By Breuddwyd
United Kingdom 
I'm planting leylandii as a privacy screen and wind barrier in front of an old hawthorn hedge in a neighbour's field which previously had grown untended to 20+ foot high and had thinned to nothing but trunks lower down till 4-foot high (now cut down to 7 foot but still very bare).
The leylandii are 2-2.5 foot tall, came in 9cm pots. I've bought sufficient to plant them about a foot apart. I'm going to plant them with compost and long lasting fertiliser granules. I want to plant them as close as is possible to the hawthorn hedge so as not to lose space in my garden. I've dug around 1-1.5feet from the hedge and explored about 9inches deep and the only roots I found belonged to nettles which I've pulled all out.
Will my new plants be able to establish that close to the Hawthorn? Sunlight isn't the issue as my side of the hedge is in full sun all day. Soil seems good until about a foot down where it gets very gritty.
17 May, 2018
Answers
I have to admit that I'm far from being a fan of Leylandii for hedging or screening but now that you have bought them there is little alternative. My main concern is that they are at least 2 feet plus tall but only in 9 cm pots. If this is the case you might find that the roots are just going round and round in the pots. You may have to tickle the roots out otherwise they will not get established. As far as planting them, I would look to loosen the ground at least 1 foot deep and plant them 18 inches to 2 foot apart. You will need to keep on the case as far as watering during the summer months. You can also thicken the hedge by planting in a zig-zag pattern.
18 May, 2018
A better solution would be the layering of the hawthorn hedge but I doubt that there are many left with that skill now.
18 May, 2018
you know that they grow enormous ie 20 foot across don't you?
17 May, 2018