Frost Protection: Considering Wind Speed and Dew Point
At our Spring mini clinics, we had several questions regarding the influence of dew point and wind speed on the timing of turning on sprinkler irrigation for frost protection. Here, I will address the considerations you need to keep in mind when determining when to turn on your irrigation to protect the vines.
Plant phenological stage: The cold hardiness of plants can change significantly as anatomical and physiological changes begin to occur due to warmer temperatures. Early phenological stages, such as tight and swollen buds, will be much more tolerant to frost damage than more advanced stages, such as hook and bloom (refer to Table 1). Frequent scouting of beds is necessary to determine the phenological stages of cultivars as the season progresses and to adjust critical temperatures for frost protection.
Phenological stage | Minimal survival temperature (˚F) |
Swollen buds | 0-19 |
Cabbage Head | 21-25 |
Bud break | 21-25 |
Bud elongation | 27 |
Rough Neck and early hook | 27-28 |
Hook | 30 |
Bloom | 30-31 |
- Wind Speed: The basis of frost protection with overhead irrigation is that the water we apply to the plants will convert from a liquid state to a solid state (ice), releasing energy (latent heat of fusion water) in this process. As long as liquid water is freezing on the plant at all times, the plant surface temperature will remain at about 32 ˚F. However, if wind velocities are high, water may evaporate from the plant surface, causing evaporative cooling and lowering the temperature of the plants, thus increasing frost damage. To avoid evaporative cooling in windy situations, we need to increase the volume of water applied while frost protecting (refer to Table 2). Frost protection by irrigation has its limitations and is not recommended if wind velocities exceed 10 miles per hour and temperatures are below 23 ˚F.
Canopy temperature (˚F) | Wind Speed (mph) | |||||
0-1 | 2-4 | 5-8 | 10-14 | 18-22 | 30+ | |
Acre-inches per hour needed for freeze protection | ||||||
27 | 0.10 | 0.10 | 0.10 | 0.10 | 0.20 | 0.20 |
26 | 0.10 | 0.10 | 0.14 | 0.20 | 0.40 | 0.60 |
24 | 0.10 | 0.16 | 0.30 | 0.40 | 0.80 | 1.60 |
22 | 0.12 | 0.24 | 0.50 | 0.60 | 1.20 | 1.80 |
20 | 0.16 | 0.30 | 0.60 | 0.80 | 1.60 | 2.40 |
18 | 0.20 | 0.40 | 0.70 | 1.00 | 2.00 | 3.00 |
15 | 0.26 | 0.50 | 0.90 | 1.30 | 2.60 | 4.00 |
11 | 0.34 | 0.70 | 1.20 | 1.70 | 3.40 | 5.00 |
- Dew point: When to start frost protecting significantly depends on the moisture content of the air, which can be measured using methods such as relative humidity (RH) and dew point. While RH indicates the amount of water vapor in the air relative to its maximum capacity at a given temperature, it alone isn’t sufficient for determining when to initiate frost protection. Dew point, which reflects the temperature at which moisture transitions from vapor to liquid, is a more reliable indicator. When the dew point is too low and the air is dry, the water applied through overhead irrigation to protect the plants will evaporate to compensate for the low humidity in the air. This will lower the surface temperatures of the plants and result in more frost damage. When dew point temperatures are moderate, some of the water applied will evaporate, but the other portion will freeze, releasing heat and warming up the air around the plants. After the overhead irrigation has been running for a while, the RH will increase, resulting in less water evaporation and more water freezing, which will lead to an overall positive net energy. In simple terms, if the dew point is too low, you must start frost protecting at warmer temperatures, since a large portion of the water applied at the beginning will evaporate, lowering the overall plant surface temperature, until the air saturates with water vapor and the water being applied begins to freeze, thus releasing heat.
To determine the temperature at which you should start frost protecting, you can refer to Table 3. First, determine the critical temperature for frost damage at a given phenological stage (refer to Table 1), and then select the dew point temperature forecasted for the frost event. The point where the row and column intersect is the temperature at which you should start frost protecting. It is generally recommended to use overhead irrigation as a method to frost protect only when in the 24 to 32 ˚F temperature range.
Dew-point Temperature (˚F) | Critical Temperature for frost damage (˚F) | ||||||||||
22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | |
32 | 32.0 | ||||||||||
31 | 31.0 | 32.7 | |||||||||
30 | 30.0 | 31.7 | 33.3 | ||||||||
29 | 29.0 | 30.6 | 32.3 | 34.0 | |||||||
28 | 28.0 | 29.6 | 32.2 | 32.9 | 34.6 | ||||||
27 | 27.0 | 28.6 | 30.2 | 31.8 | 33.5 | 35.2 | |||||
26 | 26.0 | 27.6 | 29.2 | 30.8 | 32.4 | 34.0 | 35.7 | ||||
25 | 25.0 | 26.5 | 28.1 | 29.7 | 31.3 | 32.9 | 34.6 | 36.3 | |||
24 | 24.0 | 25.5 | 27.1 | 28.6 | 30.2 | 31.8 | 33.5 | 35.1 | 36.8 | ||
23 | 23.0 | 24.5 | 26.0 | 27.6 | 29.1 | 30.7 | 32.3 | 34.0 | 35.6 | 37.3 | |
22 | 22.0 | 23.5 | 25.0 | 26.5 | 28.1 | 29.6 | 31.2 | 32.8 | 34.5 | 36.1 | 37.8 |
21 | 22.5 | 24.0 | 25.5 | 27.0 | 28.5 | 30.1 | 31.7 | 33.3 | 34.9 | 36.6 | 38.2 |
20 | 22.9 | 24.4 | 25.9 | 27.4 | 29.0 | 30.6 | 32.1 | 33.7 | 35.4 | 37.0 | 38.7 |
19 | 23.4 | 24.9 | 26.4 | 27.9 | 29.4 | 31.0 | 32.6 | 34.2 | 35.8 | 37.5 | 39.1 |
18 | 23.8 | 25.3 | 26.8 | 28.3 | 29.8 | 31.4 | 33.0 | 34.6 | 36.2 | 37.9 | 39.5 |