Preparing Dormant Annuals for the Growing Season
Plant any bare root plants in a container large enough to accommodate the roots. Move potbound plants into a slightly larger container as needed. Use a quality well-drained potting mix and set the plants at the same depth they were growing before.
Prune stems back to 4 to 6 inches above the soil surface. Only prune the upper branches on tree forms of these plants. This encourages thicker stems and more compact growth.
Move the plants to a sunny window or under artificial light and begin watering. Water thoroughly whenever the top few inches of soil are crumbly and moist.
Once the plants start to grow, you can fertilize with a dilute solution of any flowering houseplant fertilizer.
A bit more information: Geraniums and other annuals grown as houseplants for winter will also benefit from a spring trim. Pruning plants back to 4-6 inches will encourage thicker more compact growth. Fertilize these plants once the new growth begins. Your plants will be full and ready to bloom in time for the growing season.