Mobilize simplifies chill accumulation tracking

Take Georgia, the Peach State, and warm it up a bit with a little climate change, and what might you get? Possibly the Ex-Peach State. While peaches love the Georgia sunshine, they also really depend on chilling hours – as many as 850 chill hours.

According to a story in National Geographic, the temperatures in Georgia have risen 5°F in the last 60 years, and that trend is expected to continue. This would not be so bad if the date of the last frost would also back up, but it remains rather late in Georgia. What can peach growers do? Plant trees that need fewer chilling hours? Yes, but those trees will bloom earlier and can be injured by a late frost. Plant trees that resist frost? Sure, but then they have to hope they get enough chill hours.

Peaches are just one of the crops that depend on chilling hours. The problems created by warmer winters in Georgia underscores the importance of chill hours for stone fruit tree growers.

This time of year around Halloween can be very scary – for all stone fruit tree growers. It is the beginning of a crucial period in the lives of their precious trees, one that only those folks who really understand the “plant mind” would understand. It’s called: winter. But growers with a Davis EnviroMonitor system and the Mobilize app have a powerful tool in keeping their trees strong and protected.

Stone fruit trees, and other many other plants, face the threat of cold temperatures by settling in for a long winter’s nap. After summer’s fruit has been picked, the trees focus on developing new buds. Then they protect those buds by shutting down and settling into winter dormancy. During this period of “sleep,” the tree is much more immune to the effects of freezing.

Dormancy happens in two stages. During the first stage, the process can be reversed if temperatures rise. During the second stage, the trees are in deep dormancy, and they are soaking up the chill in packets of hours or units, called chilling accumulation. When chilling accumulation begins to decline and temperatures outside rise, the plants begin to wake up, grow, flower and bud, heralding springtime. (Cue violins and bird calls.)

Why do growers care about chilling accumulation? Because, depending on variety and species, if their plants don’t get enough chill units, the grower may have to deal with problems of delayed foliation, bare shoots, and weakened trees; delayed or extended bloom; weak budbreak; and even flowers that just fall off, fruits that never develop, increased risk of pests and disease, or poor quality fruit.

Too much chilling can be problematic too. It can cause the plants to end their dormancy too soon and leave them exposed to damage from a late season freeze. It can produce a short bloom season and heavier fruit, which means more work to thin.

The Davis EnviroMonitor system gives growers an automatic and easily accessible way to handle the all-important task of tracking chilling accumulation. Using temperature and humidity data from a Davis weather station or a Temperature/Humidity Sensor in an EnviroMonitor Node, the Mobilize app lets growers see exactly how close their crop is to the chilling target. They’ll see accumulated chill units as well as a four-day chill forecast. Mobilize will also alert them when an action is needed. For example, growers can know when enough chill has accumulated that they can safely begin thinning; or when rest-breaking agents are needed to mitigate insufficient chilling.

Growers using Mobilize just choose their chill calculation method and set the chilling target, set a chill start date, and start watching their crop’s chilling progress.

• Chilling Hours: The simplest method for calculating chilling accumulation is chilling hours. Chilling hours are the number of hours equal to or less than 45⁰F, starting on November 1.

• Dynamic Chilling Portions: In places like the orchard-dotted California central valley, winter temperatures are warmer, and growers are most concerned with knowing whether their trees get enough chilling accumulation. A fluctuation that takes the temperature above 45⁰F can cancel chilling effect, but would not be measured in the Chilling Hours method. This method compensates for temperature fluctuations by starting earlier, on September 1, and using a wider range of temperature, from 32 to 55⁰F. In this calculation, chilling units are called chill portions.

• Utah Chilling Units: This calculation method is based on the theory that not all chilling hours are equal. It applies different weights, or units, to every hour by temperature. For example, an hourly temperature reading below 34⁰F gets zero units, but an hour with a temperature reading of 40⁰F gets one unit. An hour that comes in over 65⁰F gets a negative one.