Asked March 26, 2015, 4:20 PM EDT

When and how do I prune the following plants:Knockout Roses,Leatherleaf Mahonia,Nandina Domestica,Hypericum,and Hydrangea[mostly mophead].

Howard County Maryland

1 Response

The rules of thumb are:
Summer-blooming shrubs are pruned in early spring (March is an excellent time, but early April is ok).
Spring-blooming shrubs are pruned after they bloom.
The exception is mophead hydrangeas.

None of these shrub have to be pruned on a yearly basis. You can prune them to remove dead parts or to shape them up.

Another rule of thumb: don't remove more than 1/3 of the shrub each year.

Knockout rose - wait to prune until you can see that leafing-out has just begun.
Nandina - If you have tall almost bare canes in a multi-cane plant, you can encourage new canes by removing a few of the older canes all the way down to the ground each year.
Mahonia - If it has a long or old stem that is fairly bare, cut back to a whorl of leaves or a good side shoot.
Hypericum - for most varieties of hypericum cut down all old growth each spring to make them more attractive

Mophead hydrangea - Mopheads are an exception to the summer-blooming shrub pruning rule which says to prune in early spring. It's because they do not set their new flower buds in the spring (after pruning), but rather in the late summer-early fall of the previous summer. So if you prune in the spring, you will be cutting off the flower buds.
The problem is, most people don't want to prune in late summer; they want to control size in spring before summer gets here. Consequently, the best solution is to plant a mophead where it has plenty of room to grow and doesn't require pruning. If you must prune, just realize that you are sacrificing some blooms.

That said, in severe winters, mopheads often suffer winterkill of many canes. If you encounter dead-looking canes in the spring, wait until the leaf buds start to swell so that you can determine what part of the cane is still alive, and then prune off the winterkilled parts down to the live part.