Keep an eye out for our Succulent City Youtube channel. Growing cactus is all about patience, but it also involves keeping a watchful eye and observing any issues that may arise. When you plant your barrel cactus outdoor, things will change. When the plant is active growing, a balanced fertilizer such as a 20-20-20 should be applied every other month. If the plant was previously placed indoors, it's better to let it out in the sun. They will need to be gradually introduced to bright sunlight after they are used to being indoors.