Answer: As far as fertilizing, I think the sticks were probably coincidental to the plants dieing, that would be about the time when the winter stress became insurmountable as they tried to do their spring growth spurt. On the other hand, I would suggest using a slow release granular fertilizer such as 10-10-10 plus minors at the lower rate on the label, you could also top dress with a good quality compost. Since these are container plants you want to keep them healthy without encouraging excess growth. Feed only in spring and early summer.
In my experience it is very difficult to overwinter plants in containers outdoors in your cold winter climate. (A much warmer climate of say winter hardiness zone 7 combined with a sheltered location and a large container providing ample insulation to the roots seem to be about the line for good chances of success.) Sun exposure as well as wind exposure add to the difficulty. The plants become stressed and often do not survive.
The best things to do are provide wind protection and insulate the roots as best you can (possibly wrap with bubble wrap or other insulating material or heap mulch around the pots), place them in a shaded spot up against a wall -- to reduce reflected heat -- and also protect them from wind and then make sure to keep the soil damp any time it is not frozen. By the time you do all that they do not look all that decorative. Even so, I would consider it an experiment. I'm sorry I can't be more encouraging.
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