Chill Hours Explained: Why Fruit Trees Need Cold

Whether you're talking about Lilacs or fruit trees, chill hours can make or break a successful flowering or fruiting season, literally because of just a few degrees in temperature!

For most plants, as long as they are planted within their proper growing zone range, which is determined by the USDA, fruit, and flower perfectly well! However, some plants have strict chill hour needs that must be met or you will have a really pretty fruit tree or lush green flowering shrub in your yard - but no flowers and no fruit!

While it may be heartbreaking to find a particular tree or shrub you want, only to find out it’s not good for your growing zone! We’re not being mean when we tell you that plant isn’t right for you, we mean it! Planting outside your Hardiness Zone will have some seriously negative effects on your plant's health and affect its flowering/fruiting abilities.

So what exactly do Chill Hours mean and when do you really need to be concerned about them?

All About Chill Hours
Types of Dormancy
Figuring Chill Hours
Knowing Your Areas Chill Hours
No Winter? No Problem!
Chill Out!

All About Chill Hours

Winter is here and baby … it’s cold outside! (For most of the US anyway!) Your deciduous trees and shrubs are asleep for the winter! Don’t feel bad for them though, even while we are wrapped up warm indoors, your plants outside are enjoying a hard-earned vacation!

Most plants benefit from a 3-4 inch layer of arborist mulch and a good drink of water right until the ground freezes.

This period of dormancy due to cold is also known as vernalization, which is a process of going dormant in cold temperatures, which helps certain plants prepare for the following year. Plants that have vernalization requirements (most deciduous plants) must be exposed to a certain number of days of cold temperatures below a certain threshold. The required temperatures and lengths of chilling depend on the plant species and variety. This is one reason gardeners need to choose plant varieties that are suited to their climate and growing zone, for the best results and the healthiest plants.

Chill hours are essentially the total number of days, at or below a certain temperature, between 32 - 45°F (0°-7.2°C) that an area consistently receives on average each winter. Deciduous plants love winter and dormancy since it's a chance to rest, recharge, and reset their internal functions.

Every fruiting bush and tree on the Nature Hills website displays that particular plant's USDA hardiness zone range and chill hour needs, so you know if that plant will do well for you! This includes deciduous fruit trees and fruiting shrubs/vines that need specific chill hours, as do flowering trees and shrubs like Dogwoods, Lilacs, Tree Lilacs, Kiwi vines, Spring Flowering Bulbs, Nut Trees, and many more, which require cold stratification or vernalization in order to grow, flower and fruit.

This chill requirement is fulfilled by the soil and air temperatures as well as day length which triggers the chemical reactions and hormones so the plant can initiate bloom. Not cold enough, or not long enough of a period of cold weather, then some trees or shrubs won’t be stimulated to bloom. No blooms = No fruit!

Not only does your fruiting plant need specific chilling hours, but also its pollinator plant, since those blooms and their pollen are what your plant needs to set fruit.

Types of Dormancy

Not only does the plant need the right temperature to enter full dormancy, but also the right temperatures to break dormancy. Plants have endo-dormancy - which is an internal working that prevents a plant from leafing out or blooming too soon. It also prevents them from blooming too early in the spring.

Then there is eco-dormancy which are external conditions that stop a plant from growing, usually including temperature. Once both of these conditions are met, a plant will sprout leaves and/or flowers.

Too little chilling and improper dormancy can also take a toll on your plant's health, resulting in fewer flowers, fewer leaves, smaller fruit sets, and reduced health and vigor. Allowing pests and disease a foothold.

Figuring Chill Hours

Luckily, with the strange weather we’ve been having lately, those chill hours don’t have to be consecutive, but their average does need to be the cumulative total during any particular winter.

If a plant's internal clock needs 500 chill hours to set fruit, then that means between fall and spring, your plant needs to be exposed to 500 cumulative hours of cold at or below 32°F and not going over 45°F. Any time the temps soar to 60°F or more, you need to subtract a half hour of your accumulated chill hours.

