Most things you use for hedges will have to be trimmed to keep them to those dimensions. There really aren't that many evergreen species that survive in Zone 4 and take well to pruning. And salt tolerance adds a whole other level of complexity...I think you are too far north for my go-to broadleaf evergreen, holly.

What about Canada Yew (Taxus canadensis) Rhododendron maximum, Mountain Laurel (Kalmia latifolia)?

They all might be hard to grow from seed.

Thanks, edlincoln. The yew looks good, but I can't find any seed. Because the hedge is over 700 foot long (expensive), because I don't really want it to look like a lot of clones, and because I would like to preserve genetic diversity, I am looking to grow it from seed... Right now, it's looking a lot like a hedgerow of Rosa rugosa 'Alba' in front of a row of Thuja occidentalis 'American Arborvitae' grown from seed--except where the Thuja would block the view (for the worse). I might mix in some exotics like Korean Fir (Abies koreana), Picea engelmannii, Pinus cembra and Pinus sibirica, and other such narrower evergreens. Just wish they were native.

Do all the plants need to be salt tolerant or only some? Does the tolerance include foliage and airborne spray, or roots and salty runoff, or both? Plants don’t make much of a difference to sound levels; only something solid like a wall or solid fence can do that. Plants can provide visual screening, however, and if planted on a berm, the berm can provide some sound blocking or reflection.

Most of what grows that is native in zone 4 isn’t evergreen except for conifers, and not many of them are salt tolerant IME. You could also try growing from cuttings if other folks you know would let you, although it wouldn’t have genetic diversity unless you took cuttings from different shrubs and trees within species.

Just so you know, Rosa rugosa isn’t native or evergreen, though it has naturalized over a fair amount of the Northeast. There are native roses such as R. palustris AKA swamp rose and R. virginiana.

For evergreens, balsam fir as it grows on my property is relatively narrow, but as with most of these plants, I don’t know about salt tolerance. Check out junipers as another hardy needled evergreen that may have some salt tolerance. I can grow Rhododendron maximum, but it does get winter foliage damage in sun. My mountain laurel AKA Kalmia latifolia Seem to do better with keeping their winter foliage looking nice, but are mostly in at least part shade, so that might be the explanation. I don’t think either is salt tolerant. Yew tend to be eaten by deer if they are an issue for your site, and often Thuja are eaten into lollipops as well, with all foliage missing as high as the deer can reach, so you may want to set up protection for them this fall, either netting on posts or a webbing of posts and high test fishing line which they have difficulty seeing.

For deciduous plants, look into the native Viburnums, some of which should be hardy enough for you, but I don’t know about salt tolerance. Physocarpus AKA ninebark isn’t evergreen, but it is densely twiggy and so provides fairly good visual screening and is a US native. Hydrangea arborescens is native, but in my garden doesn’t get taller than 4’-5’ and prefers shade in the afternoon. I have seen Amelanchier AKA shadblow or serviceberry growing in the median strip of the interstate, so it must be at least slightly salt tolerant. There are several deciduous dogwood shrubs and Ilex verticillata AKA winterberry holly that have winter interest.

If you are looking for an inexpensive source of seedlings so you don’t have to grow your own, check out the NH state Nursery. Order early next season (check in January) since I think that they may be done shipping for this year and they often sell out of popular plants. Many, though not all, are native plants.

PS This doesn’t show in the New England or Northern Gardening forums for those of us who use the Garden Web interface because you used multiple tags. It is a programming issue from combining two different websites that Houzz doesn’t seem inclined to fix. It only shows in landscape design which this really isn’t a question about. You might want to ask in the shrub forum or ask individually in the other forums.

Thanks, NHBabs, that's a lot! Thanks especially for the info on the deer! I had thought that Thuja weren't to their taste...

The hedge in question is largely up against the road, and I just don't want to upset anybody's Vermont experience planting things that look out of place in this region (I realize that Rosa rugosa is not native, but it is not listed as a
problem in northern Vermont; it's supposed to be amazingly salt
tolerant; and 'Alba' is white--which is second only to beautiful thorny rose bushes that don't bloom at all.)

In addition to spray, runoff is an issue because the property slopes downward from the road. It gets full sun (from sun up until about 6:00PM). Trees are being planted about 12 feet back (13 feet as measured on the ground). I am hoping that burlap stretched along the length will minimize the spray in winter.

Some of what you listed is stratifying now, including Amelanchier
canadensis and Ilex verticillata. I had not considered them as part of
the hedge--more as bird food (along with other things that escape me
right now).We have some Abies balsamea, Picea mariana, Picea glauca, Pinus rigida, Pinus strobus, and Pinus resinosa on the property. The pines get too wide; the Abies are too young to tell; but I have gathered seeds from P. mariana and P. glauca for use in the hedge. However, unlike A. koreana, P. engelmannii and etc. listed above, I have had no luck germinating them yet... Am now trying pretreatment soaking and sterile soil-less seed starting mix. Have you had any luck growing P. mariana or P. glauca? (On a similar note, I have had no luck with Betula papyrifera or Larix laricina. Any suggestions?)