Watering, Humidity And Fertilization Prudent watering is indispensable for the well-being of your Christmas cactus. Thoroughly water the plant when the top inch of the potting mix feels dry. Ensure proper drainage through the holes to avert root decay. During the flowering phase, elevate humidity levels slightly by misting the plant or placing a water-filled tray nearby. Nourish your Christmas cactus with a balanced houseplant fertilizer every two to four weeks throughout the growing season (spring and summer). Dilute the fertilizer to half the recommended strength to prevent overfeeding. Refrain from fertilization during the dormant period (fall and winter) when the plant is quiescent.