Chill hours and Warm Temperatures

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2021 is here and the madman has been loving the warmer than normal temperatures. He’s not fretting about frozen pipes. He’s not running around in knee-deep snow. In fact, the last precipitation that fell was liquid and he didn’t have to drag out his friend John Deere to help get us out of the driveway.

I hated to burst his bubble, but burst it I did with a simple question, “Have you thought about your blueberries?” Then the fretting started. “Shoot,” he said. “I forgot about those chill hours.”

madman's blueberry buds — The madman’s blueberry buds are counting the chill hours in mid-January.

Chill Hours

Bet you haven’t thought about chill hours for a long time – if you ever did, that is. But, a lot of your outdoor plants are counting them right now. Simply put, winter chill hours are the number of hours a plant is exposed to temperatures between 32˚and 45˚. But, in nature, nothing is that simple.

Here in Connecticut, we have a definite summer and a definite winter. In the fall, we watch our oaks and maples drop their leaves and enter dormancy – a condition we all take for granted. In the spring, we watch them come out of dormancy, but for the plant, is it really this straightforward?

No More Energy

Dormant oak tree in the madman's yard — This oak tree shed its leaves in the fall and is now dormant while patiently waiting for spring.

During the warmth of summer, plants produce energy for growth and food for storage through a process called photosynthesis. When the leaves drop off in fall, photosynthesis ends; no more energy, no more food. So, the plants have developed the ability to go dormant, or shut down for the winter. When the weather warms up in the spring, they come out of dormancy. But, what happens if the weather warms up too early? Sometimes it matters, sometimes it doesn’t – it all depends upon the chill hours.

Chill hours are a safety mechanism for the plants. Most people know about USDA growing zones. We all buy seeds and plants that are hardy in our zone. The madman and I actually garden in two zones: in Farmington, we’re in zone 6a; but at Highland Lake in Winsted, we’re in 5b although the madman says it seems like zone 4. These zones tell us the expected lowest temperatures in the dead of winter and are used to help us determine if a plant will live outside.

Plants Look Beyond the Zones

USDA Plant Hardiness Zones — All gardeners are familiar with the USDA zone hardiness map.

For most of us, this is all we look at and for the most part, this is all we need to consider. But, plants are actually a little more complicated; they respond to a number of other things such as soil composition, pH, moisture…and chill hours.

We told you what a chill hour is, but what does it really mean for plants? As we said, it is a built-in mechanism to keep them from coming out of dormancy too early. If all you grow are annuals, you will never have to consider chill hours.

chill hours map — Gardeners are less familiar with the chill hour map. Fruit growers probably know it well.

Fruit Trees Need Cold

If you grow fruits in your yard or orchard, you’ve at least heard that your trees need a certain amount of cold to produce. What they really need is a certain number of chill hours. In Connecticut, our climate gives us plenty for right now, somewhere between 1400-1800 hours on average. The madman thinks that as our winters get warmer, this number will change and not necessarily for the better.

Because of its low chill hour requirement, the madman grows this Asian pear in a pot in the étalier, protected from harsh winter weather. In the spring he carries the blooming plant outside for the pollinators; he brings it inside if a frost is predicted.

The number of chill hours a plant gets determines how well it will flower and produce fruit. Each fruit variety has a different requirement. Apples need between 300-1000; pears need between 200-800; peaches need between 50-1000; and the madman’s precious blueberries need 800-1000 while some Southern varieties need less. Interestingly, the Top Hat Blueberry, that miniature one that people grow in pots, has one of the highest requirements at over 1000. If your Top Hat has not lived up to your expectations, you might be babying it too much.

Peach blossom — This peach tree has met its chill hour requirement and buds have opened.

Peaches for Your Yard

Like everything in nature, chill hours are complicated and don’t really become an issue until late winter when a sudden warm-up might be followed by a hard freeze. We’ll use peaches as an example. Everyone loves peaches and many of you grow them in your backyards. Chill requirements for the various cultivars vary from under 100 to over 1000. Best practices are to select a variety suited to your area. Local nurserymen will carry varieties best suited for your spot in Connecticut. Mail-order companies will also steer you to good varieties for your location. Then there’s the madman.

In his early days of gardening, his purchases were determined by pretty pictures, not growing requirements. Our peach harvests were variable, but never great. Now he knows better and he has an idea of what went wrong. A plant with low chill hours has a tendency to come out of dormancy too early and risk damage by cold. “I should have considered chill hours instead of just looking at USDA zones,” he says now. Ignoring the pictures would have helped also, I think.

Chill Hours Offer Protection

A late winter snow storm can damage fruit production if a tree has already bloomed. Photo from University of Georgia Plant Pathology, University of Georgia, Bugwood.org.

But, no matter what fruit you are thinking about, chill hours come into play. A plant with a low chill hour requirement runs the risk of coming out of dormancy too soon. On the other hand, a plant with a high chill requirement may not produce well if its chill hours are not met. Fruit trees aren’t the only ones affected – our nuts and berries need chill hours, too.

And, look at the native plants in your landscape. Here in Connecticut they will be subjected to the cold of a zone 6 winter. They won’t come out of dormancy too soon. But, a non-native ornamental may break bud in a mild spell only to get damaged by a late frost. Another reason to consider planting native species – you’ll never have to consider chill hours.

redbud buds in the madman's yard — The madman doesn’t have to worry about chill hours with this native Redbud tree.

Chill Hours are Changing

But, in other parts of the country, chill hours seem to be a bigger concern with climate change. California, which contains roughly 50% of our country’s fruit-growing acreage, is seeing a measureable decline in chill hours. In 1950, its central valley experienced between 900-1100 chill hours. By the year 2000, this number dropped to 700-900, and it’s projected to drop lower as time goes on.

madman at Monsanto — Many gardeners disagree with the use of chemicals in a garden.

Does this mean the end of fruits as we know them? Hopefully not. The University of California Agriculture and Natural Resources department (UCANR) has been working on this for us. Their research has shown that spraying fruit trees with kaolin clay is an effective way to increase chill hours. Also, rest-breaking chemicals can be applied to fruit trees after 70% of their chill requirement has been met through natural temperature. Although this allows for normal fruiting, the madman is not a fan of adding more chemicals to our environment. This might be why the police were so quick to come when he tried to visit the nice people at Monsanto, but that’s another story.

A Better Plan

He’s a much bigger fan of UCANR’s long-term plan of planting cultivars with lower chill requirements as fruit growers replace existing orchards. California doesn’t own the market on chill hour worries. Michigan cherries, New York apples and Georgia peaches have sometimes faced reduced crops thanks to late spring frosts. Consumers may be unaware of this crop damage, especially if it is counter-balanced by a bumper crop in another area. UCANR isn’t the only entity looking for solutions, universities and agricultural agencies throughout the country are also working on it. The obvious solution is to develop more resistant cultivars, but it can take decades to bring a new variety to market and even longer to have it produce.

Young orchard — The madman prefers planting fruit trees with lower chill requirements over chemical use as growers face warming temperatures.

Appreciate the Farmers

As the madman stares at his blueberries, he shakes his head and says, “I’m so glad I don’t do this for a living. Thank God for the farmers.” In fact, the next time you are at a Farmers’ Market, make sure you thank those farmers.