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After the wether this past weekend I thought it may be a good idea to talk about critical temperatures for fruiting trees and plants typically grown in our home gardens.

Most of our fruiting plants and trees require a certain amount of cold temperatures (chill hours) to produce good quality fruit.  Once that requirement is met it takes a certain amount of heat units to initiate flowering.  With over 900 chill hours by mid February, we had plenty of cold to properly break dormancy in most of our fruiting trees and plants. If you have looked at most trees in the landscape you will notice that the flower buds are beginning to swell if they are not already open. 

So what do freezing temperatures mean for these developing flowers?   Different plants have different tolerance to cold, and different stages of growth are better at handling cold as well.  What are these temperatures for different crops that we like to grow? The answer to that question can get pretty lengthy.

For peaches a dormant bud can handle extremely cold temperatures.  The swollen buds that we are seeing in most of our area can handle 18-21 degrees before buds are killed.  As we begin to see a little pink from the flower the flower is more sensitive and can only handle 25 degrees.  At full bloom and post bloom that critical temperature is 28 degrees.  Those are the temperatures that we can expect to see 10% bud kill.  The temperatures that would cause 90% bud kill are much lower, and the duration of the cold event also plays an important role in bud mortality.  These same numbers can be used for plums.

For Strawberries the numbers are similar.  Buds that have not yet emerged are hardy down into the low teens.  As buds emerge they become more sensitive.  Emerged buds can handle around 25 degrees and open flowers can handle 28 if the air is dry. 

Blueberries can handle low 20’s as flowers begin to swell, and 26 degrees as the flowers are about to open. After flowers are open temperatures below 28 degrees, even for a few minutes can cause damage.

Pears have also begun to swell, but if the weather forecast is correct they should be ok.  Dormant buds can handle 15 degrees.  20 degrees can injure buds as they begin to swell.  And once you start seeing white flower parts 26 degrees can cause at least 10% mortality. 

It is still early but the cold that we experienced over the weekend definitley did damage to fruit trees. If you have questions or comments contact your County Extension Agent.

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