Cedars are coniferous, and as with most conifers, new growth does not form on old wood (or old branches). It is therefore essential to trim to the right length to preserve a uniform hedge without gaps. The best results are achieved when the new growth (the lighter green part of the hedge) is cut back by one-third to one-half its length. To avoid losing control of your hedges, it is best to trim them (or have them trimmed) every year. If you do need to do an extreme pruning on a cedar hedge, be prepared to wait 3 to 4 years for the gaps to be camouflaged by new foliage. Your patience will be rewarded!
Although we generally think of a high wall when we imagine the perfect hedge, the healthiest cedar hedges are those that are trimmed to a slightly conical, rather than square-cut shape. In order to maintain their health throughout, the entire surface of the hedge must have sufficient sunlight. Trimming your hedge slightly wider at the base will give that section of the hedge the access to light that it would not otherwise have. It is also recommended to trim the tip to a point rather than horizontally. This avoids damage from heavy snowfalls.
Applying a compost to your shrubs in the springtime will give them the minerals they need to grow. But be careful not to bury the collar under the compost.
It is also possible to use a granular fertilizer to promote growth. In this case, the first number of the formula should be the highest, for example 20-10-12. Hedges should be fertilized before mid-July…any later and you may find brown edges on your hedge the following spring. If you are fertilizing between the end of July and August, look for a fertilizer where the last number is the highest.
Source: http://www.canalvie.com/maison/cour-et-jardin/tailler-haie-de-cedres-1.1420764,
Christian Brunet, technicien horticole