Chill Out!

• The methods used to define chill requirements for fruit trees, as well as the protocols used to calculate accumulated chill from area to area, have troubled me for many years. Considering that several models are used to calculate accumulated chill, and various methods used to estimate chilling requirements, the subject of how a deciduous tree's dormancy (aka chilling or rest) requirement is satisfied can be downright confusing.

Certainly it's important to know a fruit tree's chilling needs and how much chilling occurs at a specific home garden site. This knowledge will help determine what to plant or what not to plant in your climate region.

However, because of the difficulty in determining the true chill requirement of a variety and in assessing the actual chill accumulations at a particular location, chill requirements may be the most misunderstood tool the average home gardener uses to select fruit trees.

The amount of chill needed to satisfy a trees dormant rest requirement, plus the amount of heat required to initiate growth, determines how long buds will remain dormant. In general, the lower the chill requirement, the earlier a tree will bloom.

In dormancy, deciduous trees are protected from winter cold damage. During the summer, fruit trees develop leaf and fruiting buds that will grow the next year. As days get shorter and cold weather sets in, trees drop their leaves and go dormant. With defoliation, other physiological changes occur in bark and buds to protect trees from the oncoming winter cold.

Now this can be a good thing or a bad thing. It is a good thing in a mild winter area if the fruit variety does not require much winter cold to produce fruit. Eva's Pride peach, for example, is a low chill variety rated as needing 150 hours of chilling requirement, performs well in the Los Angeles area. On the other hand, if Eva's Pride peach is grown in an area that gets very cold winters or unpredictable late spring weather, it could have real problems. This is because the flowering will happen too early and the blooms and fruit will be threatened by cold weather, especially late spring frosts.

So what is a home gardener to do?

The 45 and under model is the earliest
and most simple model and is still in use.

Simply put, every hour below 45 F. equals one chill hour received. No magic here, but it doesn't define when one starts to record chill in the fall, or when it doesn’t matter anymore in the spring.

A later model, the 32-45 model, gets a bit more definite about the cold
but much more complicated about when chilling is accumulated.

The 32-45 model says any hour of cold between 32 F. and 45 F. contributes one hour to satisfying a tree's chilling requirement. According to the 32-45 model, temperatures below 32 F. don't contribute to accumulated chill.

I find it necessary to question the chill models because even though daytime temperatures in southern California are consistently 60 degrees and higher, varieties with relatively high chill requirements grow and fruit in spite of the region’s warm weather.

All I can truthfully say is that the physical and chemical processes involved in achieving and leaving dormancy are extremely complex!

Let’s say, for example, on an average each day in Southern California we experience 10 hours of 45 degrees or less, and 14 hours of 60 degrees or more. That makes for 4 negative hours of chill a day, which by the end of the season is approximately 300 negative hours of chill. I like to call this the “I OWE YOU CHILL” model!

The complexity only increases with the more recently developed models — Hours of chilling received are expressed as "chill units." Even though accuracy may increase with complexity, the nature of the calculations and record keeping is truly daunting.

We can add yet another model to our collection, the Utah Model, but it won't necessarily clear up our problems either...

The Utah Model only adds complexity to the equation.

• The Utah Model says that
  • 1 hour of chill below 34 F. is worth nothing
  • 1 hour at between 35 and 36 F. gets 1/2 a chill hour
  • 1 chill hour is given at 37 to 48 F.
  • 49 to 54 F. gets only 1/2 a chill hour
  • 55 to 60 F. get no chill hours
  • Above 60 is all negative chill …..eeekk!!!

Here are some Google search items that may help:
NM State chill unit calculator
TAMU chilling accumulation
about chilling accumulations

• Varieties fall into three categories:
  • Guaranteed tried-and-true: time-tested varieties for the area
  • Highly Recommended Varieties: newer fruit selections that are receiving good reports for reliability in a given area
  • Experimental Varieties: selections with scant information to predict the variety’s reliability in a particular area. Planting experimental varieties will be a test

When looking for fruit varieties to grow in your area, you'll need to decide what level of risk you are willing to tolerate.

When all is said and done, it is possible to simplify the purchase of an appropriate fruit tree for the home garden by learning what to expect.

And that should take the chill out of selecting fruit trees!

Sunset climate zone map for 12 Western states

USDA climate zone map

American Horticultural Society Heat Zone Map

Arborday.org climate zones

Stone Fruit Varieties for Milder Climates with comments & observations from California Rare Fruit Growers.

— Revised July 2007 —
The original version of this article appeared in the January 2007 issue of Garden Compass magazine.

You can contact Ed Laivo to share information about other garden projects that demonstrate practical home garden use of fruit trees, composting and mulching. .....ed@davewilson.com