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Answer

Yes, you can but removing too much foliage during the growing season on any plant can damage the plants overall health and leave it susceptible to insect and disease. Below is a description of how best to prune the two types of hydrangea.

Hydrangea should be pruned as needed to keep the size and form you desire and to encourage bigger flower clusters. There are two types of hydrangea those producing blooms on new wood and those that bloom on the previous years growth. Those that bloom on new wood should be pruned during the dormant season, those that bloom on previous years growth should be pruned after it blooms. To encourage larger flower clusters reduce the number of overall canes. Before making the pruning cuts you should first decide why your are pruning, size, shape, thinning or overall health. When thinning locate the older thickest cane fallow it to the ground and remove at the base. Thinning (aka renewal pruning) should be done every couple of years to remove the old growth. When making cuts to control size or shape select each cane individually, along the cane at specific intervals there will be buds developing (the buds look like red match heads, size will very) cuts should be made just above the buds. Removing canes every couple years will encourage healthy growth, bigger flower heads and increase air circulation reducing disease.

Hope this helps!

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