Chill Unit Experiment

Experimental Protocol for determine the relationship between chill unit and growing degree days (gdd) for apples.

The purpose of the experiment described below is to determine how chill units affect the number of growing degree days needed for apples to reach the bud stage

The hypothesis is that if there are fewer chilling units, the trees will require more GDD to reach bud stage".

Experimental design

One month after full leaf drop collect from 5 different trees one branch which has a terminal bud and is a minimum of 3 to 4 inches long. The samples should be collected from the south side, outer portion of the tree and at mid height. Record the date that the sample is taken , the variety of the apple tree and put the sample in a plastic bag and label it. . Collect 5 samples from each variety available or from the top 5 varieties. After collecting the samples in the orchard , take the samples to the office or house and put them in water. Record the temperature in the house using the Hobo temperature recorder. Repeat the experiment every month until the trees in the orchard break dormancy Record the time the branches in the house reach the silver tip, green tip and half-inch green stage. See picture from this site and pictures from Vermont Record this information for each data set collect one month apart. Use the Campbell weather stations to record the chill units or install a Hope data logger in the orchard.


The chill and heat units will be calculated for each set of data and the relationship between chill units and heat units plotted. The experiment will be conducted at Farmington, NM and Albuquerque and Las Cruces for the 2000 winter season. Consequently , one of the variety at each site should be a Delicious apple.


Douglas Fir




PET for Pecans

image of PET for pecan trees graphic


Hybrid Poplar