Thursday, January 4, 2018

What are “Chill Hours” (and why is this important to my fruit trees?) By Joyce D’Agostino

Golden Delicious Apples, photo courtesy Stark Bro's Nursery

If you have fruit trees in your landscape, you may have noticed that some years the trees seem to produce abundantly, other years there is less of a crop. This can be puzzling to figure out why some years are considered a ‘good fruit year’, and others are not.

An obvious reason can be similar to what we experienced in early 2017 here in the front range of Colorado. Not only did we have some very heavy, wet snows and subfreezing weather in early and then late April, but there was two very cold weeks in April, followed by a damaging hailstorm in early May. By this time, many of the trees had already bloomed and had started to set fruit when the freeze happened and then the follow up hailstorm did further damage. For such dramatic weather incidents, trees can often bloom on time and begin setting fruit but with freezing weather and hail, it can cause a partial or total loss which can stop the whole production of the fruit for the year.

Another important factor that plays into whether a fruit tree will produce well or not for that year are the “chill hours”. Each fruit and the individual species must experience specific chill hours which is when the temperatures must be below 55 degrees F for a specific number of hours in order for it to have the required in order to complete its winter rest and dormancy.  Chill hours for the same fruit tree and variety can vary from area to area as well so be sure to check with local sources for the chill hours required in your area for any fruit you are considering. 

Areas that have what is considered a mild winter, with above normal temperatures or even record breaking warm weather and low moisture levels, then your fruit tree may not reach the required chill hours in order to have it in full rest and dormancy, and then ready to bloom at the right time. Weather changes from year to year is a big factor in successful fruit harvests.

If you find your fruit trees are not producing reliably, it may be time to investigate the fruit tree varieties you have and whether that species is suitable for your climate. Working with a garden center or plant nursery that offers trees well suited for your climate can save a lot of time and effort. Focusing on the right species will help you avoid trees that aren’t well suited or need many more chill hours than your area can offer won’t produce well and often are susceptible to diseases. 

Consult with a fruit tree expert at the Extension service or your garden center to help determine which trees are recommended for your area. Keep in mind if your state is like Colorado, with many different elevations and climates, you will find some trees that work in one part of your state may not do as well for you so it makes sense to get expert advice. 

It can take a little work to find the right tree for the right location, but the end result will be better harvests and healthier trees. 

The following provides more information on varieties and planting information: