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You can’t actually stop leylandii growing, just control where it grows. To control the height to which a leylandii (Leyland cypress) tree (or any other conifer) grows, you need to prune it carefully. Leylandii are very suitable for use as hedges as they can handle being pruned. Regular pruning once or twice a year is much better for the plant than one big pruning session when it really gets too tall or wide.
Leylandii height and width
It helps if you have an idea of how you want the finished size and shape of your leylandii plant to be. Do you want it to be a long, low hedge around a flower bed, a tall privacy barrier between you and your neighbour or perhaps a very tall statement tree?
As soon as you plant your leylandii tree, you can start trimming any branches that don’t conform to your desired shape. This encourages buds and shoots to grow within the rest of the shape and to create a dense plant.
You can learn more about how tall leylandii actually grow here, hint – some varieties grow faster and slower
When and how to prune leylandii?
Remember that leylandii can grow up to 90cm annually, especially in the first year. Don’t instantly prune out every stem that’s in the wrong place; perform pruning two or three times a year while the tree is getting established. After that, trimming once a year is all that’s needed.
Leylandii grow only from new green growth so you need to preserve the buds when you trim the plant. Basically don’t prune it too hard. Prune the trees when they’re in their dormant state so their sap doesn’t bleed out.
Cut a branch/stem 1cm above the new buds. New growth will come from the buds and create the season’s foliage. You’re not stopping the plant from growing in this way. But you are controlling its size and shape and direction of growth.
To stop the leylandii from growing any taller, you need to wait until it’s within 15cm of the desired height. Trim the branches at the top of the hedge at this point. This allows the rest of the long branches to thicken out and form the top of the hedge.
Prune early in the season. This allows the plant to recover from any shock it experiences. This is especially important if you’ve left your pruning for several years and plan to cut off a lot of the tree at once.
For established leylandii hedges I recommend trimming them with a hedge trimmer a couple of times a year to remove the new growth and maintaining a neat shape. Remember not to prune into brown bark as you will have brown patches from thereon.