In northern Delaware, southeastern, and south-central Pennsylvania, northern whitecedar (Thuja occidentalis) is widely planted in landscaping, and it generally thrives in these regions. I sometimes even see it doing well in landscaping much farther south, such as in the coastal plain of the Delmarva peninsula, in low-calcium soils that it is in theory not supposed to do well in. Adult trees in landscaping, although they grow slowly, usually seem to be healthy and be long-lived, and seem to be able to survive in a wide range of conditions, including full sun to moderate shade, and in poorly-drained sites to average moisture conditions.
However, I have struggled to grow it from seed. When I've tried sowing seed, none of it has germinated, and I have never seen seedlings coming up in the wild in these regions either. However, I have found a couple saplings (3-10 feet in height) in landscaping that look like they were likely not planted, i.e. they are coming up near large, healthy trees that are presumably parents, and they are in locations where they would probably not be planted, such as a small sapling that is offset and spaced irregularly relative to a regularly-spaced row of trees.
The regions I'm talking about are a good bit outside this species native range to the southeast. Here is BONAP's county map. It is native farther west, at high elevations in the Appalachians, and also farther north, where it is more widespread in places like the Upper Midwest and New England, and can even be found to sea level if you go far enough north along the east coast. In New England and the Upper Midwest, I have seen numerous seedlings and saplings of this species near where there are parent trees.
What is limiting this species reproduction by seed here? Why is it absent from the wild, even though it is so widely planted in landscaping? Why does it so rarely come up as seedlings in landscaping? And is there anything I can do to aid germination and/or establishment, if I want to grow some of these from seed for my own use in landscaping?