Tuesday Feb 17 2009
Ask the Master Gardeners: Fruit trees need period of cold for better yield
By: Gay Wilhelm, Placer County Master Gardener
Question: What are the chilling requirements for my cherry tree? Answer: Deciduous trees, which include fruit trees, begin a period of dormancy when daylight hours decrease in the fall. To break this dormancy and produce the necessary buds and shoots, a critical number of hours of cold temperature are needed. This usually involves 200 to 800 hours of cold temperature below 45 degrees F. This process is called vernalization. Mild winters can create havoc with fruit tree yields. December and January are the most critical months. If each month has approximately 400 hours of temperature below 45 degrees and the cold is evenly spaced, the tree will have better yields. If warm temperatures arrive for even a few days during these months, the effectiveness of chilling may be reduced. Greater seasonal totals of cold weather may be needed if warm sunny days are extended. Our foggy weather in the valley often provides temperatures under 45 degrees assisting adequate chilling hours. Through plant hybridizing, new varieties have been developed that require less chilling hours. This development has enabled fruit production in warmer winter areas. Cherries tend to have very high chilling requirements, between 700 to 800 hours or about 28-32 days continuously exposed to 45 degrees or less for sweet cherries and over 1,200 hours or 48 days for sour cherries. Be sure to check your variety for the amount of chilling hours required and appropriateness for your area. If you have gardening questions, call the Placer County Master Gardener hotline at (530) 889-7388.