It's better to wait until the new growth is well developed early in the season. The shoots from previous years are pale green, those from this season will be a lush green, and the older ones are practically dark and sometimes brownish or yellow.
This is a normal cycle that all cedar hedges go through. Here's how it works: towards the end of summer or early fall, cedar hedges and most conifers shed their older foliage to make way for new growth from the new foliage. Rest assured that a cedar hedge shedding its old inner leaves during the season will look rejuvenated in the next spring.
The time of year to trim your cedar hedge is as soon as the good weather arrives and after the leaf and flower buds start to bloom. At this time of the season, the cedar hedge has almost finished its growth for the summer, but the new shoots will still be greener and lighter than the previous year.
And it's important because ideally, you would only trim within the growth of the year. You can also trim later in the season, until the beginning of September, but at this stage, it's not as easy to distinguish between the new growth and the growth of last season's deciduous trees, as it will have lost its "spring green" color.
During cedar hedge trimming, your goal will be to remove up to two-thirds of the length of the shoots from the current year. You can use manual or motorized hedge trimmers to do the job. Young hedges will need less pruning, you want them to grow taller, after all, but once the desired height is reached, you'll need to trim them every year.
The way to preserve your cedar hedge for as long as possible is that you'll need to leave the bottom a bit thicker and fuller. This way, the leaves at the bottom won't be shaded by the top branches and will receive the necessary sunlight and therefore more light rather than browning over time and losing their foliage. Additionally, the top should be rounded or pyramidal so that it sheds snow and ice more easily, if you use hedge and shrub protection, it won't pierce through during harsh winter days.
