Trimming leyland cypress
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Yes, you may trim leyland cypress. Trimming will not kill the trees. However, leyland cypress will not fill in with new branches and bushiness after trimming. If the trees are 20 ft tall and too close together, you may actually consider taking out every other tree. That way the trees can grow to their full potential.
I suggest having a professinal trim your trees to open up the tree canopy. Pruning correctly can maintain the shape of the tree. Incorrect pruning wounds can be avenues for disease causing microorganisms.