F., or if temperatures of 32 degrees or below are forecast for the next morning.

E. Apply Dormex in combination with no other chemicals other than surfactant.  Do not apply any pesticide (including oil spray) within one week before or one week after Dormex application.  It is preferable to not spray with a dormant oil within two weeks (before or after) a Dormex application.

Special Safety Precautions & Chemistry Of Dormex

Dormex (hydrogen cyanamide) is available in one formulation only, a blue liquid.  It will not store well at room temperature (should at least be stored below 70F) but will remain in good condition for at least 2 ro 3 years if refrigerated (35 to 40F.).

Dormex is a moderately toxic chemical but not nearly as hazardous as pesticides such as parathion.  However, special care should be exercised to ensure safe use of this chemical in the orchard.  Protective clothing including head gear, etc. must be worn during mixing and application.  No alcohol should be consumed for 24 hours before and following application.  Non-target areas must not be sprayed and a non-sprayed buffer area must be maintained around the orchard to avoid problems with endangered or threatened species.  Under aerobic soil conditions, hydrogen cyanamide degrades rapidly with a half-life of less than 1 day.  Refer to the label for other particulars regarding safe use of Dormex in peach orchards.


Brand names used in this publication are for clarity only.  The mention of a brand name in no way constitutes endorsement of this brand or product by the Cooperative Extension Service.  Nor is there intended any discrimination against similar unnamed products or brands which may be just as effective.

Special appreciation is extended to: SKW Chemicals of Trostbert, Germany and DK International of Marietta, GA for funding and chemicals provided for these studies; County Extension Agents Ed Tunnell and Richard Murphy,  Master Gardeners James Rye and Gus Utter, and producers Greg Burris, Erma Anzaldo and David Delvecchio for their invaluable assistance in conducting these studies.  William Dozier, Jr., Arnold Caylor, James Pitts, and Bobby Boozer of Auburn University and Jimmy Witt (former graduate student) also made valuable contributions to this work.

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