When can I prune my hemlock trees?
On hemlocks, small, flat, dark green needles are arranged spirally around the stem. Cones are small and pendulous. Hemlock grows as a tall tree, but can be pruned or sheared as a hedge.
Thank you for your question concerning the Tsuga (hemlock). It is best not to prune after August. To maintain size, prune in late winter or early spring, before new growth begins. Prune again in mid-June. Annual shearing should be done after new growth has expanded. Follow-up shearing should continue throughout the growing season. To avoid stimulating new growth late in the season (thus preventing winter injury), do not shear after August.
Courtesy of the NYBG Plant Information Service