I would like the bushes in our front yard to grow leaves on all sides. To do this, I think they need to be trimmed to let light in.
What is the best time of year to trim them? Should we cut them back to the main stalk or trim selectively?
Front view of bushes after some selective trimming of branches:
Back view of bushes where leaves do not grow:
I think your shrubs are Osmanthus, possibly Osmanthus burkwoodii, which has fragrant, white tubular flowers in spring, and a rounded growth habit. Pruning is carried out immediately after flowering so as not to remove growths which would have flowered next spring if its done later - like now. Regardless of that, you have a choice to make - you cannot persuade the backs of these shrubs to grow as well as the front, even if you do cut them down. Over time, they will grow in the same way as they have now, because there is a structure not far away behind them (garage, maybe?) which means that side of the plants is too shaded as they mature.
It's possible they were planted to provide a screen to hide the structure behind to an extent, but whether you wish to keep them, or take them out and replant with hedging plants you can regularly clip over front and back, or lower growing plants further away from the structure to avoid this problem, is something only you can decide.
This looks to me as some kind of buxus commonly used in yards.
Once fair size is established it can be trimmed* after the year stems have grown, which is about end spring - beginning of June. Just cut the stems that grown on current year: you should be able to see the previous year's cut just edging out once done.
*If you have patience, do it manually instead of using machinery, because trimmers arm a lot of leaves that stay attached and rot on the stems.
