Frost Control in Almonds/Stone Fruit and the Critical Role of Dew Point
Spring frost events are among the most costly weather hazards for tree fruit growers in the San Joaquin Valley while temperature forecasts often get the most attention it’s the interaction between air temperature, dew point and orchard microclimate that determines when frost will actually form and when growers need to start frost protection systems such as sprinklers or wind machines. Understanding dew points is essentially the temperature at which air becomes saturated and water vapor begins to condense which is a key to making timely decisions during cold clear nights.
What happens on a frost night?
On calm clear nights the ground loses heat rapidly through long wave radiation, the air near the ground cools faster than air aloft and if the temperature drops below dew points moisture will condense onto leaf surfaces including leaves and fruit. This process releases a small amount of latent heat as water changes from vapor to liquid which actually slows further cooling.
Critical Low Temperatures for Almonds and Stone Fruit
These temperatures reflect commonly cited UC damage thresholds for spring frost events. Exact injury depends on duration of exposure, tree health, and cultivar.
Almond (Nonpareil-type sensitivity typical)
| Growth Stage | 10% Kill | 90% Kill |
| Tight bud | 18°F | 5°F |
| Pink bud | 25°F | 15°F |
| Full bloom | 27°F | 24°F |
| Petal fall | 28°F | 25°F |
| Small nut (Jacket split) | 29°F | 26°F |
Practical field threshold (most growers protect at): 28°F during bloom.
Peach
| Growth Stage | 10% Kill | 90% Kill |
| Swollen bud | 18°F | 2°F |
| First pink | 25°F | 15°F |
| Full bloom | 27°F | 24°F |
| Petal fall | 28°F | 25°F |
| Shuck split | 29°F | 26°F |
Apricot
| Growth Stage | 10% Kill | 90% Kill |
| First bloom | 25°F | 18°F |
| Full bloom | 27°F | 24°F |
| Petal fall | 28°F | 25°F |
Sweet Cherry
| Growth Stage | 10% Kill | 90% Kill |
| First white | 25°F | 17°F |
| Full bloom | 28°F | 25°F |
| Petal fall | 28°F | 26°F |
Sprinkler Turn-On Temperatures Based on Dew Point
Important Correction on Dew Point & Timing
When dew point is LOW (dry air), sprinklers must be started EARLIER.
Why?
- Low dew point = large wet-bulb depression
- When sprinklers first start, evaporative cooling occurs
- This evaporative cooling can temporarily drop tissue temperature several degrees below the air temperature
- If you wait until air temperature reaches your critical temperature, in dry air, evaporated cooling can quickly drop the temperature below your critical level before air is saturated and starts warming.
That is why, under very dry conditions (dew points in the teens or low 20s), growers in the San Joaquin Valley often start protection 4–6°F above the critical damage temperature, not 1–2°F.
Practical Rule for San Joaquin Valley Growers
For almonds at full bloom (28°F critical):
- Dew point above 32°F → Start around 32–34°F
- Dew point 25–30°F → Start around 33–35°F
- Dew point below 22°F → Start around 34–36°F
Dry air requires a bigger buffer.
Also, words of wisdom are. When you have micro sprinklers, the spaghetti tubing going to the sprinkler, water inside these small tubes will start to freeze at 32F. This will plug up most of your sprinklers in the low areas. So, it’s always wise to start the water before 32F on cold nights with micro sprinklers, and even higher with dew points very low.
Is drip irrigation enough to keep above critical temperatures?
It can give you one to two degrees of protection. However, they need to be turned on even earlier, maybe even a day before the frost event to moisten the ground around the drip emitter. This moist darker soil absorbs more heat energy from the sun during the day and then releases more heat at night. Then keep the water on through the night to help saturate the atmosphere in the orchard to slow the temperature dropping.
Table 3 below can help you determine the temperature when to turn sprinklers on when dew points are low.
Table 4 can help you calculate your approximate dew point if there are no dew point in your forecast in your area but you do have relative humidity.
