Frost Protection for Berries
Posted: April 19, 2012
The temperature at which damage occurs varies with the crop and growth stage. Table 1 lists commonly-accepted critical temperatures for strawberry and blueberry blossoms at different stages of bud development. These values are not absolute, and within reason, it is better to err on the side of safety when protecting crops from frost damage.
Table 1. Critical temperatures
Critical temperatures (degrees F) for cold damage of flower buds based on stage of development. Note with blueberries, there is considerable variability in temperatures at which damage was reported for these growth stages.
|Strawberries||Critical Temperature||Blueberries||Critical Temperature|
|Bud emergence||10||Bud swell||15-20|
|Tight bud||22||Tight cluster||18-23|
|"Popcorn"||26||Separate flowers visible||22-25|
|Open blossom||30||Late closed blossom||25-26|
|Green fruit||28||Open blossom||27|
Sources: Strawberry Critical Temperatures - K. Perry and B.C. Poling, North Carolina State Univ.; and Richard Funt, Ohio State Univ.; Blueberry Critical Temperatures - Fruit Crop Advisory Team Alert, Vol. 18, No. 3. "Protecting Blueberries from Frost", E. Hanson and M. Longstroth, Michigan State Univ.
Sprinkler irrigation works well on all small fruit crops, but needs to be used carefully. Because sprinkler irrigation use can result in the application of large volumes of water to the crop, use should be delayed until greater than 10% of the blossoms are in danger of being damaged. This does not necessarily mean that 10% of the blossoms are open. Sprinkler irrigation for frost protection works because water gives off heat when it changes from a liquid to a solid (i.e., freezes).
A common recommendation is to start the system when the temperature at plant level falls to 4° F above the critical temperature (for example, 34° F for open strawberry blossoms). If the dew point is below freezing, irrigation must be started at a higher temperature. Under conditions with wind or low humidity, damage can occur when the air temperature is several degrees above the freezing point because of evaporative cooling. Because of this, the wet bulb temperature is often a better indication of when the irrigation system should be used rather than dry bulb (standard) temperature. Irrigation should be operating by the time the wet bulb temperature equals the critical temperature.
Most overhead sprinkler systems are designed to deliver 0.1 to 0.2 acre-inches of water per hour and are useful for radiant freeze or frost protection when wind speeds are light and temperatures are not below the mid-twenties. Microsprinklers provide more uniform distribution than those having larger droplets and/or those covering a larger area.
However, the rate at which water freezes depends on several environmental factors, including air temperature, humidity, and wind speed. When breezy conditions (5 mph) are forecast overnight, water supply lines should be moved closer together. At 5-10 mph, protection will be spotty. When wind speeds exceed 10 mph, the risks for crop damage from evaporative cooling may outweigh the potential benefits.
Overhead irrigation pipes and sprinklers can be set up on row covers, and irrigation started after the temperature under the row covers drops near the critical temperature. This is the safest way to protect crops in the case of advective freezes, and greatly reduces the amount of water used regardless of the type of frost event. Because of the necessity of and time required for removing and re-applying the row covers (they can just be gathered into the row middles in which the irrigation pipes are located), this method is best suited for small acreage plantings. Be sure to uncover the plants as early in the day as possible so that drying of the foliage and pollination can take place.
And, on to some questions!
Q1. I only have a well available for water. What recharge rate do I need to be able to frost protect?
A1. How much water you need varies of course with the temperature, plant growth stage, wind speed, and relative humidity, but 45 gallons of water per minute will get you through most frost events if you have one acre to frost protect. “Most frost events” are ones where you need 0.1 inches of water per hour or less, assuming you have sprinkler heads that give you this rate. That would mean that your plants would be protected on calm nights down to 24 degrees at 75% relative humidity, or on nights with a light 2 mile-per-hour breeze down to 27 degrees. See tables in Appendix A of the Berry Guide to see where these numbers came from.
Q2. I have an irrigation pond, but it’s shaded and the water doesn’t warm up much. Does that matter? Should I cut down some trees to warm the pond up?
A2. No, unless you want the wood for heating. The heat released from having warmer water is miniscule compared to heat released from the water when it freezes on the plant. You get 1 BTU released per degree that the water cools for each pound of water, but 144 BTUs released per pound of water as it changes to ice.
Q3. I put my min-max thermometer in a bowl of ice water to calibrate it, and nothing happened. The temperature stayed the same as it was. Did it break?
A3. The bulb is actually at the top of most mix-max thermometers, so partially immersing yours oriented the same direction as you would read it doesn’t work. Either put it in upside down (which can give your neck a workout if you try to read it), or better yet, lay it flat in a tub with the ice water slurry to calibrate it.
Q4. I have a device that gives me the web-bulb temperature, but I don’t know exactly what that is telling me. Can you explain?
A4. As explained in the article above, the web-bulb temperature basically tells you the temperature the plant will be after evaporation has taken place, so it tells you the temperature your plant will be at after you turn the irrigation on. You should turn the irrigation on when the web-bulb temperature is a degree or two above the critical temperature for your plants’ growth stage. That way the system is doing some good by the time the critical temperature is reached.
Q5. I’ve read that the dew point is important. Why? What does it tell me?
A5. On a clear still night, the temperature drops like a rock until it hits the dew point, and then stabilizes for a while as the moisture in the air condenses and dew forms, because heat is released during the process of condensation. If the dew point is below freezing, so that condensation and heat release does not take place until below freezing, temperatures can drop to damaging levels extremely rapidly. In this case, the white crystals typically seen in a frost or freeze may not form, a condition sometimes referred to as a "black frost".
Q6. What do you think of row covers vs. overhead irrigation?
A6. I like them. Of course, they are more practical when you have less acreage – maybe two acres or less. I like using two layers of row cover way better than just one. Instead of throwing away an old row cover, keep it for an underlayer, then pull your new row cover over the old row cover. If you have two layers to pull on, it’s unlikely that you’ll ever need overhead irrigation for frost protection again. If you don’t have enough for two layers of row covers, I’m a big fan of irrigating overtop a row cover (see the article above).