Knowing Your Areas Chill Hours

The good news is there is really no need to do much math here if you have planted your tree or shrub within its recommended growing zone range. However, those in Microclimates, at higher or lower elevations than their surrounding areas, and on the edge of a growing zone range may need to break out the calculators.

Microclimates and elevation considerations can be figured with the help of their County Extension Office for assistance with calculating and fine-tuning their chill hour availability and totals for the area.

The continental US is massive! From the Southern US with zero chill hours, up to 3,400 chill hours up in the northwestern US!

Googling your zip code, checking out a chill zone map, or giving your Extension Office a call, will give you all the info you need for a successful planting!

Once you know your growing zone and chill hours, choose a plant with the same or fewer chill hours than your area provides to ensure success. If you are on the edges of a growing zone or right on the ‘line’ between two growing zones, always go with the lower of the two.

No Winter? No Problem!

Not everyone lives in cold growing zones and has any cold winter weather. Areas that need low-chill fruit trees and flowering shrubs still have options! This includes growing zones 9 through 11, where winter is not something anyone thinks about much. But there is now a wide array of fruiting and flowering plant alternatives that do great in areas with little to no winter temperatures!

But the lack of chilling temperatures unfortunately means that some highly sought-after plants are not suitable for these conditions. Lilacs are one we have many folks that moved to warmer climates sad to hear they won’t enjoy those fragrant flowers! But there are equally fragrant alternatives like the gorgeous Gardenia, gorgeous Camellia, long-blooming Crape Myrtle, or the California Lilac for your enjoyment!

Luckily, Avocados, Tropical fruiting trees, and Citrus trees prefer things on the warm side and need protection from freezing temperatures.

Chill Out!

Choosing the right plants for your area and your growing zone will make or break that plant's ability to fill your landscape and vases with flowers, and your pantry full of delicious fruit!

Plant hybridizers are busy coming up with new and improved varieties to give every growing zone its own niche plants that will thrive in your area best! Let Nature Hills Nursery help you choose the right shrub or tree for your growing zone and just the right amount of chill! Head over to the NatureHills.com website or give our knowledgeable staff a call for assistance! We’ll make sure you have a successful planting and harvest from your plants year after year!

Happy Planting!

Frequently Asked Questions

Why do fruit trees and shrubs need chill hours to produce fruit?

Fruit trees and shrubs require a certain number of hours between 32 and 45 degrees Fahrenheit each winter to complete their dormancy cycle, a process called vernalization, which resets their internal functions and prepares them for the following growing season. Without meeting these chill hour requirements, even a healthy, well-established plant may leaf out beautifully but fail to flower or set fruit entirely. Always check the chill hour requirements for any fruit tree or shrub before purchasing, and match them to your region's average winter temperatures to ensure a productive harvest.

How do I find out how many chill hours my area receives each winter?

To find the average chill hours for your area, check with your local cooperative extension service or university agricultural program, as they typically maintain detailed records for your region. The USDA Plant Hardiness Zone Map is a great starting point, but chill hours are more specific than zone ratings alone since they measure the total number of hours at or below 45°F (7.2°C) accumulated between fall and early spring. You can also search online for chill hour maps specific to your state or region, which break down averages by county or climate zone. Once you know your area's average chill hours, cross-reference that number with the chill hour requirements listed for any fruit tree or flowering shrub you plan to purchase at naturehills.com.

Do chill hours have to be accumulated consecutively to count?

Chill hours do not need to be accumulated consecutively to count toward a plant's seasonal requirement. They are tallied as a cumulative total of hours spent at temperatures between 32 and 45 degrees Fahrenheit throughout the entire winter season, even if warmer days occur in between. Keep in mind that temperatures below 32 degrees Fahrenheit generally do not count toward chill hour accumulation, so hard freezes do not speed up the process. Track your local chill hours through your county extension office or a weather monitoring service to ensure your chosen plant variety is a good match for your area.

Can warm winter temperatures interrupt chill hour accumulation and hurt my harvest?

