One of the most important aspects of winter care for fruit and nut trees? Make sure they are planted in a location where they will receive their recommended amount of chill hours.
Cooler temperatures act as a reset button for fruit trees that go dormant, like apple trees, cherry trees, nut trees, and more. The fruit trees can take a break and recharge before producing next year’s harvest.
A Chill Hour or chill unit is equal to one hour that a fruit tree spends in cooler temperatures, ranging from 32 to 45 degrees F. Many fruit trees vary in the amount of chill hours that they need. Some fruit varieties, like low chill peach trees, only need 50 to 100 chill hours, while others need up to 1,000. Some tropical fruit varieties don’t require any chill hours.
Some fruit tree varieties grow their buds for next year’s fruit towards the end of the summer. As the temperatures drop, trees slow their processes, like photosynthesis, in order to go dormant in the winter. The fruit buds then “go to sleep”, so that they will be protected from harsh winter temperatures. Sudden warm days in the middle of winter won’t cause the buds to break dormancy.
Once temperatures start to warm up and a plant has met all of its chill hours, dormant buds wake and grow. This allows the buds to prep themselves to bloom at the proper time during the spring.
Too Many Chill Hours?
There is a possible risk involved with planting a low chill hour plant in a high chill area. For example, if a tree that needs 100 hours is planted in an area that receives 1,000 hours, it runs the risk of waking up too early on a warm day. If the tree breaks dormancy too early, late freezes will damage the new leaves and flower buds, ultimately causing the tree not to fruit.
Not Enough Chill Hours?
More risks are associated with planting a high chill plant in a low chill area. If a tree or plant that needs 1,000 chill hours is planted in an area that gets 500 or less chill hours then it may have trouble blooming or breaking dormancy. No blooms mean no fruit.
This can lead to a later bloom time and a longer bloom time. When blooms stay on a tree too long, they become more susceptible to fungus and diseases. This can also lead to a reduced fruit set and poor fruit quality.
If a plant with male and female flowers doesn’t get enough chill hours, then the male and female flowers run the risk of not blooming at the same time. This can make it difficult for the blooms to become pollinated.
Remember: Chill hours aren’t an exact science. If you have a fruit tree that needs 600 hours, it doesn’t need exactly 600 hours. 600 is just the average amount of hours that your tree needs, a few more or few less hours won’t hurt. However, it’s best for high chill plants to be planted in Northern areas and it’s best for low chill plants to be planted in warmer, Southern areas.
Growing Zones and Hours
The USDA Plant Hardiness Zone map lists the average lowest temperature in different regions of the country. Knowing your growing zone will help you determine which plant varieties will survive the winter temperatures in your region. However, the USDA Plant Hardiness Zone doesn’t list the average amount of hours for each zone.
Where Do I Find My Hours?
To discuss how many hours your plant needs and or if it will do well in your area, contact one of our plant specialists or call your local agriculture center. Also, refer to the map below to see the average amount of chill hours for your region.