Know the chill factor before you plant

Fuyu persimmons bear more fruit when there's more chill.
(Photo: Debbie Arrington)

Chill hours are a key to success for many fruit trees and shrubs


By Debbie Arrington

It’s nature’s cue to take a nap. Lower temperatures in fall signal many plants to wrap things up, drop their leaves and drift off into dormancy. They need to rest.

But it’s the winter cold – measured in chill hours – that really makes a difference in plant performance in the year ahead.

Autumn, which officially starts Saturday, is a wonderful time to plant deciduous trees, shrubs and perennials in Sacramento. They appreciate the lingering warmth of summer while putting down roots and gearing up for a big spring.

Chill hours are an important factor when choosing which varieties to plant. Some need more chill than others.

Chill hours are that kiss of cold that cues blooming and fruiting in a wide range of plants, such as apples, almonds, blueberries, peaches, grapes, persimmons and peonies. Measured from Nov. 1 through Feb. 28 or 29, that chill time is roughly the number of hours below 45 degrees each winter (but above freezing). The total subtracts hours above 60 degrees, which offset the chill effect.

According to the UC Cooperative Extension master gardeners, Sacramento gets on average about 600 chill hours each winter, some communities much more. The Fair Oaks Horticulture Center, home to the Sacramento County master gardeners, recorded 792 hours below 45 degrees last winter; 762 were in that sweet spot above 32 degrees. That also means the Hort Center experienced only 30 hours below freezing.

Chill hours can fluctuate widely, which often accounts in crop changes, too. For example, the Fair Oaks station measured 1,028 chill hours in the winter of 2012, but only 480 in 2014.

The University of California’s Fruit and Nut Research and Information Center developed a very handy set of online tools to calculate chill hours. Its database includes both cumulative chill hours during the current season as well as wealth of historical information. Find it here: https://bit.ly/2NPObh0. Or got to the center’s website, http://fruitsandnuts.ucdavis.edu/, and search for “chill calculator.”