In your sentence, "winter" is not a modifier of "shrubs." It is a modifier of "pruning." In other words, "Winter shrub or hedge pruning" means "pruning shrubs or hedges for the winter season."

تقليم الشـجيرات والسـياجات الشجرية لفصل الشـتاء

Otherwise, the rest of the sentence would not make much sense, especially the verb "should be completed." It is the pruning that should be completed, not the shrubs.

ترجمة حرفية