I wonder if elderberry would work in a large container on a third floor deck in Toronto? And survive the winter? The container has styrofoam insulation and deck has a watering system but it is hot.
This is a challenge every fall for those of us who love to grow in containers. Here’s the basic issue: it is usually the freeze/thaw cycle that kills the plants, not the initial freeze up. So container size is important. The more soil there is in the container, the better. In a large (at least 18 inch x 18 in by 18 inch) container, there may be enough soil that it is unlikely to thaw out on your deck during the occasional warm day we have in the winter. The Styrofoam insulation will also prevent the freeze / thaw cycle but it may also prevent the plant from going dormant. I would have the plant in a non –clay pot( do not use a clay pot as it may shatter when the moisture captured in it freezes) and once the weather has gotten cold enough to have the plant go dormant, I would then insert the pot into the Styrofoam container as this should prevent it from thawing prematurely. Keep the pot in a sheltered spot as you mentioned that the deck is hot ( I am assuming South-facing?).
The article attached suggest that a plant stored in a container should be 2 zones hardier than the zone it is in, Elders (Sambucus) are hardy to zone 4; Toronto is zone 6a. Keep in mind that they prefer well drained, moist soil.
Here is a useful site on container gardening; 7 Secrets to Successful Container Gardening