Chilling hours and cold weather

Chilling hours and cold weather
(Source: Pexels)

With a few weeks until our average last frost in mid-March, my pear, peach, and mayhaw trees have already bloomed.

Gardeners quantify cold weather temperatures in three ways. First, we work with the absolute coldest temperature that we should expect to have each winter. Where I live, that temperature is between 15- and 20-degrees F. These minimum expected temperatures are what places us in the USDA cold hardiness zone. For almost all of Angelina county, we are in USDA Hardiness Zone 8b.

The second way that gardeners cope with cold-weather is the first and last frost. We know that the date of our average last frost in our area is just a few weeks away on March 15. Mind you, that’s just an average date. By definition of ‘average’, we will still have a 50-50 chance of having a frost after the 15th.

The third way that gardeners handle cold weather is with chilling hours. “Chilling hours” is a measure of the amount of cool weather required by plants (most specifically fruit trees) before they decide to bloom, begin spring growth, and bear fruit. Fruit growers select fruit species and varieties based upon their adaptation to a region.

Now even the horticultural experts disagree on what a chilling hour is. There’s at least five models that I’m aware of. One identifies chilling hours is anything below 45°. Another one (and the one that I was taught was the best one) defines chilling hours as anything between 45 and 32°. The thought with this one is that anything below 32° is indeed a freezing hour. There are a few other models but I won’t try to explain them.

Just like folks need varying amounts of sleep each night in order to function the next day, fruit varieties need a certain number of chilling hours before they “wake up” from winter dormancy and bloom. A peach tree adapted to Houston should only need 250 chilling hours, and a tree adapted to Amarillo should expect to get 1,000 chilling hours.

Historically, much of East Texas falls into the 600-hour zone. According to whichever map you study, Angelina County normally has 450-750 hours. As of mid-February, we’ve already surpassed our annual average… and we still have a few weeks left before winter is typically over. As mentioned earlier, March 15 is the average last date of a killing frost.

As I type this, Lufkin has 702 chilling hours on record. Other locations around the county are Hudson at 745 hours, Huntington at 726 hours, Pollock at 635 hours, and Zavalla at 571 hours.

You can find the chilling hours for your location by going to the website, Getchill.net. It takes a little bit to work because you must follow the link to a map of the United States and find a weather station near you. But once you understand the site, it is an excellent resource for the serious fruit grower.

So, if I am already just over 700 chilling hours at my farm in Clawson, it makes perfect sense why my fruit trees are “waking up” after getting their required chilling hours.

Looking ahead to more winter weather and another expected frost, homeowners can protect their few trees with bedsheets and turning on the sprinkler to keep them from freezing. To be clear, freezing weather on already formed fruit will severely reduce fruit production.

To learn more about chilling hours and other factors affecting fruit production, be sure to attend the upcoming East Texas Fruit and Vegetable Conference on Friday, March 1. It will be at the at the Pitser Garrison Convention Center at 601 N 2nd Street in Lufkin. This is an all-day event starting at 8:30 am with lunch included. Topics include vegetable production, growing grapes, solving pecan problems, building better soils, and much more. Cost is $30 a person and $50 a couple if registered by Tuesday, Feb 26. There is a $10 late fee if you pay at the door.

Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is cw-sims@tamu.edu