December 1, 2012

1 - Plant chilling requirements help plants measure the length of the winter and break dormancy in the spring.

Yard and Garden December 1, 2012


I am trying to find out what the "chill hours" are in the Mimbres, by San Lorenzo, NM. Would you be able to help me with this information? I have a list of the fruit and berries that grow in our area, but I wondered if there was more information. Also, is there a map that would give me the information for our area? through local NMSU Extension office

Grant County, NM


Thank you for including the information that you are located in the Mimbres. There are 2 San Lorenzos in New Mexico. Yours is in Grant County, the other is in Rio Arriba County!

Chilling requirements of various fruit and berry plants are important and can determine whether or not you are able to produce a crop with some varieties. In your location, in the mountains, you probably have sufficient chilling conditions for most fruit varieties. Your list of fruit and berry varieties growing in your area will be a very helpful resource.

Chilling units (or hours) are the number of hours of cool, non-freezing temperatures experienced during the winter. As temperatures cool and days shorten in the fall, plants become dormant. Each variety of plant has a different chilling requirement that results in the breaking of dormancy in the spring and resultant flowering. There are biochemical changes that occur in buds during the winter that, simply put, measure the length of the winter. If the winter is long enough, if there are enough chilling hours, the plant can bloom in the spring. Our fluctuating temperatures complicate matters somewhat since the temperatures experienced have an effect on the biochemical changes. At temperatures below freezing, the biochemical changes stop and there is little or no measurement of the chilling. At temperatures above 50 degrees, or higher for some plants, the biochemical reactions back up and require additional chilling to overcome the period of warmth. The ideal temperatures are in the high 30 degree range. This is the temperature range at which the biochemical changes occur most rapidly and most quickly overcome dormancy. Various fruit production books and online sites can provide equations to help you calculate the chilling units accumulated by various fruit varieties at your location. You need to measure the temperatures and apply them to the equations. The specific chilling requirements for various fruit varieties are usually listed in catalogs selling these plants.

Because you are in a mountainous region, a map or table of chilling hours will probably not be accurate. Your location makes a great difference in the temperatures experienced. Valley locations are different from slopes which differ from hill and mountain tops. North-facing slopes accumulate chill units at different rates than south-facing slopes. You will need to measure your daily high and low temperatures and calculate the chilling units experienced at your orchard location.

Because of your location in the Mimbres, you can probably expect most fruit and berry varieties to overcome dormancy. Mid-winter cold temperatures or spring freezes may be a more important determinant of which varieties will do best for you. Those with high chilling requirements that break dormancy and bloom later may be your best choices, but that depends on your specific environmental conditions.

By the way, this information is relevant for the San Lorenzo in Rio Arriba County and most other parts of New Mexico as well, but the statements about mid-winter and spring cold is probably even more relevant.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email:, office: 505-865-7340, ext. 113.


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