Full Marks!

its a beauty

Growing shrubs in a pot can be challenging! While those are handsome pots, I can see several problems you will have. The narrow base will make them more prone to tipping over when there is wind with the arborvitae on top. You have a quite restricted amount of room for roots, width-wise, and since the bottom part of the pot is filled with rocks and styrofoam, there isn't much root depth. The less soil and the narrow pot means that temperature and moisture will fluctuate more than with a bigger pot. If I were you, I might try getting a wider (& as deep as possible) pot and filling it with potting soil, not garden soil, and replant your surviving arborvitaes. Garden soil tends to be more dense than potting soil, and without enough air spaces, the plants' roots don't get enough air. (In the garden, soil gets aerated by worms, moles, etc as well as having organic matter constantly added from on top naturally.) Also, potting soil is lighter in weight than garden soil if your situation has weight limits. Water deeply when the soil feels dry when you stick in your finger to the first knuckle. Since some of your plants get more sun than others, you will have to check each plant separately. Too much water for your shady plants may be what killed them. Also, in general, arborvitae are sun plants, so will want at least 6 hours of sun to grow really well and full, though they will grow with less, just not as densely. They won't be happy in full or almost full shade. I am assuming that you are planting to block the view beyond your railing or to give some privacy. If you decide your conditions aren't good for arborvitae, some other plants that are tall enough and full enough to help give you a green wall and are tolerant of living in pots include ornamental grasses (there are some quite full, tall grasses, though they will need sun), bamboo, and vines on an obelisk or trellis (some clematis will tolerate part shade, and there are many annual vines, though most of those like sun.) I've also heard of folks fastening bamboo screening to a fence, which lets in some air, but provides some privacy, and then planting in front of it. Since the plants don't need to be quite as dense to ensure privacy, you will have more choices. There are container and balcony garden forums here on Garden Web where you may get more info. Read their FAQ's (if they have a set) and search for arborvitae and evergreen. See what info you can get just from reading a bunch of posts, and then ask for information that you still need. You can also find books and magazines in your local bookstore or library that will have photos to inspire you with a variety of choices. Researching roof gardens will probably help give you ideas as well. Just remember that whatever you plant should be about 2 zones hardier than where you actually are if you want them to survive the winter in pots. Good luck and let us know what you decide to do.

It's a cottony scale. Squish or brush off.