College of Agriculture & Natural Resources Department of Horticulture

Critical Temperatures for Blossom Buds

THE TEMPERATURE at which fruit buds are injured depends primarily on their stage of development. They are most hardy during the winter when they are fully dormant. As they begin to swell and expand into blossoms, they become less resistant to freeze injury. 

Not all blossom buds are equally tender. Resistance to freeze injury varies within trees as it does between orchards, varieties,and crops. Buds which develop slowly tend to be more resistant.  As a result, some buds are usually killed at higher temperatures while others are resistant at much lower temperatures. To indicate this range, a new set of temperature values has been developed. It shows the average temperatures required to kill 10 percent of the buds and 90 percent of the buds as found in research at the WSU Research and Extension Center, Prosser from 1964-1970. 

The range in temperatures at which buds are killed is a factor in determining the need for frost control. Orchards with a large number of buds should be able to stand more frost than those with only a light set of buds. In using the temperatures, consideration should also be given to weather conditions preceding cold nights. Prolonged cool weather will tend to increase bud hardiness during the early stages of bud development. 

These new values do not replace the temperatures previously given in EM 1616, but provide additional information on critical temperatures.

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CRITICAL TEMPERATURES (°F) FOR BLOSSOM BUDS*  APPLES

Washington State University Extension Bulletin 0913

^* For Red Delicious. Golden Delicious and Winesap approximately 1 degree hardier; Rome Beauty, 2 degrees hardier; After petal fall all varieties are equally tender.

** Bud development stages: 1 - Silver Tip; 2 - Green Tip; 3 - Half-inch green; 4 - Tight Cluster; 5 - First pink; 6 - Full pink; 7 - First bloom; 8 - Full bloom; 9 - Post bloom

¹ Critical temperatures as previously published.
² Average temperatures found by research at the WSU Research and Extension Center, Prosser, to result in 10% and 90% bud kill.
³ Average date for this stage at the WSU Research and Extension Center. 

CRITICAL TEMPERATURES (°F) FOR BLOSSOM BUDS Apricots

Washington State University Extension Bulletin 1240

** Bud development stages: 1 - first swelling; 2 - tip separates; 3 - red calyx; 4 - first white; 5 - first bloom; 6 - full bloom; 7 - in the shuck; 8 - green fruit.

¹ Critical temperatures as previously published in WSUEM 1616.
²Average temperatures found by research at the WSU Research and Extension Center, Prosser, to result in 10% and 90% bud kill.
³ Average date for this stage at the WSU Research and Extension Center. 

CRITICAL TEMPERATURES (°F) FOR BLOSSOM BUDS* Sweet Cherries

Washington State University Extension Bulletin 1128

*For Bing. Lambert and Rainier are approximately 1 to 2 degrees hardier through Stage 6. 

** Bud development stages: 1 - first swelling; 2 - side green; 3 - green tip; 4 - tight cluster; 5 - open cluster; 6 - first white; 7 - first bloom; 8 - full bloom; 9 - post bloom.

¹ Critical temperatures as previously published in WSU EM1616.
² Average temperatures found by research at the WSU Research and Extension Center, Prosser, to result in 10% and 90% bud kill.
³ Average date for this stage at the WSU Research and Extension Center. 

CRITICAL TEMPERATURES (°F) FOR BLOSSOM BUDS Tart Cherries

Michigan State University Research Report 220

* Bud development stages: 0 - dormant; 1 - first swelling; 2 - side green; 3 - green tip; 4 - tight cluster; 5 - open cluster; 6 - first white; 7 - first bloom; 8 - full bloom.

¹ Wide range, depending on time of year.

CRITICAL TEMPERATURES (°F) FOR BLOSSOM BUDS* Peaches

Washington State University Extension Bulletin 0914

* For Elberta.

** Bud development stages: 1 - first swelling; 2 - calyx green; 3 - calyx red; 4 - first pink; 5 - first bloom; 6 - full bloom; 7 - post bloom.

¹ Critical temperatures as previously published in WSU EM1616.
² Average temperatures found by research at the WSU Research and Extension Center, Prosser, to result in 10% and 90% bud kill.
³ Average date for this stage at the WSU Research and Extension Center. 

CRITICAL TEMPERATURES (°F) FOR BLOSSOM BUDS* Pears

Washington State University Extension Bulletin 0978

^* For Bartlett. Anjou is similar in hardiness but may bloom earlier and therefore may be more tender than Bartlett at the same date.

** Bud development stages: 1 - Scales separating; 2 - Blossom buds exposed; 3 - Tight cluster; 4 - first white; 5 - full white; 6 - first bloom; 7 - full bloom; 8 - post bloom. 

¹ Critical temperatures as previously published. 
² Average temperatures found by research at the WSU Research and Extension Center, Prosser, to result in 10% and 90% bud kill.
³ Average date for this stage at the WSU Research and Extension Center. 

CRITICAL TEMPERATURES (°F) FOR BLOSSOM BUDS* Prunes

Washington State University Extension Bulletin 1186

* For Italian Prunes and Early Italian Prunes. 

** Bud development stages: 1 - first swelling; 2 - side white; 3 - green tip; 4 - tight cluster; 5 - first white; 6 - first bloom; 7 - full bloom; 8 - post bloom.

¹ Critical temperatures as previously published in WSU EM1616.
² Average temperatures found by research at the WSU Research and Extension Center, Prosser, to result in 10% and 90% bud kill.
³ Average date for this stage at the WSU Research and Extension Center.