|Will Westmont Arborvitaes survive in containers on a deck?
I live in Maryland and the lowest winter temperature is about 20 degrees; the highest summer temperatures are around 98.
|In my experience, it is very difficult to keep evergreens healthy in containers year round. You may find that they need to be replaced come spring.
Although recent winters have been mild overall, I would expect an average winter low of closer to zero in your area. Nominally in zone 6, this puts you closer to the cold end of this plant's hardiness range and that means that the plant is going to more cold sensitive for you. These zone ratings are based on plants set into the ground and very often roots are far less cold tolerant than the topgrowth. Unfortunately, the winter lows make a big difference in root temperature when the plants are kept in containers, as does being exposed to cold from the bottom due to being on a deck -- there is no insulation from below.
If you want to try to keep them, your best bet would be to sink the containers in the ground or set them in a sheltered location out of the wind and heap mulch around the containers. These methods help to insulate the roots the way they would be if they were planted in the ground.
Your other problem will be heat-related in that sun hitting the containers can raise the interior soil temperature quite high -- and cook the roots. Again, insulating the containers or using larger tubs will help, if nothing else try to shade the pots.
Larger containers will also help with keeping the soil evenly moist whenever it is not frozen. Watering can also be tough, in very hot weather you will need to water more often so that the soil stays evenly moist. These trees actually prefer a moister soil than many evergreens, so this is of concern as well.
I'm sorry I just can't be more encouraging.