Canadian Hemlock are the best choice if in zone 6a or lower for a privacy screen in shady locations. Belonging to the Tsuga genus, Canadian hemlock is also known as Tsuga canadensis. They grow less than two feet per year, to be big evergreen trees, and can be pruned smaller and denser. They do not grow as fast in shade. More on best hardiness zones, soil types, pruning, watering, growth rates, spacing, and one pest which is trouble.
Canadian Hemlock grow best in zones 3 to 7, according to all web sites I have found. By my own observations, they actually do best in zones 3 through 6. In North Carolina, for example I only see them growing naturally in the mountains, where it is zone 6a. My advice if you are in zone 6b or higher and have a shady location to screen, consider Nellie Stevens Hollies instead.
These trees do not like dry or compacted soils at all. They need acidic soils that drains well. If your screen location is elevated and dry, you should add a ring of mulch over your tree's root zone and consider installing a drip irrigation system to keep your tree looking its best. Don't bank the mulch or dirt after planting up against the trunk itself, however. Planting Canadian hemlock in sandy, clay and chalky soil with a ph of between 5.0 and 6.0 is ideal for as it does best in moderately acidic soil - weakly acidic soil. Canadian Hemlocks must have a soil rich in organic matter--like compost or peat moss--that is able to hold moisture. If you plant in clay type soils, add organic matter and the plant will do well. For a thriving plant, condition sandy soils with organic matter and watered regularly. Hemlocks prefer a soil that is acid so make sure and perform soil samples before adding lime near the plants!
Twenty five foot spacing would allow you to let them grow with no pruning or topping. Ten or Fourteen foot spacing is good for Canadian Hemlock privacy screens, if you plan to prune them and also top them before they reach 4 times whatever distance you spaced them. The zig zag pattern allows you to plant that far apart but get closure in ½ the time as a single row. For the zig zag pattern, plant two parallel rows , space trees 14' apart on center along each row and offset so there appears to be a tree every 7 feet. Once each tree reaches seven foot wide, you will have the beginning of your visual screen.
To manage height, complete your pruning by April before the new growth starts. This way the new growth will fill in any spaces you open up during pruning, and the new growth that develops will keep the plant soft-looking, even though you have cut the plant like a wall. Watering Their root system should not remain wet, but moist with frequent watering. All tree roots need to aspirate or breathe, so water every other day at most.
Pests - Hemlock Wooly Adelgid.
Canadian hemlock isn't often bothered by pests or diseases, there is one pest that does bother them, but is easily handled. The hemlock wooly adelgid hides inside a woolly sac and is a small, aphid-like insect. Regular inspections of your hemlock will help prevent serious damage from this pest, provided you check for them at least once a year. The insect looks like a small pieces of cotton, and develops on the underside of the needles. October is the best time to treat these pests, using either insecticidal soap or horticultural oil. You can use a product called "Bayer Advanced 12 Month Tree and Shrub Protect and Feed II Granules ". When this product is applied to the base of the tree yearly will keep the tree insect free. It has systemic action; just pour on the ground below the tree and it will be absorbed by the root system and distributed to the top of the tree. This product is available everywhere. The active ingredient is Imidacloprid. Always follow label instructions, The label may say "not for sale to NY".