Many of these shrubs tolerate severe pruning. I prefer a more moderate approach. This allows you to control growth and encourages sturdier stems. Cut all the stems of potentilla and spirea back halfway to the ground. Then remove half of the older and thicker stems to ground level.
New shoots will emerge in spring. The older remaining stems will provide support for the thinner often floppy new growth. The result is greenery from ground to stem tip and a sturdier neater plant for you to enjoy.
And be sure to wait to prune lilacs, forsythia and other spring flowering shrubs until right after bloom.
A bit more information: Spring flowering shrubs like bridal wreath spirea, lilac and forsythia form their flower buds the summer prior to flowering. Pruning these shrubs any time other than soon after flowering eliminates the following spring's floral display. The plants will be fine, but you eliminate the reason (flowers) most of us grow these plants.
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