According to work by Dr. Ted DeJong in the Plant Sciences Department at UC Davis, the first 30 days after bloom can tell a grower quite a bit about 1) the sizing potential for a stone fruit crop and 2) the time to harvest.  See back ground info and predictive model at:  http://fruitsandnuts.ucdavis.edu/Weather_Services/Harvest_Prediction__About_Growing_Degree_Hours/. 

It has been just over a month since full bloom.  So, what can we learn from the first 30 days of the 2011 prune crop?

Given a 50% bloom date of March 28, 7000-7600 growing degree hours (GDH) accumulated in much of the Sacramento Valley (Colusa, Nicolaus, Durham) in the 30 days that followed.   So, the model (available free on line at:  http://harvest.ucanr.org/) predicts harvest between Aug 29 and Sept 1 in 2011.  Since the model is always long when applied to dried plums, I’m predicting a harvest date of Aug 18-21 for Sutter/Yuba region.  That is when I predict prunes in an orchard with a good crop – say 3 dry ton per acre -- will reach 3-4 pounds pressure.  DON”T take that to the bank, but I suspect it will be pretty close.  What do you think?  Send me a comment (see below), please.

Now for the bad new-- fruit sizing potential for an orchard with a "normal" cropload could be less this year compared to the same sized crop in the same orchard in the last few years.  Why?  The heat unit accumulation in the first 30 days after bloom also helps give growers an indication of the relative sizing potential of a given crop.   A relatively high GDH 30 (accumulated GDH in 30 days after 50% bloom) means a smaller sizing potential.  A smallish GDH 30 means a better sizing potential.  In the past decade, GDH 30 has ranged from 5000-9000.  In 2004 it was almost 9000.  In 2006, a great year for sizing, it was around 5000.  This year, at 7000-7600 GDH 30, we fall in the warmer side of the range.  This suggests that the sizing potential of the crop will be less this year compared to the last few years.  What does this mean to a grower?  Count fruit/tree in a block as soon as reference date arrives – maybe as early as pit hardening.  If you have to thin, thin hard and early.

A post on cropload evaluation post by Bill Krueger follows this one.