Friday, February 24, 2012

Winter Peach Bud Development

  We've now finished winter pruning of the apples and pears, and are just starting on the peaches.  Warm days like the past couple days are great weather for pruning, but the warmth has caused some peach flower buds to start to swell already, so I'm glad for today's colder weather to hold them back.
   Peaches and some other stone fruit trees, especially apricots and Japanese plums, tend to bloom very early in spring, when there's still a strong chance of frosts that could kill open blossoms and developing fruit.  Apricots bloom so early that we don't grow them, as a crop can be expected only one year in ten in this area.  With peaches, we choose varieties that require the most winter chilling, which is the number of hours with temperatures between 32 F and 45F.  (Colder or warmer temperatures don't satisfy the chilling requirement.)  Once a variety's chill needs are met, it must accumulate a certain number of heat units above 45F, then buds will start swelling and developing until they bloom.
Our peach trees are still dormant, but some flower buds have begun to swell.
   Many peach varieties need just 700 to 850 chilling hours, and these tend to bloom earlier than high-chill varieties.  We've chosen only varieties that need 950 to 1100 chilling hours, which is about the highest among peach varieties.  Even so, with a mild winter like we've had so far with many hours with temperatures between 32 F and 45F, our peach varieties' chilling needs have already been met.  So we hope that nights stay cool for the next month or so, to keep flower buds from developing too fast.
The branch in front shows the rounder flower buds on each side of the branch starting to get plump.  Pointed leaf buds, often found between two flower buds, don't swell until later, so peach leaves usually don't emerge until after bloom.
   We are thankful that so far this year we haven't had the extreme cold of Feb. 2011, which killed all the peach flower buds while they were dormant, so they never bloomed. With earlier peach bloom expected this year, we'll most likely have spring frosts during bloom.  A light frost down to 28F during bloom is actually beneficial, as it kills some peach flowers but not all, reducing the amount of fruit thinning we have to do later.  We just hope we don't get severe frosts of 25F or below during peach bloom, as that could eliminate the crop.