In general, yes, it is true that leaders or other major branches lost on arborvitae will not regrow. This is because the older wood (branches that are a few years old or older) has no dormant leaf buds on it, unlike evergreens like boxwood and holly, so pruning heavily (or heavy deer browsing damage) will permanently remove foliage in the trimmed/eaten areas. However, on very young plants, you might get enough regrowth that eventually the original damage will be hidden or replaced with new growth.

Arborvitae should be hardy enough to not need winter wrapping in our area, though careful attention to watering if rain or snow is lacking during fall and winter can be very important, especially for young plants still getting established in their first two or so years in the ground. During mild spells in winter when the ground isn't frozen (or before a predicted prolonged freeze), feel the soil about five inches deep next to the root ball and water well if it's become somewhat dry to the touch at that depth. Unfortunately, over-watering and poor soil drainage that rots roots can also create the same symptoms of branch dieback that can occur with under-watering, both of which may also look like winter damage.

If you want to share photos of the plants and their symptoms, we can get a better sense of how extensive their damage is and if replacement is worth trying, or if they should grow out of it. (We can get back to you early next week as to their prognosis.)

Miri

Thank you for the photos. Given how much growth was lost, and the fact that a corresponding loss of roots might have occurred, it may be more practical to replace them. If you prefer to keep them, prune off all dead branch ends and anticipate that some with extensive dieback might have a more rounded shape than the upright habit they normally would have.

Miri

You're welcome.