Answer: These summer blooming spireas frequently suffer extensive winter damage. The old branches help to protect the lower portion of the plant, so it is better to leave them intact until spring. In the spring, trim them back as needed to remove the winter damage and remove dead stems/branches, and restore a somewhat symmetrical look to the plant. Usually, you will remove half to two thirds of the bush. Then reach into the center and individually remove some of the oldest stems, cutting low down on the plant. This will help thin the center and stimulate new vigorous growth from the center. These shrubs bloom on the new growth, so this will actually promote flowering. After the main bloom period,lightly shear the plant to deadhead (remove the faded flowers) and encourage additional flowering later in the summer.
The recommendation to cut spireas to the ground as a renewal pruning method refers to the old fashioned, larger growing, white flowered spireas that bloom in the early spring. These are different in that they bloom on old wood from the year before. To renew these (a very old shrub that is neglected and terribly overgrown) you can be drastic and cut it down very short in late winter (and lose a year of bloom.) Or, you can prune over a three year period to gradually remove all of the old stems by cutting them at the base -- taking out a third in year one, half the remaining in year two and the rest of the old stems in year three. You would do this in the spring either before or after it flowers. This allows the plant to continue flowering while you work on renewing it.
I hope this helps.
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