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Damaging Temperature Thresholds

Posted: April 13, 2011

What a difference a year makes! Last year at this time many areas were in full bloom or past full bloom on many tree fruit species. I was down at FREC the first week of April to see peaches in full bloom. At Rock Springs we had apples, peaches, and sweet cherries all come into full bloom around the 20th of April. Thankfully, this year, bloom seems to be a little bit closer to our normal time. (We will be applying our Bordeaux spray tomorrow.) If you remember last year you will also remember that we did have some cold weather later in May around the 8th and 9th. In many cases this caused damage to the young developing fruit. Hopefully, this type of late frost/cold temperature situation will not repeat itself. The hardest hit areas seemed to be the western and northern portions of the state. While most of you do not have provisions for frost protection, it is still useful to review temperatures that can cause damage to flowers. The table at the end of this article is from the current Tree Fruit Production Guide and can serve as a guide on what to expect should temperatures get close to freezing.
2010 Freeze damage from May 8th-9th cold temperatures

2010 Freeze damage from May 8th-9th cold temperatures

Dr. Rob Crassweller, Penn State Professor of Tree Fruit

I need to add a couple of comments about those temperature thresholds. First, not all flowers on a given tree are equally sensitive—some flowers can be a little more hardy than others. The same is true for trees of the same cultivar—some trees may be less sensitive and of course trees that had a heavy crop the previous year may be more sensitive to cold temperatures. Normally to achieve the damage in the table flowers must be exposed to the threshold temperature for at least 30 minutes. Conditions leading up to a cold event will impact damage to the flowers. Exposure to low temperatures and dry conditions prior to a frost event can induce greater hardiness and resistance to damage. Remember that the coldest part of the night usually occurs just before sunrise. The dew point also influences how rapidly the temperature will drop at night. Dew points above 320F will not cause temperatures to drop as fast or as low, however, dew points below 320F will cause a more rapid drop and to a greater depth.

For those of you that do not have any provisions for frost protection there are a couple of actions that can help reduce the potential for temperature drops. First orchard floor conditions make a great deal of difference in potential frost damage. Heat is absorbed by the soil during the day and released to warm the blossoms at night and early in the morning. Maximum exposure of the soil to sunshine is necessary to provide optimum frost protection. The following is a table of relative temperature differences as influenced by orchard floor conditions:

Bare, firm, moist ground                                                         warmest
Shredded cover crop, moist ground                                        ½°F colder
Low-growing cover crop                                                     1°-3°F colder
Dry, firm ground                                                                  2°F colder
Freshly disked, fluffy ground                                                  2°F colder
High cover crop                                                                 2°-4°F colder
Where cover crop restricts air drainage                                  6°-8°F colder

Second, dark colored soils absorb a greater amount of heat during the daytime and can potentially store more heat. Gravelly soils also have a greater capacity to absorb solar radiation.

Third, compacted bare, but moist, soil can store a greater amount of daytime solar radiation heat than a covered dry soil. Thermal conductivity and heat content of soils are affected greatly by the soil water content.  On a daily basis heat is transferred into and out of approximately the top 1 ft of soil.  When the soil is wet, heat transfer and storage in the upper soil layer is better, so more heat is stored during daylight for release during the night.  Considerable differences between thermal conductivity and heat capacity are observed between dry and moist soils.  However, if the soil water content is near field capacity, wetting the soil is unnecessary.  Wetting the soil to a depth below 1 foot is unnecessary because diurnal temperature is insignificant below that level.

Lastly, heavier soils with more clay retain heat better than sandy soils. Sandy soils are also often lighter in color and hence tend to reflect more sunlight rather than absorb it in the form of heat.

Stage of

10% kill

90% kill

development

0F

0F

Apples

 

 

Silver tip

15

2

Green tip

18

10

½ inch green

23

15

Tight cluster

27

21

First pink

28

24

Full pink

28

25

First bloom

28

25

Full bloom

28

25

Post bloom

28

25

Peaches

 

 

First swelling

18

1

Calyx green

21

5

Calyx red

23

9

First pink

25

15

First bloom

26

21

Full bloom

27

24

Post bloom

28

25

Pears

 

 

Scales separating

15

0

Blossom buds exposed

20

6

Tight cluster

24

15

First white

25

19

Full white

26

22

First bloom

27

23

Full bloom

28

24

Post bloom

28

24

Sweet Cherries

 

 

First swelling

17

5

Side green

22

9

Green tip

25

14

Tight cluster

26

17

Open cluster

27

21

First white

27

24

First bloom

28

25

Full bloom

28

25

Post bloom

28

25

Apricots

 

 

First swelling

15

-

Tip separates

20

0

Red calyx

22

9

First white

24

14

First bloom

25

19

Full bloom

27

22

In the shuck

27

24

Green fruit

28

25