2021 California Pistachio Chill Update & Frequently Asked Questions

January 28, 2021

In this post we'll find out how 2021 California pistachio chill is progressing and answer some of your most frequently asked questions related to chill accumulation, chill portions, and chill hours including:

How is pistachio chill accumulation progressing in 2021?

The quick answer? Chill accumulation is progressing slightly better than last year for most of California’s Central Valley, but we are not out of the woods yet, especially in the Bakersfield area.

Last season many pistachio growers told us they experienced a poor leaf-out, with some wondering if it was due to a lack of chill accumulation. With 2020 still fresh on everyone’s minds, growers are paying close attention to chill this season - especially with this year’s ongoing La Niña event. 


Figure 1. Chill portions accumulated up to Jan. 20, 2021 (A), and the difference in chill portions accumulated for the same period year (B).

chill portion accumulation across California 2021


It looks like we're on track with a month and a half still to go. however, given the La Niña effect, it's something you'll want to monitor closely.

What’s the difference between chill hours and chill portions?

In pistachio production cool temperatures during the dormant season are important for leaf and flower bud development. The cooler temperatures crops rely on can be described as chill hours, chill units, or chill portions (see Understanding Chill in Pistachios)

What are chill hours?

The chill hours model counts the hours a plant is between 32°F and 45°F.

In Figure 2, there are 13 instances where temperatures are between 32°F and 45 °F resulting in 13 accumulated chill hours on this date.


Figure 2. Total Chill Hours Accumulated in One Day

Total chill hours accumulated in one day


What are chill portions?

Chill portions is a more complex and dynamic model for tracking chill. Since it cannot be calculated in a single equation, the Semios Crop Management Platform automatically calculates and forecasts chill portions and hours for growers on a per acre basis (see Figure 3).


Figure 3. Chill tracking, forecasting, and previous season comparison tools on the Semios web app.


chill tracking, forecasting, and previous season comparison tools on Semios web app

The general rules the chill portions model follows are:

  • Some chill is accumulated at any temperature between 32-54°F but maximum chill is accumulated at 43-47°F.
  • The chill accumulated in the most recent two days is provisional and can be negated by subsequent warm temperatures.

Figure 4. Dynamic Chill Portion accumulation in the San Joaquin Valley for 2010-2021. Chill portion requirements are indicated in purple and the blue line shows when chill requirements should be met by (mid February).

Dynamic Chill Portion accumulation in the San Joaquin Valley for 2010-2021. Chill portion requirements are indicated in purple and the blue line shows when chill requirements should be met by (mid February)


Can you convert chill hours to chill portions?

No. Although you can calculate both chill hours and chill portions for a given season, there is no correlation between these values.

In Figure 5 we see the calculation of chill hours and chill portions for hundreds of places and dozens of years. In the right panel we can see how on average 1 chill portion corresponds to 14 chill hours. However, at times, 1 chill portion can correspond to as much as 40, and as little as 0 chill hours.

Figure 5. Calculation of chill hours and chill portions

calculation of chill hours and chill portions

Source: Luedeling & Brown 2011


What model should be used to track chill? Hours or portions?

Chill portions have proven more accurate than hours in predicting the effect of insufficient chill on  bud break or leaf out.

Let’s use the 2013-2014 season as an example (see Figure 6). In this season, pistachios displayed severe symptoms of insufficient chill. Remember that pistachios need at least 60 chill portions by the time they begin the leaf out process. 

The chill hours model did not anticipate this catastrophe while the chill portions model singled out this season as being problematic.


Figure 6. 2013-2014 chill hours vs. chill portions

2013 to 2014 chill hours vs. chill portions

Source: Katherine Jarvis-Shean

When should I use chill hours?

Chill hours should be used with extreme caution, we should not expect to be able to identify when there is going to be insufficient chill using the chill hours model.

How much chill do pistachios need?

Each variety has a different chill requirement and this is typically related to how late they wake up. 

Figure 7. Chill portions requirements by pistachio variety, calculated using the dynamic model

chill portion requirements by pistachio variety calculated using the dynamic model

Click here for detailed information about different crop and variety chill requirements.

What are the impacts of low chill accumulation in pistachios?

Pistachio buds that leaf out unevenly can negatively impact synchronization with male trees. In turn, this uneven leaf out will lead to uneven nut development. 

Trees with poor chill accumulation tend to have branches that look like “poodle tails” (see Figure 8), where only terminal buds come out. 

If chill accumulation is very poor, yields can be severely impacted. However, once the chill requirement has been met, additional chill will not increase yields. 


Figure 8. Pistachio trees with ‘poodle tails’ as a result of poor chill accumulation

Pistachio trees with ‘poodle tails’ as a result of poor chill accumulation

Source: Katherine Jarvis-Shean

Is there anything I can do to improve chill accumulation?

Some growers use hormone analogs, nitrogen cocktails (e.g. calcium ammonium nitrate) and hydrogen cyanamide (aka Dormex). These solutions can be effective in mitigating the effects of insufficient chill. 

Dormex is the most common solution worldwide and has been approved for use in pistachios, apples, stone fruit, almonds, berries.

Although these solutions can help with uneven bud break, they may be less effective when chill accumulation is extremely low.

Note that Dormex has phytotoxicity and should be used with caution. 

UC Davis: Fruit & Nut Crop Chill Portions Requirements
AgFax: 2020 Leaf Out Problems

Related Posts:

Understanding Chill in Pistachios
4 Benefits of In-Canopy Weather Stations in California