Dormancy begins with defoliation and ends with bud break

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Dormancy begins with defoliation and ends with bud break

  • Dormancy begins with defoliation and ends with bud break

  • Lack of ‘good chilling’ results in straggly bloom, weak flower buds and poor leaf out

  • Buds don’t develop good vascular connections, so aren’t strong ‘sinks’ for nutrients and PGRs

  • Strategy to overcome lack of chilling includes ‘pushing’ trees into dormancy (chemical defoliation) and compensating for some of the chill requirement with dormant oil

How to measure chilling?

  • The Dynamic Model was developed in Israel where there are also warm winter periods

  • Unit of chilling = Chill Portion (CP)

  • Chill hour model doesn’t adjust for heat cancellation

  • Based on four trial years, the broad definition of 850-1500 chill hours as a requirement for ‘Bartlett’ pear should be considered as 56 to 66 chill portions for a minimum to adequate requirement, using the Dynamic Model, forced bloom in mid-winter and full bloom dates

When and Why to defoliate?

  • Fall defoliation may help to induce early dormancy

  • Inducing early dormancy may help to satisfy chilling requirement

    • Reducing some of the ‘need’ to chill
    • Shortening the dormant period
  • Fall urea applications increase cold hardiness in some crops

Defoliation treatments in 2006-2007

  • 1% CuEDTA + 2% urea applied 0, 2, 4 CP

  • Earliest defoliation on 26 Oct, 0 CP

    • fruit weight and firmness; no diff. in Brix
    •  #1 fruit and estimated total yield
    •  #2 fruit by approximately 50%
  • Defoliation on 3 Nov, 2 CP

    • slightly decreased rat tail flowers
  • All timings  #2 fruit, no  in #1 yields

  • Tendency to thin fruit most with earlier defoliations

Fall Defoliation 2007-2008

  • Chelated Copper + Urea = Fertilizer grade urea (2% v/v) and chelated copper (1% of Monterey Copper-All; Monterey AgResources

  • Bartlett/Winter Nellis, planted in 1960 spaced at 12’ x 20’ (182 trees/acre) on the Polder Ranch Orchard on Russell Road. The soil was Columbia silt loam over Sacto silty clay

  • CRBD, 3 blocks, 1 acre = 5.5 rows, total trial ~8 acres

Fall Defoliation Results, 2 trial years

  • Defoliation treatments in 2006 and 2007

    • Slightly delayed primary bloom
    • thinned primary bloom
    • improved fruit size and yield (more than doubled %crop picked at first harvest)
    • later defoliation more beneficial (3-4 CP in 2006, 7 CP in 2007)
    • Defoliation in 2007 at 7 CP thinned rat tail bloom
  • This may be a tool to

Dormant Oil Timing for Control of Bloom

  • Timing of dormant oil is usually mid-Dec to Jan 1 for convenience

  • This doesn’t account for state of dormancy in the tree and how much chilling has accumulated

  • Polder Ranch, trees planted 1960 and 1970; 10’ x 20’ (218 trees/acre)

  • Clean Crop Dormant Plus applied in CRBD, 3 reps of 10-12 rows each (rows varied from 207 to 408 trees per row)

  • 2007-2008 timings for dormant oil applications: Dec 31 (35 CP), Jan 7 (40 CP), Jan 14 (45 CP)

  • Collected bloom data (primary bloom, rat tail bloom) and yield data

Trial 2005-2006 Trial 2007-2008

  • 23 Dec, oil @ 30 CP  #1 & #2 fruit,  total yield and  Brix (too early)

  • Rat tail flowers reduced by treatment at 43 and 54 CP (2 last timings)

Where to get chill data and information about calculating chill portions

  • Fruit and Nut Center Website (How-to Guide on the Dynamic Model and dataloggers)

  • Your farm advisor!


  • Our appreciation to our grower cooperators

  • Joe Green Ranch

  • Greene and Hemley Ranch

  • Van Loben Sels Ranch

  • and to the California Pear Advisory Board for ongoing support

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