Damaging Temperature Thresholds

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.