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Apple Bloom Cycles and Harvest Predictors

3/30/2017  By Cris Hales
Apple Bloom Cycles and Harvest PredictorsThere is a science behind determining the harvest of our Washington State apples. Apple harvest is between August to early November with Red Delicious falling in the middle to later part of the harvest window. On average there are 143 days between full bloom and the first harvest. Additional seasonal adjustments are made to predict harvest based on the average minimum temperatures for the 15 days following full bloom.
The development of apples from dormancy to fruit set is measured in 9 stages as shown in the photo.
Washington State University is forecasting that our bloom and harvest will be significantly later than it has been the past few years.
Daily high temperatures are used to predict bloom based on degree days (DD) above 43F. As of March 11, 2017 Bloom DD in Wenatchee were 78 DD and in the Mattawa area (typically earlier ground) 118 DD, this indicates that on March 11th the trees were still in full dormancy. To predict silvertip 331 DD is needed, 600 DD for tight cluster and 920 DD predict full bloom. On March 1st, 2016 Mattawa was already at silvertip, and by March 11th at green tip. On March 1st in Wenatchee we were 217 DD behind 2016, and 166 behind normal.
In both 2015 and 2016 full bloom was recorded on April 9th; using 91 years’ worth of data, the average bloom date for Red Delicious is April 27th. Two different growers spoken with have both indicated that this year looks a lot like 2009 and/or 2011. In 2009 the bloom date was May 1st and in 2011 May 7th (The latest recorded was May 16th in 1922).
Since Red Delicious is a relatively late blossoming variety, other apple varieties on warmer sites will bloom up to a week earlier. Elevation will be a large factor, lower elevations may see some bloom up to a month earlier. Apricots are typically the first tree fruits to bloom, followed by peaches and nectarines, then cherries and pears, and finally apples.
Source: WSU Extension, Tree Fruit