Yes, warm spells during winter can interrupt chill hour accumulation and significantly impact your harvest. Chill hours are only counted when temperatures remain consistently between 32 and 45 degrees Fahrenheit, so periods of warmth above that range do not contribute to the total and can even reset progress in some plant varieties. Fruit trees and flowering shrubs like lilacs that do not meet their required chill hour threshold may leaf out and look healthy but fail to produce flowers or fruit. To protect your harvest, choose plant varieties with chill hour requirements that match your region's reliable winter averages, and consult your local extension office for accurate chill hour data specific to your area.

What happens if a fruit tree does not get enough chill hours in my climate?

If a fruit tree does not receive enough chill hours, typically defined as hours between 32 and 45 degrees F during winter dormancy, it will fail to flower or produce fruit even if it appears healthy and leafy. This is one of the most common reasons gardeners in warmer climates, particularly in USDA zones 8 through 10, end up with beautiful but unproductive trees. Before purchasing any fruit tree, check the chill hour requirement listed on the plant's page and cross-reference it with your area's average annual chill hours to ensure a match.

How do low chill fruit trees perform differently from standard varieties in warm climates?

Low chill fruit trees are specifically bred to flower and fruit successfully with fewer hours of cold temperatures between 32 and 45 degrees F, making them ideal for gardeners in zones 8 through 10 where winters are mild. Standard varieties require anywhere from 700 to 1,000 or more chill hours that warm climates simply cannot provide, which means they may leaf out erratically, fail to flower, or produce little to no fruit even when they appear otherwise healthy. If you garden in the Deep South, Southern California, or similar warm regions, always check the chill hour rating listed on the plant's page at naturehills.com and select varieties with chill hour requirements that match or fall below your area's average winter accumulation.

Which fruits and berries grow best in low chill hour regions?

Gardeners in low chill hour regions, such as USDA zones 8-10, will have the most success with varieties specifically bred to thrive with fewer than 300 chill hours. Top performers include low-chill blueberries like Sunshine Blue and O'Neal, strawberries, figs, pomegranates, and citrus, as well as low-chill peach and apple varieties developed for warm winters. Always check the chill hour requirement listed on each plant's product page at naturehills.com before purchasing to confirm it matches your local average, which you can find through your county extension office.

How do I choose the right fruit tree for my specific chill hour zone?

Start by determining your local average annual chill hours, which represent the total number of hours between 32 and 45 degrees F that your area receives each winter. Every fruit tree listed on naturehills.com includes its specific chill hour requirement alongside its USDA hardiness zone rating, so you can match both figures to your location before purchasing. Visit your local cooperative extension office or check online chill hour maps for your region to get an accurate number, then use that figure to filter your selections at naturehills.com and choose only varieties whose chill hour requirements align with your climate.

Can I grow fruit trees successfully if my winters are unpredictable or mild?

Growing fruit trees successfully with mild or unpredictable winters depends heavily on whether your area accumulates enough chill hours, which are the total number of hours between 32 and 45 degrees F that your region receives each winter season. If your winters are consistently mild, standard fruit tree varieties may fail to flower or fruit even if they appear perfectly healthy. The good news is that many low-chill varieties have been developed specifically for warmer climates, particularly in USDA Zones 8 through 10, requiring far fewer chill hours to bloom and produce. Check the chill hour requirement listed on each plant's product page at Nature Hills, then compare it against your local cooperative extension service's recorded average chill hours for your area before purchasing.

How do I track chill hours in my own yard throughout the winter season?

Tracking chill hours in your own yard is straightforward with a min-max thermometer or a digital data logger placed in a shaded, representative spot away from heat-reflecting surfaces like walls or pavement. Starting in late fall, count the cumulative hours when temperatures fall between 32°F and 45°F (0°C to 7.2°C), since temperatures outside this range do not count toward your total. Many gardeners also rely on local weather station data or university extension services, which often publish seasonal chill hour accumulations by region. Check your running total against the chill hour requirements listed for your specific plants on naturehills.com to confirm your yard is on track before spring arrives.

#ProPlantTips: All About Chill Hours! - Why It’s Important!