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The very best time to prune hemlocks is in late winter, just before spring growth begins. If the hemlock is growing vigorously, it can be pruned again in late spring or very early summer to keep it from getting too large.
Emergency pruning can be done any time of the year. Never leave damaged, broken or diseased wood on a shrub; it is an invitation to disease and insect infestation. - Never prune a hemlock in the summer and fall if it can be avoided. Pruning in late summer may force the shrub to produce new growth, which will still be weak and susceptible to damage when winter arrives. Pruning in the fall may leave open cuts that the shrub cannot heal before winter.
- A formal hemlock hedge can be sheared with hedge shears to tidy the overall surface. However, more selective thinning should be done early in the season to "open up" the shrub and let sunlight reach the inner branches. A hedge should be shaped like a wedge, so that the base is wider than the top. This lets sunlight reach the lower branches, which prevents the bottom branches from dying off and giving the entire hedge a scraggly appearance.
- An informal hemlock hedge needs very little pruning, since the natural form of the hemlock is part of the hedge's beauty. Prune this kind of hedge to improve the health of the shrubs by removing any damaged growth or branches that rub against each other. Thin the hedge by removing a few of the oldest branches in the center of the hedge each year.
- Keep the use of hedge shears to a minimum. Although hemlock tolerates shearing better than most narrowleaf evergreens, it will suffer from frequent shearing. Use loppers or a pruning saw for thinning and shaping a hemlock hedge. Make sure all cutting tools have sharp blades, so they can make clean cuts without ragged edges. If you are cutting out diseased wood, sterilize the blades after use so that disease is not transmitted to a different shrub.
