Answer: If they are very crowded together they may be shading themselves at the base, this can cause a bare-legged effect. (They should be roughly four to six feet apart so sun can reach all parts of each plant.) Otherwise it may be that they have never been pruned at all.
The best time to prune these is in late winter to very early spring before they start to grow again for the year. Cut them off short (literally near the ground) to stimulate branching at the base. Or, remove about a third of the stems (take out the oldest, thickest ones) by cutting them at the base. This will stimulate new growth from the roots and from lower on the plant. Every few years you can prune again if desired. This seems drastic but they regrow quickly.
If you want to increase the density of your hedge you can actually take your trimmings and poke them firmly down into the soil, just place them upright with the cut end downward and far enough in so they are self supporting. Many of these will root and grow.
Watering -- when needed to supplement rain -- is the most important thing you can do to help these shrubs become established. As far as soil moisture, they will tolerate average soil moisture levels but are native to damp to wet areas and so prefer it damp. You can check by digging into the soil with your finger. If it is still wet or moist you do not need to water yet, when it begins to dry you can go ahead and water. When you water, water slowly and deeply so it soaks in to encourage deep rooting. After watering, wait a few hours and then dig down and see how far the water went, sometimes this can be surprising.
Keeping them mulched with several inches of organic mulch will help keep the soil more evenly moist and also help feed the soil slowly over time as it breaks down. This is probably the second most important thing after correct watering.
As far as fertilizing, the best way to know what to use is to run some basic soil tests and then fertilize as indicated by the results. Your county extension can help you with the tests and interpreting the results.
Otherwise, an annual top dressing with a good quality compost and/or an application of granular or slow release 10-10-10 fertilizer per the label instructions is fine. Overfertilizing is no better than underfertilizing and can in fact stimulate overly weak, lank, leggy growth.
Keep in mind it takes several years for newly planted shrubs to become established and begin to grow their best. The larger/more mature the plants at planting time the longer the recovery period as well. I think in several years your plants will naturally be big and thick just as you envisioned.
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