Blue Mist Spirea (Caryopteris incana), Dark Night Spirea (Caryopteris x clandonensis), Russian Sage (Perovskia atriplicifolia) and all Butterfly Bushes (Buddleia sp.) look much better if pruned low to the ground, 2" to 6" tall, every spring in March or April. All of the shrubs listed will survive if you choose not to prune them back. However, by pruning them to the ground, you're removing all of the dead wood from the previous year and the new growth will look much cleaner and the summer flower will look more attractive.
I also like to prune some of the true Spirea species to the ground in spring. Spirea tend to accumulate years of deadwood and pruning them to the ground in spring will produce a brand new shrub and flowers in the summer. Some of the more common varities include Spiraea x bumalda 'Anthony Waterer', 'Goldflame', 'Limemound' and 'Magic Carpet', Spiraea japonica 'Gold Mound', 'Little Princess' and 'Shirobana'.
Potentilla varieties can be pruned the same way, tight to the ground, in March or April. Potentilla look especially scraglly if you allow the deadwood to accumulate. Some of the more common varieties include Potentilla fruticosa 'Dakota Sunspot', 'Gold Drop', 'Mckay's White', 'Pink Beauty' and 'Mango Tango'.
Spring favorites you shouldn't prune ... yet.
Shrubs such as Forsythia, Lilacs, Quince and Viburnum should not be pruned until they are finished flowering for the season. If, for example, you were to prune a Lilac in March you will have cut the flower buds off and you will not see a flower until 2010. Viburnum have some of the most fragrant flowers available and they are often pruned too early and the flower is removed.
If planted in an appropriate spot, with room to grow, most shrubs require minimal pruning. You should always have a specific reason to prune ... usually to remove dead, diseased or damaged branches. Don't prune your shrubs just because your neighbor said you should or just because the previous homeowner pruned the shrub a certain way.
