In the spring have you ever seen a lilac bush and for some reason maybe there are only blooms at the very top and the sides are barren of flowers or maybe wondered why the poor lilac didn't bloom at all? Maybe the same thing happened to a forsythia bush or any number of other spring blooming shrubs.
Usually the likely cause is mistimed pruning. It happens, either we get the pruning bug in the fall during fall garden clean up, you went on a pruning spree in the middle of winter (which is a great time for pruning many things), or it starts to get warm in the spring and out come the pruners. Sometimes we just can't help ourselves. When it comes to pruning shrubs it's important to understand when to prune so that we can still enjoy their floral display.
Shrubs are easy to classify into three different bloom times: spring, summer, and fall blooming and depending on when they bloom tells us when we need to prune them. In our lilac example, which is a spring blooming shrub and that means that they start developing flower buds the previous year which means they bloom on old wood. If you prune them in the fall or during the winter, you are removing the flowers buds. Make sure to wait until they are done blooming in the spring then you can bring out the pruning equipment.
Lilacs, forsythia, viburnums all do well if you remove 1/3 of the oldest growth. Prune out the oldest stems down to just above ground line if possible. Do this every year for three years and you'll have a shrub that will also bloom better and be in better shape. You can then let it be for a few years. The other benefit to pruning like this is that it can reduce the overall size of the shrub which is great for when they are overgrown or just a bit too big for their current location.
Sometimes I think it's easy to forget about shrub pruning and then we realize that they have become more like shrub monsters then shrubs. Even when I look back at pictures of the house that I live in from 2012 to pictures from last summer I can't believe how big they've gotten and it's a reminder that I need to do some shrub pruning myself. And sometimes over grown shrubs can call for drastic measures. I've taken extremely overgrown viburnums down to the ground because it was nearly impossible to get in and remove 1/3 of the oldest growth or sometimes you need them shortened faster than over a three year period. If you have to it's okay to do that sometimes, but before you think about it make sure you know if the shrub can tolerate that kind of massive cutting and removal.
If you want to pull out the pruning equipment right now, it's a great time to prune summer blooming shrubs which bloom on wood produced during the current growing season. A great example would be spirea – these summer blooming shrubs can sometimes get a bit overgrown but due to the nature of growth, the 1/3 oldest stem removal plan doesn't work. Instead you take the entire shrub down to 4-6 inches above the ground and let it grow back from there. Fall blooming shrubs should be pruned in early summer. When pruning shrubs make sure to have a plan and know what's the best way to prune before you make that first cut.