Height determines spacing. The first question is how tall do you need them to grow? If a 14' row of Thuja Green Giant will provide the privacy screen you need, you should be sure and "TOP" them at that height. To do that, you must let them grow a foot or so taller than the desired height, then just clip off the central leader, or main trunk. For evergreens, they will be finished growing tall, and spend their energy thickening out. Additional height above what is needed is a disadvantage. One reason is that during stress times, like a drought summer, or winter, the tree has to "decide" whether to send the moisture it does have to the upper limbs and truck or the lower limbs, it will always send the moisture to the top growth areas and starve out the lower limbs. Many people say their row of Thuja Green Giant was doing fine, then "all of a sudden" this year they started showing brown on the lower needles. This is because the height reached the point relative to spacing that results in stress.
RULE of fours. You should space the trees so the target height needed is no more than 4 times the distance between the trunks. One example is you need a 20 foot tall row to block your neighbor's house or windows, you could space as close as 5 feet apart on center. That is provided you will follow through, and when they reach 21 or 22' tall, top them back to 20' height. That means each tree will get the moisture from a 5' diameter ground area without competition from the tree beside it. It also means there will be enough room for a strong 5' diameter root system to secure a 20' tall tree against the wind.
Thuja Green Giant are shaped differently than Leyland Cypress, the top half of a Thuja is thinner than a Leyland. This would be even more reason to plant the zig zag patern so you have some overlap in the visual screen.
ZIG ZAG pattern. This is a super solution if you can surrender some "width" of your property for the privacy screen. Let's take an example; someone needs a 30' tall privacy screen. If they use the rule of 4's, and plant a single file row, they should space at 8' on center, 4 times 8' spacing = 32' max target heigth. If they are choosing to start with ten' Thuja Green Giant or Leyland Cypress, they will be 4' wide at planting time. That will leave them with 4 foot gap in between each tree and waiting a long time to close together. If you plant two rows, with trees planted 8 foot on center, but staggered there will appear to be a tree every 4 feet you will have closure much quicker, yet still have the trees spaced for strength and low stress. In this case the closest row will be 4 feet from the property line, and the next row will be 8 feet from the first. If space is tight, you could make the second row 6 feet back from the first row. One note is that the appearance of a tree every 4' is only when you are exactly perpendicular to the row. Also remember a ten foot Thuja Green Giant or Leyland Cypress may be 4 foot wide at it's widest point, but also they get skinnier as they get tall. In this situation, you still need the 8' spacing based on the rule of fours, and the zig zag pattern will get you closure much sooner than a straight line row. If they decided on 12 foot trees, they will be 5 feet wide at the widest point closure would come much quicker. Use and distribution of this article is subject to our Publisher Guidelines whereby the original author's information and copyright must be included.
David Watterson - tree nursery
About the author: Watterson tree farm is a family business, David, his wife and sons have developed proven techniques for growing trees. See our website for information about: proper spacing approach, fall planting considerations, tree staking, fertilizer selection based on season, also how to plant on a slope(hills), and the effects of shade on evergreen trees. http://wattersontreefarm.tripod.com