Hi Kayla, I see your question still hanging here. I haven't grown them from seed, have grown them though. We cut a row back low from the roof line several weeks ago, trying to stop raccoons from climbing on my roof :)

I would think in Z5 you should have plenty of winter left for the chill. Freezing isn't necessary, just an average of approx 40F, so days in the 50's, nights in the 30's would still be fine.

And Druse (Making More Plants), says moist cold 30 days, followed by light, 55F for germination in 14-60 days. But even though fast growing for a conifer, if you are looking for a privacy screen you'd be in it for the long haul starting these from seed - from what I can find, those 2 gal 4-5' plants at Home Depot/box stores are about 6-8 yrs, and I'll bet they are cutting grown. Our Home Depot offers them frequently for less than $10, compared to cost of materials for privacy fence, not too bad...but I don't know how many you are talking about, what that would come to.

Another thought might be buy one (or find a friend growing them) - have you ever tried cuttings? These are supposed to be pretty reliable, one grower who used to participate a lot on the propagation forum says: bottom heat at 75 to 80 degrees, rooting hormone, strip wound (shallowly slice of a strip of bark from end to insert in the medium), peat/perlite in equal amounts mixed for medium. No misting to keep humidity up as for some types cuttings. 1 1/2 zones warmer than you, states he/she gets typically 100% success with thuja cuttings taken mid-Jan. The timing might just work for you to try it now if that's an idea that interest you at all.

My own cutting experience can be hit and miss, I never count on it but can be happily surprised :)

I already ordered two varieties of arborvitae from Sheffields. I will try them. It will be a fun trial anyway. If not, I'll have to purchase from a nursery.

OK, let us know how they do. If closer, I could have given you a pickup load of cutting material a couple of months ago. Raccoons looking down in @ 2 AM from the over-bed skylight wasn't amusing after the first time :)