“Every decision you’ll make while pruning will be determined by why you are pruning in the first place. Learn why you need to stop pruning in late summer. “Some shrubs like lilacs and viburnums form flower buds in late summer, so extensive pruning of these shrubs can remove the next year's flowers.” Add to Google settingsPhotographer, Basak Gurbuz Derman // Getty ImagesWhen is the best time to prune shrubs? The bottom line is that pruning should always be done for a reason, and pruning is not a must.