I have a thuja occidentalis hedge that is 8 feet tall . It has been planted around 20 years ago ,pretty closed together ,1 feet apart. The hedge is 4-5 feet wide. The previous owner has always pruned it pretty square so the top is flat and during winter the snow accumulates there . Is it possible to prune it so that is will grow taller. In my dreams I would wish it would grow to 16 feet but maybe that's impossible once it has been 8 feet for years. Is there a way to prune it so it will grow tall? Should I just not prune the top anymore ? or prune a little and eventually it will grow more tall year by year? or should I try to pick a leader branch on each plant and prune it in a special way? Any advice would be welcome.
The taller you allow the hedge to grow, the more chance you have of the snow breaking parts down as the weight of the snow can become very heavy indeed.
If you want the hedge to grow taller, all you need do is keep pruning the sides of the hedge but, leave to top (FLAT) area to grow onto the hight you require,
Make sure as the hedge grows taller, you have the bottom of said hedge grow wider as taller tops require width at the foot of hedge for better support or the top heavy hedge will come crashing down with either wind or snowfall laying on it.
If you find after several years the top is thin and week looking, you will need to get a ladder out and hand prune some tips from the top growth to encourage side shoots to form.
I think your dream of a 16 feet hedge is wonderful but don't know IF you have neighbours to consider, this is a very tall screen to look at IF it's not your own making, there are other things to consider too, like you may have to insert stakes and wires to run the length of this hedge as it grows taller to offer stability due to the extra hight, once a tall hedge starts to sway in windy weather, believe me it can become dangerous.
The stakes should not be seen and the wires are hidden and taut the length hidden within the hedge . make sure you feed the hedge as you allow it to grow upwards to help strengthen the plants as they grow heavier and taller.
Thuja sometimes will not sprout where it has been pruned back so much. Does yours have new growth on top? If so, you can let that grow. Some guidance so the most vertical, best attached parts become the woody structure will be very important. You won't want a branch that is starting off sideways to become the new leader. That sideways attachment is weak. It is OK to let that fill out and add to the density.
Diana is correct re cutting into the wood as you prune, some types of these tree's / shrubs can take any cutting into Brown wood, that is as you prune, you can only cut off greenery,and not expose brown branches as the brown part cant regrow to form fresh new greenery like other shrubs / trees so you would spoil the shape and leave empty holes in the hedge if cutting into the brown areas, when these plants grow very tall, sometimes the snow pulls out large areas of branches and you see gaping big holes in the hedge, you would need to tie these loose branches back into place and to prevent this, you need to offer some wire supports to help prevent this problem as mentioned in my last reply.
If you ever require help from someone when this tall hedge needs pruning, you need to REMIND them NOT to cut into the brown wood. I have a friend who's next door neighbour asked her IF she would object if they pruned off some side branches on their side of the hedge as they were causing shadow's onto their lawn, my friend agreed to some careful pruning and when she returned from work, the guys who were employed to do this pruning actually used a saw and cut right back to the trunk, what a mess, neighbours side showed all brown and my friends side was thinned out, could see birds nests laying all over the place and believe me, after a couple of years, there was no sign of new shoots regrowing on the neighbours side, they eventually asked my friend IF they could remove the hedge and out of disgust she refused and said they would have to live with the disaster they caused as she herself was living with a thin, half dead looking hedge, it was beautiful before all this and she used to sit at her study watching the birds go in and out, so be aware, you need special care with this large hedge.