Canadian Hemlock can be a great choice for a privacy screen in shady locations. Belonging to the Tsuga genus, Canadian hemlock has a botanical name of Tsuga canadensis. They grow slowly, about one foot per year, to be large evergreen tree with flat needles, naturally airy shape, but can be pruned smaller and denser. They will grow more slowly in shade. Let's discuss the best hardiness zones, soil types, hardiness zones, growth rates, spacing, pruning, watering, and even one pest which is easily handled.
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. Sandy soils, like those on Cape Cod, should also be conditioned with organic matter and watered regularly to have a nice plant. 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. Plant two parallel rows with the zig zag pattern, space them fourteen feet apart on center along each row but staggered so there appears to be a tree every seven 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 stay moist, but not wet, with frequent watering. All tree roots need to aspirate or breathe, so water every other day, not every day.
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 is a small, aphid-like insect that hides inside a woolly sac. 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 at the base of the tree and it will move up through the root system to the top of the tree without spraying. This product is available everywhere. The active ingredient is Imidacloprid. The label may say "not for sale to NY", always follow label instructions.
David Watterson and Family have been growing trees in NC since 1995 and now grow trees in VA and have an affiliate nursery in GA. We specialize in privacy screen trees, Leyland Cypress and Thuja Green Giant are favorites, also have Cryptomeria, Maples, River Birch and many others. We are wholesale and have a $2,500 minimum order. Contact us for trees from 7' to 25'. http://www.leylandcypress.org