Fruit Trees
By Jiordana
Stark, Master Gardener
Q: Can you tell me what could have
happened to my fruit trees this year? Most of my
apples, peaches and apricots had very few blossoms and even less fruit. My care for them was the same as always.
I usually have to give the fruit away, there is so much, but this year I
had to buy fruit.
Jerry L. Atascadero
A: You are not alone with a deficient
fruit crop this year, Jerry. As you might recall, we
had an unusually mild winter. Nighttime temperatures
did not drop low enough or consistently enough to give our deciduous fruit and
nut trees the “chill units” they needed to adequately enter a dormant, or
“rest” period after leaf fall. Many home orchardists do not give enough credence to the importance
of winter chill units. These are the total number of
hours of temperatures between 32 and 45 degrees from November 1 through mid
February, and are required to effectively break dormancy. Generally,
during the winter there should be fifty to sixty days with a mean temperature
of 45 degrees or lower. December and January are
usually the most critical months. If each of these two
months has approximately 400 hours of fairly evenly distributed temperatures
below 45 degrees F., then troubles related to mild winters are less likely.
Stone fruit trees, such as peaches, develop their vegetative and fruiting buds
in the summer. As winter approaches, the already
developed buds go dormant in response to both shorter day lengths and cooler
temperatures. Once buds have entered dormancy, they
will tolerate temperatures much below freezing and will not grow in response to
mid-winter warm spells. However, periods of a few days
to a week or more of mild weather may offset or reduce the effectiveness of
accompanying periods of good chilling weather, and require more chilling hours
than are usually necessary. Last winter is an example
during which periods of adequate low temperatures were interrupted by longer
periods of warm sunny days.
Trees are kept in dormancy due to natural growth inhibitors that prevent growth. Dormancy is broken in the spring when sufficient chilling
units have accumulated to break down these natural growth inhibitors within the
tree. As long as there have been enough chilling
units, the flower and leaf buds develop normally. If
the buds do not receive sufficient chilling temperatures during winter to
completely release dormancy, trees will develop one or more of the
physiological symptoms associated with insufficient chilling: 1) delayed
foliation with late straggling spring growth, 2) reduced fruit set with buds
opening over a long period and large numbers of buds failing to open at all,
and 3) reduced fruit quality.
Chill unit requirements vary among fruit varieties. Temperature
during winter and spring is the key element in choosing fruits and nuts for
coastal and inland areas. Spring frost is the main
hazard for early fruit and nut varieties in inland areas, killing both new
leaves and flowers. Conversely, too few hours of
chilling causes problems with fruit production along the coast. Insufficient chilling is probably the most limiting
climatic factor for apple, apricot, nectarine, peach and sweet cherry. Due to the many micro-climates in the coastal areas, it is
difficult to set hard and fast rules on suitable varieties. Lists,
however, of suggested varieties for both inland and coastal orchards are
available at .
For more information on chill hours for your area, or on tracking your own
chilling hours, contact the