The upcoming week will see temperatures drop to the mid-20’s. With the current almond crop stage, damage and losses may occur if mitigation practices aren’t utilized. Below is a list of previously written entries to provide guidance on when to start, how much to irrigate, and other practices to utilize to mitigate frost damage.
The point to turn on irrigation is dependent on dew temperature and the expected low temperature. Starting the irrigation too late when the dew temperature is low can increase the risk of damage. Turning off too early can also increase the risk of damage. Techniques utilized to determine when to start and turn off irrigation usually revolve around the use of a “wet bulb.” More can be found in this article written by Dr. Richard Snyder, a bio-meteorologist at UC Davis.
Irrigation application rates need to be high enough to provide an increase in air temperature. Application rates should exceed 30 gallons per minute per acre. Rates less than 15 gallons per minute per acre may lead to freezing of irrigation lines/spaghetti tubing. More on this can be found within this article.
The critical temperature of damage will vary by bloom stage and variety. Tests to determine the sensitivity of some varieties has been conducted and can be found on this table. At full bloom, temperatures at or below 27-28F can cause crop loss. As trees leaf out and nuts begin to develop, the sensitivity to cold temperature increases.
There are a few practices that should be implemented to reduce damage. These are outlined in this article. Please note that in flood and drip-irrigated orchards it may not be possible to have high enough discharge to have a warming effect of the water, but adding moisture to the soil can increase the warmth of the field – which is why mowing any vegetation is advised within these systems. Mowing may not be as critical in orchards that are able to apply irrigation water over the top of a cover-crop.
Alec Earle
February 18, 2018David, thank you for the information on frost protection…very timely! Should we irrigate ahead of the projected freezing temperatures to get the 1st foot of soil moist? Then run our microsprinklers prior to the temps actually hitting the “causes damage temperature”? We’re also trying to balance info from your recent article on not irrigating too early in the season. Thank you.
David Doll
February 19, 2018Alec,
The irrigation system usually needs to be started 2-3 hours before the coldest temperature – depending on the dew point and wet bulb data. This will create moist conditions for the soil. Wet soil does increase its ability to hold heat, which does suggest that having some moisture would be a good idea…but a balance approach must be taken. With the dry winter, applying several nights of irrigation (usually around 8-12 hours) should be okay as there is plenty of room in the soil profile.
David
Sara Savary
February 18, 2018I have been asked about and have heard about copper applications going out on almonds. What’s your take on copper for frost protection?
David Doll
February 19, 2018Yes, Sara, this was an oversight on my part. Copper can prevent frost damage by reducing populations of ice-nucleating bacteria, but pose a significant risk of phytotoxicity.
Applications should be made within 12 hours of a freeze event for maximum effect. This may also reduce blast, but the rate should be dependent on tree stage.It should be added that copper has not been shown to reduce freeze damage in the field. Samples from the field taken into the laboratory has shown a reduction of ice nucleating bacteria, but due to the inconsistency of freeze events, this was never demonstrated in the field. As an excellent consultant has indicated to me, the risks of phytotoxicity of spraying copper at bloom often outweigh any theoretical benefit of frost protection.
David
Bob Beede
February 24, 2018David! Thank you for assembling this valuable information for Almond growers faced with the stress of frost affecting their livelihood.
Rick Snyder’s Fast Fact would be MUCH more valuable to growers and consultants attempting to implement wet bulb temperature estimation for reducing the duration of frost water on soils already near field capacity IF TABLES 3 AND 4 WERE NOT IN CENTIGRADE! This makes these tables virtually worthless to invididuals trying to use them quickly. Any chance of getting them converted to Fahrenheit?? Thanks!
David Doll
February 26, 2018Bob,
Great idea.
I will put this on the to-do list.
David
Bob Beede
February 24, 2018David: I am concerned that growers might interpret your comments about copper mitigating frost susceptibility by reducing ice-nucleating bacteria to mean that copper can be applied DURING BLOOM without phytotoxicity. Wouldn’t copper applied at rates sufficient to reduce ice-nucleating bacteria populations cause severe damage to flowers past bud swell??
David Doll
February 26, 2018Bob,
You are correct. I have dug into the research and have found out a bot more about copper and ice nucleating bacteria. It seems that reductions in populations only occurred with weekly applications and frost prevention has not been shown. This was confirmed by both trial locations which were in Butte and Stanislaus County. I will correct the above comment to indicate that copper shouldn’t be applied at bloom due to the risk of phytotoxcity.
David