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Using Irrigation To Protect Strawberry Blooms From Frost
There's nothing colder than a strawberry field on a frosty spring night. Strawberry plants bravely bloom in early spring, often before the last frost. The blooms are close to the ground, and the ground, covered with straw, doesn't radiate much heat. That's why many strawberry growers pull a few all-nighters each spring to run the irrigation system and use a thermodynamic principle to protect their crop from frost injury. Why does it work? As water is in the process of freezing, enough heat is released to protect blooms from injury. As long as water is constantly freezing, the temperature of the bloom will not fall below the critical point. What equipment is required? Irrigation systems should have the capacity to irrigate the whole field at one time. Sprinkler heads designed for frost protection should be used, because they apply lower volumes of water. The nozzle head should be metal rather than plastic, and the spring should be covered to prevent freeze up. Sprinkler rotation should be somewhat faster than 1 revolution per minute. When to start irrigating: See Table 1, Critical Air Temperatures for Strawberry Tissues for critical temperatures at different bud stages. Tight buds can be damaged by very low temperatures, but strawberry flowers are most sensitive to frost just before and after they are open. Just when to turn the irrigation on depends on cloud cover, wind speed and humidity.. A rule of thumb during bloom is to start irrigation when air temperatures in the lowest part of the field reach 1.1ºC. As the irrigation begins there is rapid evaporative cooling of about 1-2ºC. For this reason, the recommended start-up temperature is several degrees higher than the critical temperature for plant injury. What is the significance of dew point? The dew point is the temperature at which moisture condenses from the air to form dew. The dew point is related to relative humidity: when the air is humid the dew point occurs at a higher temperature than when the air is dry. Once dew begins to form, the air temperature begins to drop more slowly. When temperatures reach freezing, the dew turns to frost. If the air is very dry, then the dew point will be very low, and it is often lower than 0ºC. If this occurs, frost injury can occur without the appearance of frost. Growers should not wait for frost to form before they turn on the irrigation system. Growers can use dew points to estimate how low and how quickly the temperature might drop in any given night. Dew points can be obtained from:
| Top of Page | How much water to apply: The amount of water applied per hour is based on the amount of wind and the temperature, Table 2 - Amount of Water to Apply for Frost Protection. A rate of 2.5 -3 mm is considered adequate to protect to -5.5ºC with no wind. More water is required if it is windy or colder. The ice should be clear if enough water was applied. If the ice is cloudy or milky white, there is not enough water to protect the flower. At wind speeds above 16 km/hr or at temperatures below -7ºC sprinkler irrigation can do more harm than good because of rapid freezing. When to stop irrigating: Irrigation can be stopped when ice on the plants begins to melt, usually after sunrise. Monitor carefully to make sure that the ice continues to melt and the temperature remains above freezing. Changes in wind speed could change temperatures near the plant surface. Irrigation should be started up again if water begins to freeze. More information: Irrigation for frost protection is tricky.
Improper procedures can do more harm than good. Growers who are not
familiar with the practice of irrigation for frost protection should
refer to a more detailed reference of information, such as:
Table 1: Critical Air Temperatures for Strawberry Tissues Twenty minutes or more at critical temperatures can cause injury. Duration of temperature for injury varies with wind speed, humidity, variety, etc.
Source: Martsoff and Gerber in Ohio Strawberry Manual Bulletin 436. Cooperative Extension Service, Ohio State University. | Top of Page | Table 2: Amount of Water to Apply for Frost Protection
0.25 cm/hr = 2,250 imperial gallons water/acre/hour Source: Martsoff and Gerber in Ohio Strawberry Manual Bulletin 436. Cooperative Extension Service, Ohio State University. Related Links
| Top of Page | For more information:Toll Free: 1-877-424-1300 Local: (519) 826-4047 E-mail: ag.info.omafra@ontario.ca |
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