David Doll2 CommentsMarch 5, 2023General, Horticulture

Frost damage to almonds

Almonds are susceptible to frost once they begin to flower. As a flower progresses through bloom (i.e. dormant > pink bud > full bloom > petal fall > small fruit), it becomes more susceptible to cold temperatures. Frost killed flowers/small fruits are easy to identify as the color of the tissue changes. Flowers affected by bloom often have wilted petals or blackened pistils. At petal fall and later, the damage often appears as brown or black tissue within the ovary. This can be identified by cutting the flower/nutlet in half. The photo provides a comparison of flowers affected by frost and a healthy flower. Note the darkened, off-color tissue of the three flowers on the right. All of these have been killed by the frost. Flower sensitivity to cold damage has been studies. Additional information on this can be found here.

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David Doll4 CommentsMarch 6, 2022General

Considerations for March 2022

March is a month full of surprises. Weather can be highly variable, with rain, risk of frost, and heat waves all being possible. Below are some points to consider as the season progresses. Ongoing frost concerns. Frost conditions can occur through the end of March. Maintain a watchful eye on the weather conditions to reduce the risk of any frost event. Critical temperatures after petal fall are around 28F (-2.2C), and temperatures at or below 28F for an extended period will cause crop loss. Micro-irrigation can increase orchard temperature by a few degrees, with higher flows of water providing a greater warming effect. More on frost mitigation can be found here. With irrigation occurring during multiple frost events, it is common to see trees push slow and with pale green growth. This is due to saturated rootzones that lead to loss of fine feeder roots and low micronutrient uptake. Soil irrigation levels should be monitored, and irrigation withheld until stored soil moisture begins to drop. Furthermore, nitrogen applications should not occur during the period of frost risk as running water for extended periods of time will leach nitrogen out of the rootzone. Crop losses from frost are hard to define in early March. Frost affected flowers will have blackened tissue inside the ovary, or flowers may appear blighted and remain on the tree. Trees will slightly compensate for loss of flowers with a slight increase in fruit set percentage of the remaining flowers as well as greater nut size. Therefore, in areas of minimal damage, the impacts might not be observed. In areas with losses greater than 10%, yield reductions should be expected. If frost damage is suspected, it is important to contact any insurance company as soon as possible and trim the early season fertilizer applications until a better

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David Doll3 CommentsApril 30, 2018Horticulture

2018 Almond Frost Event – What was learned

Field note written by Allen Vizcarra, Staff Researcher for UC Cooperative Extension, Merced County Almond frost damage begins when temperatures in orchards drop below approximately 28 °F, but the exact temperature depends on variety. There are a few but important practices to mitigate frost damage in orchards. Common practices include running irrigation water, mowing vegetation, and increasing air flow to the orchard (e.g., using fans to mix cold and warmer air). Implementing any or all these strategies could be just enough to increase the temperature by 1 or 2 °F, which can be enough to avoid or mitigate frost damage. Almond sensitivity to cold temperatures progressively increases from buds to small nuts, emphasizing the concern for cold temperatures during and after bloom. This past year, we observed survivability of almonds within orchards that were colder than the critical temperatures. This could be due to an increase in flower set of the remaining flower buds that opened after the freeze event. It could also be due to improper estimation of the cold temperature thresholds that we use as guidance. Finally, and more than likely, the varieties commonly planted within the field may be more tolerant to cold temperatures than we thought. Many of these were not ever evaluated for cold sensitivity. A recent observation near Chowchilla, CA in an orchard using micro sprinklers was the formation of icicles hanging from the lower canopy. There are always questions regarding if this is good or bad. The answer is a “it depends,” but generally it is better for the flowers in the upper canopy than the ones that are covered in ice. The icicles formed when irrigation water hit parts of the canopy by either unleveled sprinkler (i.e., spray not orientated perpendicular to the ground) or from a high angled spray pattern. Water

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David Doll8 CommentsFebruary 17, 2018Horticulture

Almond Frost Warning and Protection Methods – 2018

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

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