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A straight overhead canopy of shade is not the preferred situation for Leyland Cypress and Thuja Green Giant trees. The absolute worse situation is an overhead pine or evergreen canopy, because the trees under an evergreen canopy even in winter don't get sunlight. If that is your application, don't plant there! Sometimes it cannot be avoided. For example, in Long Island and in the Hamptons area, some neighborhood have rules about what can be cut and are very strict about cutting native trees to replace with anything not native to the area. Even so, I am not recommending planting Leyland Cypress or Thuja Green Giant trees under a canopy. I have planted these varieties under a canopy that was very high and still it effects the trees.
Southern locations do not fare as well with decidious tree shade as northern locations! If the trees bring the shade are deciduous tress, at least after those canopy trees shed their leaves, the Thuja Green Giant trees will get full sun all winter. This effect is seen from PA and northward than in southern states, because they shed about five weeks earlier in fall than southern trees and don't green up untill four or five weeks later than southern deciduous trees. On Long Island, Thuja Green Giant or Leyland Cypress trees planted under a partial canopy of scrub oaks may get full sun for seven months of winter before leaves re-appear, whereas southern states with a similar situation may only provide five months of full sun. Shade on one side of your row is normally fine, because your Leyland Cypress trees will still get the powerful straight overhead sunlight. Thuja Green Giant or Leyland Cypress trees can grow three feet per year if fertilized properly, so remember if there are slower growing trees shading on one side, The Thuja Green Giants or Leyland Cypress will out-grow the shading trees in most cases and will have more sun after that time.
The main symptom of too much shade on Leyland Cypress and Thuja Green Gants is thinning. At first planting if field-grown Ball and Burlap trees, they will start out thick. Over the years, shady applications will take their toll and they will thin out somewhat. The second sympom is slow growth rate. As I stated above, thinning is not caused by shade to one side, lack of straight overhead sunlight is the problem.
Providing light by pruning the offending shade trees is a great idea! If you cut limbs off an evergreen tree that is shading your Leyland Thuja Green Giant screen, those limbs will not grow back so it should be a one-time effort. If your arborists trim deciduous trees like Oaks, Maple trees, it will likely need trimmed again at a later date. If the limbs on deciduous trees are cut all the way back to the trunk, on the side that shades your Leyland Cypress trees you may solve it completely. It is always best to do the tree surgery overhead before you plant the privacy screen row below. During limbing the arborist can let the limbs to fall if it is done before installing the Thuja Green Giant row, but if you wait till after planting, the arborist will likely have to rope the limbs down one by one to prevent damage on your privacy screen below.
A good substite variety is Nellie Stevens Hollies for shady applications. Also remember, if you are planting in an established wooded area, the trees nearby not only rob sunlight but will have established root systems that will compete with your new plants for moisture.
