The worst time of year to trim a tree depends on the type of tree. The tree's growth cycle also plays an important role in determining the best and worst times to trim it.
The worst time to trim a flowering tree is when the flowers are budding and blooming. Summer is also a poor time to trim most trees, as freshly trimmed areas are very vulnerable to high heat and bright sunlight. Trimming trees during the summer diverts growth energy, frequently resulting in hindered growth.
The ideal time to trim a tree is the early phase of its dormant cycle. During this time, the tree has plentiful energy reserves that promote quick, efficient healing of the trimmed areas.