When pear blooms begin to swell in the spring and open, they become vulnerable to cold temperatures and frost. Frost occurs when temperatures fall below 00 degrees Celsius. These conditions may damage the pear tree blossoms but will not necessarily reduce overall crop yield. The percentage of blossoms damaged will determine how much fruit the tree will produce.
Pear blossoms can withstand temperatures of -4.44 degrees C as long as they are in a tight cluster, but as they begin to open and bloom, they become more susceptible to cold. Blossoms in the first bloom stage, for instance, require temperatures of -1.67 degrees C or higher to remain damage-free. In the post-bloom stage, the blossoms require temperatures of -1.11 degrees C or higher.
When pear blossoms are damaged by frost, the injured tissue turns dark, becoming blackish-brown with time. If the blossom's pistil (the centre part of the blossom) is damaged, it will turn brown, indicating the flower is dead and will not develop to become fruit. This does not necessarily result in lower crop yield. Pears and other large-fruited fruit trees can suffer as much as 75 per cent flower loss and still produce a decent yield.
Covering the pear trees when a heavy frost is expected can help to prevent injury to the tree's blossoms. This will protect the trees from direct contact with frost. Cover the trees with cloth, such as cheesecloth or bed sheets. Commercial orchards heat their orchards during bloom time, but this is not a practical method for home gardens.
Some varieties of pear are more cold hardy than others. These varieties tend to blossom later. Examples of cold hardy pear tree varieties include Summer Crisp, Parker, Luscious, Gourmet, Williams, Bon Chretien and Onward. Invincible, also known as Delwinor, is a hardy pear tree that flowers twice, with the second flush of blooms often missing the latest frosts.