Answer: The Canadian Hemlock (Tsuga) makes one of the most beautiful hedges. You've made a great choice!
The best time to shear any hedge is when it's making its fastest growth. This should be done a minimum of twice a year. In order to have a tall, tight hedge, you should first develop a small, tight hedge and let it grow larger gradually. It's tough to tighten up a large, loose-growing hedge. One of the best guidelines is to just trim it when it looks like it needs it! It will be thicker and better looking if you cut frequently, removing only small amounts at a time. It will be healthier that way too.
Be sure to shear the sides as well as the top. Always shear a hedge so the top is a bit narrower than the bottom. It can be so slight that it's hardly noticable, but do do that! This way the sun gets to the bottom and sides as well as the top.
Get started as soon as possible so your hedge doesn't get out of control.
Q&A Library Searching Tips