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The mystery of fruit tree dormancy explained

A cherry tree blooming.
A cherry tree blooming.

Q: While my stone fruit trees are looking good, my pear tree has not budded out yet. It is May. What is the problem? — Suzie in San Luis Obispo

A: Your pear tree might be suffering from “delayed bud break.”

All deciduous fruit trees have a period of winter dormancy, the stage between leaf drop in the fall and bud break in the spring, which protects the tree buds from freezing weather.

Scientists have identified two distinct stages. Endodormancy is the first stage, which is related to limiting growth factors in the tree buds.

Ecodormancy, the second stage, is related to external factors — temperatures that control growth. Stay with me, because this lifts the veil of the dormancy mystery.

In order to wake up your pear tree, a certain amount of cool weather is needed during the first stage. We refer to this as a chilling requirement. Different varieties need different amounts of cold hours, roughly between 30 degrees F and 60 degrees F.

Stone fruit trees, apricot and peach, need an average of 700 to 1,000 chilling hours below 45 degrees F. Pome fruit trees, pear and apple, need an average of 1,000 to 1,500 chill hours. The chilling hours are needed to end the first stage of winter dormancy.

In the second stage of winter dormancy (ecodormancy) warm temperatures “wake up” the buds and the tree starts budding, then blooming. Just as the cold temperature is needed to facilitate the end of the first stage, warm temperatures are needed to begin growth and bloom.

The unusual weather this year might have contributed to the delayed bud break on your pear tree; not enough chill hours during the dry warm-winter, followed by unusual cool temperatures with storms in April.

There is a good chance that your tree will catch up with the warmer weather we are having now.

Please join us for our Drought Tolerant Plant Sale from noon to 2 p.m., Saturday, June 21 after our Advice to Grow By Workshop. It will be at the Garden of the Seven Sisters, 2156 Sierra Way, San Luis Obispo. We will feature numerous drought-tolerant plants, California natives, and Mediterranean-climate plants. Master Gardeners will be available to answer plant questions.