3 feet should be good. The fence should be even with the foliage. Even if you white wash the trunks, it helps. Think this thread will help you understand my strategy Full Size Pear Tree Regret . Pear tree trunks develop very thick bark, unlike apples and other fruit trees. The rodents can not damage mature trees. The one in the photo is 30 feet tall.
Pear trees that produce bushels of fruit and avoid disease
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