Hello Lois,

Thank you so much for reaching out to us at the UNH Extension Yard & Garden Infoline.

The following research has been conducted by Master Gardener volunteer, Debi Green:  My name is Debi Green and I will be happy to help you find the perfect privacy hedge for your property!

Thank you also for your patience in receiving an answer to your question as there was a break for the holidays.

Although Leland Cypress is a great choice for a privacy screen, it does have some problems and it can be damaged by New England weather. A couple of its susceptibilities are root rot and stem cankers, both of which are untreatable. They are only hardy to zone 6, which means that they are only marginable hardy here. Extreme weather would be risky for them. See the link below for further information. I've included an excerpt for faster reference.

https://marylandgrows.umd.edu/2018/04/11/why-is-leyland-cypress-turning-brown-winter-took-its-toll/

"Leyland cypress (x Cupressocyparis leylandii) has a lot going for it. It’s fast-growing with evergreen, feathery foliage and a pleasing, slender profile. It makes an excellent specimen tree or screening plant. A cross between two Pacific coast species, the Leyland cypress thrives best in moist, cool climates with moderate temperatures. These trees are hardy to zone 6, however, they do not tolerate sudden temperature fluctuations. Since Leyland’s are often used for screening and windbreaks they are frequently exposed to temperature extremes and windy conditions that lead to drying out and cold damage. One of the weaknesses of Leyland’s is their shallow root systems which makes them susceptible to stress through desiccation. Another weakness is the dieback and death of the water-conducting tissue and cambium layer just under the bark during extreme winter temperature fluctuations."

That said, if you have your heart set on them there are things you can do to give them the best chance possible.

Protecting and Choosing the Best Plants for Winter Tolerance
https://Extension.unh.edu/BLOG/2019/09/how-do-I-protect-my-trees-shrubs-winter-damage

"Even when you do everything you can to protect plants, winter damage is still a possibility. Some plants are simply better adapted to survive than others. Many winter injury issues can be solved by choosing appropriate plants, and hardiness is the first thing to consider. Trees and shrubs should be hardy enough to survive in the zone where they are planted without too much extra care. In most of New Hampshire, this means selecting plants which are hardy in Zones 3-5."

I am also including an article that discusses the updated 2023 USDA Hardiness Zones map: https://extension.unh.edu/blog/2023/12/new-2023-usda-hardiness-zones-explained

With your county being Merrimack, according to the 2023 map, the following hardiness zones are included in your county: Zones 5a (-20°F to -15°F), 5b (-15°F to -10°F) and 6a (-10°F to -5°F). 

Here are a few alternative trees you may wish to consider:

1) Eastern Red Cedar (Juniperus virginiana). It will grow 30 to 40 ft. tall - in sun, part shade, or shade. Many perks include: It is native to our area, deer resistant, drought, salt, and pollution tolerant, bears a favorite food for wildlife and it is a fast-growing, tough tree and will grow just about anywhere. Below are some links for more information about them:

https://extension.unh.etdu/blog/2020/05/which-evergreen-shrubs-privacy-are-deer-resistant

https://extension.unh.edu/blog/2019/09/urban-community-trees-eastern-red-cedar

2) Another option is Green Giant Arborvitae - Eastern White Cedar (Thuja occidentalis) or Western Red Cedar (Thuja plicata). Eastern White Cedar will grow between 30- 40 ft tall and Western Red Cedar will grow 50-70 ft tall. Eastern White is loved by deer though, so you may have to take some precautions there. The Western Red Cedar is somewhat deer resistant. They can handle light afternoon shade.

https://plants.ces.ncsu.edu/plants/thuja-occidentalis/

https://plants.ces.ncsu.edu/plants/thuja-plicata/

3) Chinese juniper (Juniperus chinesis) is another needled evergreen and is similar to Eastern red cedar in many regards. It also has scale-like foliage on mature branches and is highly tolerant of deer, drought and dry soil and part shade. While plants can grow into large trees, a great number of smaller shrubby varieties can be found at garden centers. Chinese juniper is also susceptible to cedar apple rust and should not be grown in the vicinity of apples.

4) Inkberry (Ilex glabra) is a native evergreen holly species that is popular as a screen in garden settings because it is adaptable to both well-drained and wet soils. It has small, glossy, dark green leaves that are spineless, and produces small black fruit that are enjoyed by various songbird species. Inkberry is easy to grow in most landscapes, provided there is full sun or part shade. It will be at its best when planted in full sun in consistently moist, acidic soil. Inkberry is likely the best native shrub to grow as an informal hedge. Plant height varies considerably, depending on variety, so make sure to choose a form that will suit your landscape needs.

https://extension.unh.edu/blog/2020/05/which-evergreen-shrubs-privacy-are-deer-resistant

I hope this has been helpful and if you have any further questions please don't hesitate to contact us again. Happy New Year!