We planted these 43 ten' Leyland Cypress for a privacy screen around the property border on 3 sides of a home in Garner, NC Oct 2009 and spaced them based on "rule of 4". David Watterson has found that Leyland Cypress or Thuja Green Giant for privacy screen can be spaced so the height is 4 times the distance between the trunks. Example is 4' spacing planted on center, between trunks is ok if the trees will be topped at 16'. To do that, let them grow to about 18', climb up and snip off the central leader at 16' height. Same is true for 5' spacing, and 20' height. Many times your privacy screen doesn't need to be 40' tall, and that height can be a disadvantage. If your trees need to be sprayed for bagworms, for example, and you have le them grow to 40', you will probably need a professional to reach those heights. If you have topped your row of trees at 16 or 20', you can buy a sprayer from Lowe’s or Home Depot and use liquid EIGHT and handle it at low cost. If you are to allow your Leyland Cypress or Thuja Green Giant row to grow to full height, they should be planted 8 to 10' apart. They other approach is the zig zag pattern, where you plant two parallel rows, 8' apart with trees 8' on center with trees staggered along each row so there appears to be a tree every 4 '. You can see from the pics the closure will be quick with this size tree planted spaced as we did.