Why has the trunk of my lilac tree split? How can I help it?

This is probably caused by freezing and thawing of the bark during the winter. The split part is probably in the southern side of the tree because that is the warmer side. There is nothing directly to do to help the wound. Keep the tree healthy with watering and some fertilizer to help it heal.

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Related Resources:
Protecting trees and shrubs in winter
Winter Damage to Trees and Shrubs