Understanding chill hours – what they are and what they signify – can be a very useful tool for a gardener.
Every fruiting plant has a chilling requirement.
And every geographic area has an annual average of accumulated hours of chill time.
If you’re growing any fruiting, perennial plants such as peaches, blueberries, apples, etc., it means a great deal.
In fact, understanding the concept of chill hours and how it applies to what you’re growing may mean the difference between success and failure.
The concept of chilling hours is pretty simple.
Let’s use a peach tree as an example:
The spring and summer is prime time for that peach tree. It’s just going to town maturing this year’s peach crop and vigorously putting on new growth.
And part of that new growth is next year’s peach crop, since the buds that become peach blossoms next spring are formed this summer.
So the summer rolls swiftly by, as summers always seem to do, and fall soon arrives. The days become shorter, the air a bit nippier, and before you know it the first frost shuts down the growing season.
If our tree continues to put on new growth, old man winter will slap it down hard. And the survival of the species depends upon those tender young peach buds surviving the harsh winter.
So what’s a peach tree to do?
Simple: just go into hibernation mode. Stop growing, stop the flow of sap through the tree’s extremities, and allow those young peach buds to become tough and hardy.
In hibernation mode, our peach tree will be able to survive just about anything that nasty Mr. Winter throws at it.
How does our peach tree know when to emerge from hibernation?
The timing is critical, because if it ‘wakes up’ and starts growing too early, it risks total destruction of its offspring (peaches) from freezes.
Too late, though, and the tree may not have enough growing time to mature the peach buds for the following year before growth shuts down in the fall.
Fortunately for our peach tree (and for all of us who love peaches!), Mother Nature has provided an alarm clock that, more often than not, ‘wakes up’ the peach tree at the optimum time.
Or at least, that’s how we humans measure it.
The tree just ‘knows’ when there’s been enough cold weather to signify the end of winter – at least in a normal year. It’s not a foolproof system, because there are years when chilling hours accumulate at a faster or slower pace than normal.
But more often than not our peach tree’s internal alarm will awaken it at just about the right time.
Let’s stay with our peach tree a little while longer.
And let's assume that you are growing that peach tree in your garden.
Now let’s also assume that the number of chilling hours that the tree must accumulate to break dormancy is 800.
And let’s assume one more thing: that the average number of chilling hours in your area is 1000.
You’ll probably have a healthy, nice looking peach tree, but that’s about it. You’ll rarely ever see a peach on your tree.
Because it’s going to ‘wake up’ and try to grow way too early in the springtime. Its requirement for chilling hours (800) will be satisfied long before winter is normally finished in your area.
And if the situation is reversed (your area averages 800 chilling hours, tree needs 1000), that’s going to be a problem too.
Because your tree will be forced to get going long before it’s ready. The result will be poor growth and terrible peach crops. (How well do you perform on a half night’s sleep?).
So as you can see, it’s important that you purchase peach trees (or blueberries, or apples, or plums, etc.) with a chill hour requirement that matches the average chill hours in your area.
Which leads back to the question we opened this page with…
To avoid unhappy plants – and unhappy gardeners – due to mismatches between chilling time requirements and averages, it’s obviously important to know both the plant’s chilling requirement and your chilling hours yearly average.
Finding out about the plant’s requirement should be easy enough.
The nursery you’re buying from should be able to give you that information for any variety of peach (or any other fruiting plant) that they sell. If they can’t, you might consider taking your business elsewhere, because that’s a basic piece of info that any reputable nursery should be able to supply.
And finding out the chilling hours average for your area? Well, that should be pretty easy, too.
You can just Google “chill hours map area” where you replace the word “area” with your state or region. Here’s an example for Texas.
You can also call the closest extension office and ask for the average chilling hours in your area – they’ll be able to tell you. And the nursery you’re buying from should also be able to give you that information.
When you’re considering whether or not to try a particular plant variety, the chilling hours don’t have to match exactly. If I wanted to try a plant that had a chilling hour requirement that was 750 in my 800-hour area, I wouldn’t hesitate.
If I REALLY wanted to try a plant that was way off, I might try that too. But I’d know it was an experiment that would probably be doomed to failure. (Hey – sometimes it’s fun just to throw caution to the wind and give something a try!)
But you’ll have the best chance of success if you assure that the chilling hour average of your area at least approximately matches the chilling hours requirement of any fruiting tree you plant.
If you’re already a gardener, you know that Mother Nature frequently tosses curve balls your way.
But by understanding the concept of chill hours and how it impacts you as a gardener, at least you can avoid throwing yourself a curve ball!