February 01, 1999
The following two articles excerpted from Olive Notes, Steve Sibbett, editor
A mass of cold arctic air, moved into California and the Central Valley during the third week in December. Olive is a subtropical tree and can be damaged by freezing temperatures. Generally, small fruitwood and small branches can be injured or killed by periods of temperature below 22 deg. Expect only minor damage to next season's table olive crop. At this writing (1/4), only occasional bark splitting and defoliation have been observed. Oil content of fruits left on trees for oil extraction will not be affected. Tests conducted by Hartmann in 1952 found that freezing did not affect oil content of mature fruits. However, because frozen fruits can dehydrate readily, an appearance of increased oil percent occurs as fruit water is lost. Oil quality may be reduced as acidity increases following freeze damage to fruits.
In a similar freeze in 1990, cold hardiness was affected by:
Olive variety:
Of the table varieties grown in California, Manzanillo suffered the most tree damage in the '90 freeze. Of the other varieties grown, Ascolano emerged as the "hardiest" with only minor damage reported for Mission, Sevillano, and Barouni.
Acclimation:
Extent of damage can be affected by the degree to which trees were acclimated going into the freeze. The fall of 1998 was characterized as being cooler than normal providing some acclimation.
Cultural practices:
Pruning, irrigation, and fertilization can influence the amount of damage a tree sustains. In 1990, trees that were pruned in early fall lost the protective effect of the canopy.These were severely damaged while unpruned trees had minimal problems. (Note, freeze causes bark to split resulting in openings that can be infected by the "Olive Knot" bacterium. Additional copper sprays are recommended for disease protection.) Trees that were well watered, especially after harvest, sustained more damage than trees where water was withheld - little or no irrigation after harvest apparently encourages "hardening off" and more cold tolerance. Trees well irrigated the previous summer, however, recovered faster the next spring than those not well irrigated We believe excessive fertilization or Nitrogen applications late in the season that stimulate excessive vegetative growth will cause more sensitivity to cold temperatures. Young trees were more severely injured than mature ones. It's of interest however that few of these trees were killed outright - most survived and returned to normal growth the following season.