Remove the geranium from its pot and gently brush off the soil clinging to its roots.
Examine the roots and gently pinch them between your fingers. Healthy roots should feel stiff and turgid. Remove any roots that are brown and mushy to the touch.
Cut unhealthy roots from the geranium's root system with a sharp, sterile root cutter or garden knife. Make clean, straight cuts. Jagged cuts are difficult for the plant to heal.