We have successively winterized our geranium plants indoors in a dormant state. Is there a good fertilizer that can be applied to better sustain the plants and give them an added boost for quicker return in the spring. When blooming outdoors we apply “Jacks” regularly.
You shouldn’t feed then at all during the winter while they’re in their rest period. Wait until late winter to start light feedings with your Jack’s.
More on that here:
http://blog.timesunion.com/gardening/getting-ready-to-come-inside/6934/