Winter Chill

Friday, February 3rd, 2012 by Jenny Watts
    • Pluots are a cross between plums and apricots. Their meaty fruit has a wonderful flavor. Bare root trees can be planted now.
    • Spray for peach leaf curl with copper sulfate. Peach and nectarine trees may suffer from this fungus disease without a protective spray.
    • Start an asparagus bed so you can enjoy their young, tender shoots straight from the garden.
    • Plant strawberry plants now for delicious strawberry shortcake this summer.
    • Cabbage, broccoli, lettuce and other cool season vegetables can be started now from seed. There are many wonderful varieties available on seed racks.

Chill Out!

When fruit trees drop their leaves in the fall they go into a winter dormancy. This is important for the tree for several reasons. Shutting down for the winter protects the trees from getting damaged by cold or freezing weather. It also gives them the rest period required before the trees will bloom.

When you read descriptions of fruit trees, there is often a number of hours listed at the end of the description. This is the chilling requirement of that particular variety.

Deciduous fruit trees first need some cool weather and then some warm temperatures to start growing. Different tree species need different amounts of chilling and/or warm temperatures to begin bloom.

In general, the lower the chill requirement, the earlier a tree will bloom. Once the tree has accumulated the required number of chill hours, it will bloom during the next warm period. This can be very difficult in areas like ours where we may have an early warm spell followed by more freezing weather. A variety with a low chill requirement will flower too early and the blossoms or immature fruits will often be damaged by cold weather, especially late spring frosts. Selecting a high chill variety in warm areas will result in little or no fruit production.

Apples have the highest chilling requirements of all fruit trees, followed by apricots and, lastly, peaches. If a tree does not get a sufficient amount of winter chill, it can result in a loss of up to 50% of expected harvest.

There are several models for determining the amount of chill that your orchard receives. The “45 and under model” is the simplest and is still in use. It holds that every hour below 45°F equals one chill hour received.

The “32-45 model” says any hour of cold between 32°F and 45°F contributes one hour to satisfying a tree’s chilling requirement. According to the “32-45 model,” temperatures below 32°F don’t contribute to accumulated chill.

Other models are even more complex, but they still don’t make good predictions all the time. It is very difficult to figure out just what Mother Nature’s formula is.

Determining the exact amount of chill you receive is very difficult. Terrain can affect the chill hours too. Open slopes may receive more chilling hours than sheltered areas next to warm buildings. Locations at about 1000 feet above the valley floor may receive substantially less winter chill than the valley below.

In the Willits Valley, we receive over 1000 hours of chilling each winter. The nearest recording station is the Hopland Field Station, which receives an average of 1024 hours below 45°F and 987 hours between 32°F and 45°F.

When choosing a fruit tree for this area, varieties with at least 700 hours will do best. This is particularly important for peach trees as they tend to bloom early. But if a peach is rated for 800-1000 hours, it will usually remain dormant until the beginning of March. It will have to take its chances after that.

You will have the most success with your home orchard if you choose trees that have the proper chilling requirement for your area.