I moved house and the garden has a row of cedars that are about 6-7ft tall and have been there for approx 10 years (looking at old satellite photos). The are planted with gaps between the trees that I would like to fill. The gap varies across the row, but many of them I can easily step through to the other side. Can I plant new cedars in the gap to create more of a hedge or will it kill the established ones? Or is there an alternative that i can plant that will add height? I would like privacy without having to fence.