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Chilling Hours – What They Are and Why They Matter

August 3, 2020

Written by Dave G.

Gardening, love of good food, and a desire to protect our own and our family’s health have all come together in the biggest gardening trend of the moment – growing food at home. The joy of seasonal eating – enjoying food in its natural cycles – and a love of freshness; a desire to know what exactly has been done to that food on your table; interest in eating more healthily, with an emphasis on vegetables and fruits; the pride and satisfaction of growing your own, all these wishes have fueled an enormous interest in turning at least part of your garden (or balcony) from ‘pretty’ to ‘productive’.

Growing lettuce, salad vegetables and tomatoes is one thing – a quick return and a straight-forward process. Growing fruit trees or bushes is another – hugely rewarding but, frankly, a lot trickier. The key to success with that is to grow types and varieties of fruits that will succeed in your area. The best gardener in the world will fail with the wrong plants. It is easy to choose what to grow based on their size, flavor, yield or harvest season, but bushes are living things, and they have their own needs. Unsatisfied, those needs will thwart the best of plans.

The Natural Cycle of Blooming

Fruits are, by definition, the seed-carrying parts of a plant, and before the fruit comes the flower. So, the first step in growing fruit has to be helping your bush or tree to bloom. Understanding the key factors that control blooming is central to success with growing almost all fruit trees and bushes.

Nature has developed a near-universal system to regulate the blooming of plants. In most plants that grow in areas with noticeable winters and summers, blooming takes place in spring. Now for us, spring is a time of increased warmth and longer days, so we naturally assume that plants are following the same signals. To an extent they are. It does take warmer weather and changes in length of the days to bring those flowers out on the branches of your apple or peach tree (or raspberry bush), but there is more to it than that. We are all well-aware of the ‘warm spell’ in winter, when a particular weather pattern causes a few warm days to interrupt the weeks cold weather. Welcome to us, who can put our heavy coats back on when the cold returns, reading a few warm days as a moment to bloom could be fatal for a plant, because one opened, or even simply stirred into life, flower buds will then be killed by the return of freezing weather.

To overcome this hurdle, plants have evolved an ingenious mechanism to know when warm weather is a sign of spring and when it is merely an aberrant weather pattern. They count the hours of cold – yes, they really do. During the summer months plants develop their buds for the next season’s leaves and flowers. As the cooler days of fall arrive, these lower temperatures cause plants to produce a chemical that slows and matures the growth of those buds. Short periods of cold are not enough to make this slowing irreversible – the early return of warmer weather will allow more development to take place. But after a while that chemical accumulates and eventually triggers full dormancy – plants go to sleep. Cut a branch of a tree in fall, after the leaves have gone, and put it in water in a warm place. What happens? Absolutely nothing. The branch will not die, but neither will it grow. Do the same in March and bingo – the leaves or flower buds will quickly expand and grow. What has happened?

By mechanisms we don’t fully understand, the plant has counted the time that has passed with low temperatures. Only after the accumulation of enough cold will the buds be able to start into growth when warmed. This ingenious trick protects a plant from ‘false springs’ and ensures that it only flowers in spring, not during a winter warm spell. Plants generally count the hours when temperatures are between freezing (32 degrees Fahrenheit) and 45 degrees. Days below freezing are not registered, and neither are warmer days. (This is an approximation we humans use to track these hours, plants are probably more sophisticated in their true responses than this). These periods are called ‘Chilling Hours’ and they provide a simple measure to ensure that your fruit bushes will be a success. This need is almost universal among all plants that grow in cooler places – which is why, for example, trying to grow a Japanese maple bonsai, or some tulip bulbs, indoors will fail. Plants that don’t receive enough hours simply won’t produce any new growth, so once the existing growth dies, their life is over.

There are some chemicals that can reduce this need, but home gardeners are basically trapped by this requirement, and it is fixed for each plant. Apple trees need the longest number of hours, which is why they grow well in colder states – peaches need the fewest, which is why Georgia is the peach capital of America.

Luckily individual plants of a particular kind – say apples – can and do vary in the hours they need. There are apples that need over 1,000 Chilling Hours, while others that just need a few hundred. The same is true for most fruit crops, from blueberries to peaches. This means that particular varieties of trees have different chilling hours, so if you choose a variety with chilling hour requirements that will normally be met in your climate zone, you will have success.

Measuring Your Chilling Hours

Choosing fruit trees and bushes is therefore a two-step issue. First you need to know the average chilling hours where you live. Second you need to know the chilling hour needs of your chosen tree.

Use the map at the top of this blog to get an approximate idea of the chill hours where you live – locate yourself on the map and see what zone you are in. In rural areas you can ask your local USDA Extension specialist, and there are more complex calculators available, but for most situations that map should be sufficient.

Once you know that magic number, the rest is simple. When choosing fruit trees or bushes, find the Chilling Hour requirement, and as long as it is equal to or less than your area, it is a good choice for you.

A Caution. . .

Try to match your varieties as closely as possible to your local chilling hours. Don’t grow a bush that needs only 400 hours if you are in a zone that has 1,000 hours. Why? Because your bush will begin to flower too soon, in the first warm spell of late winter. The chances are that if that happens the flowers will be destroyed by the return of cold weather, and you won’t even see a bloom. So, if your zone is 1,000 hours, try to find varieties that need 800 to 1,000 hours of chill.

Chilling Hours are an important consideration when choosing fruit trees and bushes. Don’t forget to check it, it’s the best way to avoid disappointment.