Question
First of all I should tell you that I live in the Portland Oregon region. I planted a hedge about three years ago of Giant greens. I planted them about 6' a part. I bought them online and they showed up bare rooted and were no bigger than about 7". The first year they just survived and got established and not much growth. The second year they got to be about 3'. This year they took off and they are about 10' tall, some are shorter. They are not super dense but they are providing a bit of a screen. A couple have two main competing main tops to them. I understand they reach like 50' so that would be too tall since we have power lines in the way. So I will have to prune them. Not sure what time of year to do that. They are so pretty and I am scared about how to prune them. Since some are taller than others do I take them all down to a common height and how much is too much to take off these fairly new plants? Since they are already pyramidal right now meaning the bottoms are much wider than the tops do I really need to prune the sides now? If so how much do I take off? What problems should I watch out for after pruning? They have such a beautiful form and the new growth gets this kind of golden color to it which is so nice, I am wondering if I prune them are they going to fill in and look nice or look like butchered trees? Does anyone have any pictures of a Giant Green hedge?

Answer
This is a long answer so I will refer you to to web link that describes pruning of evergreens in detail. Generally the pruning is just a light hair cut rather than cutting limbs back. The aim is trying to keep the same shape but keeping the size smaller. The top can be trimmed to reduce the height. I would wait to do any pruning 'till late winter or early spring just before new growth begins. Yes, you can prune both tops and sides. How far back depends on what you want to achieve by pruning. I would suggest that you not shear them by giving them an overall close "haircut", but rather use the "thinning" method of pruning which will result in a better appearing, healthier and more resistant to storm damage specimen. If you remove about 20% of he branches each year by thinning you can maintain an orderly rate of growth resulting in a densely branched sturdy shrub.

http://extension.unh.edu/Pubs/HGPubs/PrunEverg.pdf