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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?

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  • What did you do to try to grow from seed (where did the seed come from, how old was it, what time of year was it sown, sown into what, temperature, soil etc...)
    – Bamboo
    Commented Nov 3, 2021 at 22:40
  • @Bamboo I collected the seeds and sowed the same year, in late fall, and I sowed them outdoors with little delay, in an area with moist soil and an abundance of leaf litter, hoping for germination the following growing season. The source trees were landscaping plants, which is all there is around here.
    – cazort
    Commented Nov 4, 2021 at 12:46

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Thuja occidentalis is not too keen on growing from seed, which is why it's usually propagated by cuttings. The best average germination rate from shop bought seeds is about 37%, and estimated at around 1% of seeds collected in the wild, but it's still worth a go.

The steps missing from your description of what you did are 1) you did not sow in pots or trays and 2) you did not cold stratify them over winter in a refrigerator. Seeds from cones you've collected need to be cold stratified so they are more prepared to start growing when spring comes, if the seeds are viable. Further info on when to collect seed and precisely how to sow it here https://homeguides.sfgate.com/grow-arborvitae-seed-21603.html

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  • In my area (zone 5, Midwest US), they're actually weeds in the landscape, but ONLY in areas with acid soil—acidic enough to grow blueberries & wintergreen. They stratify themselves because we get below 0 F for 5-14 nights per winter. Perhaps your soil's pH is too high? Also, these seeds are not covered by soil, only by leaf litter. This, too, may be a reason for your lack of success. I would guess that the 1% germination figure is about right, though—my 3 arbs put out a ton of seed, but only a few dozen seem to sprout—always in deciduous shade, and not under their parents, oddly enough,
    – Jurp
    Commented Nov 4, 2021 at 18:23

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