I have just plantd 6 leylandii at approx 3 ft apart. They were 6ft tall when I bought them last month. I want to grow them to about 15 ft and want them to merge together with a nice thick healthy growth. Can you suggest when I should trim, and do I need to control the height growth at the same time to allow the width to establish?
The advice is to leave hessian in place when planting (though I personally never do unless the roots have already penetrated the hessian). You now need to go out and reduce the height of the plants by half - I know it sounds like a lot, but it is necessary to encourage shoots from the base to make a thick, dense hedge down to the ground. Do not prune again this year, but next year you will need to clip or trim over fairly lightly at least 4 times, including the tops of the plants. When clipping over, bear in mind the base of the plants should be left a little wider than the tops. Keep doing this until the hedge has reached the height you desire, then after that, its a case of pruning back 3-4 times a year to maintain the shape and keep the height down.
Given the weather we've been having,the ground is currently very dry - keep your plants well watered for the rest of the spring and summer, right up to November. You may need to take care of watering during next spring and summer too if it's dry and warm - after that, they should cope without being regularly watered.