The main reason that we need to trim and prune is to keep our hedges and shrubs happy 
and healthy. Spring pruning is required to remove any winter damage or dead limbs, and it is also important to remove any diseased sections. This stops the spread of disease and allows more light and air to get through to the healthy limbs of shrubs and hedges. This job is best done by getting right into the shrubs, having a look and then taking out the limbs that are affected or dead by using your hand held pruners, or secateurs.
When to Prune Shrubs and Trim Hedges
There are a few rules of thumb to help you decide the best time of year for trimming and pruning and it comes down to when the shrub and hedge flowers. If the shrub is spring flowering then trim after the flowering is complete in the early summer. This would be the case with Rhododendrons, Azaleas and Himalayan Sweetbox.
For all non-flowering shrubs and hedges such as boxwood and cedar it is best to wait until the buds have fully developed with leaves. That said, it is still fine to do minor trimming and pruning all summer long to keep your shrubs and hedges looking good. Choose overcast days for this type of work to avoid sun scorching the leaves.
How to Trim a Hedge
Let’s start with informal hedge pruning. As I mentioned, it is important to cut out the dead branches right at the base of growth and although the plants may be flowering plants, informal hedges are allowed to grow naturally so that their shape isn’t spoiled, however that doesn’t mean that they never get pruned. If neglected they could soon grow too tall or spread out of their allotted space. To keep informal shrubs and hedges in good shape, it is a good rule to remove old stems with secateurs or to cut larger branches using loppers to keep the plant within bounds at least once per year.
Formal hedges are quite common in Vancouver, and are usually grown using types of conifers such as yew or cedar, or broad-leafed evergreens such as English laurel, holly or boxwood. Conifer hedges, which include cedar and yew hedges, perform best when they are trimmed in both June and September. You never want to prune back into the old wood since the conifer will not grow back from there again, the exception being Taxus, also known as yew. Yews can be cut into old wood and they will rejuvenate from there by pushing out new branches and leaves.
When trimming, it is ideal to make the hedge a little wider at the bottom to allow all areas of the hedge to receive sunlight. All too often you see yews or cedar hedges that are dead or brown at the bottom. A lack of sun is the most common reason, as the bottom is in the shade of the top of the hedge.
In the broad-leaf evergreen category, English laurel and holly also benefit from both a late spring and early fall trimming. Laurels are tough and you could cut them to the ground and they will still come back. Holly is also a fast grower and you can prune it quite hard.
For boxwoods the best time for Vancouver gardens to trim for shape is spring just around the end of May to the first week of June and then again in the fall. It is really important with Boxwoods to not cut them in hot sunlight as they have a tendency to get scorched which looks terrible as the hedge takes on a brown crispy-leaf look.
When trimming, always leave some foliage so the shrub has enough leaves to live. On most plants (yew, privet and rhododendron are exceptions), if you cut back into bare wood, new leaves won’t sprout.