Growing any large plant - like the arborvitae - in a container acts as a natural dwarfing mechanism. The normal root run can't develop in the confines of the container so the plant grows slower than typical and stays smaller. It's a bit like bonsai in that regard :-)

Personally, I'd go with the three per planter for the same reason you state - faster screening, plus an uneven number just "looks" better than paired plants.

I hope you understand that these shrubs will need ongoing attention in a container, even large planters like you describe. In addition to using a high quality, durable potting mix (no Miracle Gro, please!) and providing necessary water and fertilization, you will need to plan on repotting, root pruning and refreshing the potting mix periodically, probably every 3-5 years. Without this attention, the roots will eventually consume all the space available, water will just flush through rapidly and the plants will begin to fail.

Thanks for the comment.

I agree with you on the "dwarfing" mechanism. I planted a weeping river birch and Japanese maple at my last place in the pots they came in (burying them in the ground). People thought I was crazy and they would die. I did so because 1) they were next to a pond with a rubber liner I didn't want ruined by roots and 2) I didn't want them to get too big. They did phenomenal for years.

My question is, will planting them that close just limit their growth or potentially kill them?

==>>> its always about the relatives ... its all relative ....

plant spacing is usually reduced... when the GOAL ... is screening ...

NOTHING will die.. from that alone ...they will just grow together faster ... which is kinda the point of that goal ...

what gal said as to all your other potential problems... but since you didnt ask .. i wont go further ...

be nice to know where you are ... and what kind of winter issues you might also have .. but again.. you didnt ask ...

ken

I'm in Chicago.

I had them at my last place planted in a very unfriendly environment and they did make it the first 2 years (then I moved). I actually had them planted in between the joists of a deck, so they had less than 12" depth. They were 5'-6' tall.

They are going to be on the 4th floor, so I am a little concerned about wind. I get a lot of sun up there.

They cost $20 (good sale) for 6'-7' plants, so if I get a few years out of them, I won't loose any sleep. It more the effort to get them home and to the roof.

You might wish to investigate insulation during the winter. Plants in containers are exposed to air temps, as opposed to soil temps. Air gets much colder than soil. all else being equal.

+oM

A late friend had the siding on his house warp and start to come off one winter in Illinois, when it was 40 below. So, yes it might get too cold in boxes up in the air even for arborvitaes.

I've read they are hardy to 40 below.

As I said, they are cheap too, so if I can get a few years out of them, I'll be happy.

AFAIAC .... figure that one out ... lol

its more about winter winds freeze drying the leaves... rather than the magic min cold number ...

on sunny days.. tissue can thaw ... then come sun down or so.. 20 to 40 mph artic winds... blow thru ...

i like to say.. get them dormant.. keep them dormant ... and you win ...

but when tissue can go in and out of dormancy... sooner or later ... tissue dies ...

same if the media thaws.. and freezes.. repeatedly ... but then its root death ...

soooo ... dont fixate on min temp.. that is really.... the least of the issues ... yet the easiest one to track ...

maybe get some foam board... up against the planter.. to keep winter sun off the planter ... and burlap them also ...

with trees that big... in such a small container.. you will have to be fert'g ... but no fert after about 8/1 ... these plants need to be slowing down.. and hardening off for winter ... thats kinda the whole point of fall in our area ...

too much fert.. too late in the season... and they might not harden off properly .... watch zones.. on anyone who says otherwise ... the warmer the zone.. the lesser this issue is ...

ken

I've seen them wrapped in burlap for the winter here. Not real common, but not sure if that is to protect from the cold/wind or from salt from plows.

In general plants in containers are roughly about 20 degrees less hardy than the same kinds established in the ground. Up in the air on a deck the bottoms of the containers are not in contact with the ground so they don't even have the benefit of soil warmth where the bottoms of the containers touch the ground.

That sounds about right. I don't think there is going to be anything compact that is an inexpensive screen though that will fare any better.

Yes, winter cold may be more of an issue AFA the root system is concerned rather than the top growth. Roots are typically less able to withstand severe cold - like your typical Chicago winter - than the above ground growth. If they are surrounded by the insulating capacity of the soil mass, this is usually not a concern. But in a container where the insulation factor is minimal and subzero air temperatures surround them, they could be quite vulnerable to damage. Wrapping the container sides in bubble wrap may be something you want to consider.