When you’ve planted lots of fruit trees in central Alabama as we have, you begin to pay attention to chilling hours. Chilling hours are nature’s way of letting fruit trees rest in preparation for the busy spring and summer growing season. All fruit trees need some chilling hours over the winter, and if they don’t get them, they won’t bear fruit.

Unfortunately, chilling hours have become shrouded in mystery. They shouldn’t be. The concept of chilling hours is simple, and they are critical to the performance of the fruit tree. Let’s take the questions about chilling hours in order:

What is a chilling hour? In simple terms, a chilling hour is an hour of time during which the temperature falls between 35 and 55 degrees Fahrenheit.

How many chilling hours does a fruit tree need? It varies according to the fruit, and even according to the variety. Some varieties of blackberry need as little as 150 chilling hours, and most varieties of apple need 900 or more. In general, blueberries and muscadines have low chilling hour requirements, which is one of many reasons why they tend to be the easiest fruits to grow in central Alabama. Pears have higher chilling hour requirements, apples even higher, and cherries the highest of all. This explains perhaps why we in central Alabama have almost no success getting cherries to set fruit.

Whoever sells you your fruit trees should be able to tell you their chilling hour requirement. If they can’t, move on; they’re not serious about providing quality trees.

How many chilling hours will I get where I live? ACES has a nice handout with a little map of Alabama, with chilling hour zones drawn on it. Noble effort, but confusing and too static. It’s difficult to develop the confidence that you’re using the right map and following those little squiggly lines correctly. And as the handout points out, chilling hours are different each year. Try this site instead. And once you’ve found it, bookmark it on your browser. For reasons that elude me, it doesn’t show up on search engines. Look for the measurement site closest and most similar to you, and assume your chilling hours will be in the same range.

Which column on the chart do I use? It depends. If your question is whether a particular variety will bear reliably for you in your area, you should use the standard chilling hours, because all the nurseries are quoting chilling hours on the “standard” scale. So, for example, if you’re reading on the Petals from the Past site about Tifblue blueberries needing 650 chilling hours, that’s 650 standard hours.

On the other hand, if you’re looking at the chilling potential in your orchard from year to year, you should use the modified chilling hours, which will always be lower than the standard hours. Thanks to the research of Dr. Arlie Powell (the father of Jason Powell, who owns Petals) and many others, we know now that standard chilling hours are not always a good measure of the time needed to satisfy the rest requirement of a fruit tree. The difference has to do with when to start counting those hours each winter and how much attention to pay to temperatures below freezing. You and I don’t need to understand it fully; all we need to know is that modified chilling hours are a more accurate measure, even though they’re rarely used by nurseries.

Do chilling hours vary from year to year? You bet your best pruners they do. As you view the chart here in the winter of 2011, beware of paying too much attention to this year or last year. Winters were unusually cold both of these years, but if you look back at the winter ending 2009 or 2008, you’ll see significantly lower (and perhaps more typical) chilling hours.

What happens to a tree if it doesn’t get enough chilling hours? It’s not good. A fruit tree that gets fewer chilling hours than it needs arrives at the growing season still tired. It is slow putting on foliage. It either bears no fruit or bears fewer and smaller fruit of lower quality. Long term, a tree that needs more chilling hours than it gets year after year will be more vulnerable to disease, fungus, and insect damage. And let’s face it: I read Matthew 7:19 just like the next guy, and after 2-3 fruitless years, I’m looking for my saw.

Is it ever a bad idea for a tree to need too few chilling hours? Yes, but less often than we might think. Yes because if a tree satisfies its chilling hour requirement by January 15 and there’s a warm snap in late January, the tree may bloom early and sap up, and then be vulnerable to a late frost. Less often than we might think because trees also have a less well-known requirement called “Growing Degree Hours.” Once a plant has met its chilling requirement, it must have some number of hours during which the temperature exceeds 41 degrees Fahrenheit. Muscadines, grapes, figs, and some peaches have a low chilling hour requirement but a relatively high growing degree hour requirement. The net effect is that these plants are relatively unlikely to bloom too early and suffer from a late frost.

This is all so complicated. Can’t I just plant by zone and be done with it? Yes, probably. If you’re in central Alabama and don’t want to mess with understanding chilling hours, just plant rabbiteye blueberries, muscadines, and figs and quit worrying about it. You’ll be fine.

When in doubt, plant another fruit tree. Hard to go wrong.