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

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

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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

frosted mums

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 areathermometer 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

dormant flowering bush

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 infographic

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!

chill hours map

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!

california lilac

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! woman planting an apple tree

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!

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