Answer: Plants in need of renewal pruning are best pruned in very early spring (this will sacrifice the year's blooms.) Simply cut the plant off at the base as close to the ground as possible. This will result in a thicket of shoots which are then thinned by removing any that are thinner than a pencil. That is drastic but usually works.
Alternatively, over a period of several years you can rejuventate it by selectively removing the oldest stems or trunks. This cna be done in very early spring or right after they bloom. Start by removing one third of the oldest wood (stems/trunks) by cutting it at the base. The next year, remove half of the remaining oldest stems. The third year, remove the rest of the original stems. Each year, remove any shoots thinner than a pencil.
It is possible that the different varieties have different mature heights and would not grow as a symmetrical set.
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