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About Fruit Tree Chilling, Jan 31, 2014
– By Dave Wilson Nursery
As deciduous fruit and nut growers know, adequate winter cold for the varieties planted is essential for strong bloom and satisfactory fruit set.
This winter, in most southern California low-chill locations, fall and early winter fruit tree chilling was considerably lower than for the previous year. Between November 1st and January 31st, many locations received less than 60% of the chill hours (hours below 45°F) that were received during the period one year ago.
At the Pomona data station, for example, 252 chill hours were received between November 1 and January 31 this year, compared to 427 a year ago. In much of southern California, the low-chill varieties definitely earn their keep – especially after the warmest winters.
In northern California during the recent November 1 to January 31 period, many locations received chilling greater than a year ago in terms of hours below 45°F, but the effective chilling may be lower, as indicated by the lesser “chill portions” received, per the Dynamic Model for fruit tree chilling. The Dynamic Model is a more exact way of estimating fruit tree chilling, taking into account the effectiveness of chilling at various temperatures and when they occur during the dormant period. For example, the data station at Fresno recorded 913 chill hours this year vs. 754 for the previous year, November 1 to January 31, yet chill portions at January 31 were lower: 49 for this year vs. 54 last year.
To evaluate fruit tree performance from year to year and variety to variety, to assess pollenizer performance and requirements, and for making certain pruning, fertilizing and irrigation decisions (less crop expected means fewer early season inputs required), fruit hobbyists and for-profit growers should be diligent in their annual note-taking with regard to bloom time and duration, as well as crop size, as this information can be especially valuable when correlated to chilling data. Much of fruit tree care is centered on crop management; too much crop burdens both tree and grower, and, conversely, there’s little reward in having a poor crop yet more pruning work and wasted nutrients due to the resulting excessive vegetative growth.
Home fruit growers who track their varieties’ bloom seasons will know for certain which varieties bloomed early or late relative to a hard frost, whether cross-pollinating varieties had overlapping bloom periods and whether the bloom for each variety was all at once or prolonged, which could indicate inadequate chilling was received. The answer to a key growing season question is often in the bloom notes.
For fruit growers who may be interested in taking a look at some comparative fruit tree chilling data, a summary of UC Davis data for representative California locations is provided: see Fruit Tree Chilling, Jan 31, 2014. Included are chill hour and chill portion data for this year and last.