You should also make sure you subscribe to our Succulent City Youtube video channel. Growing cactus requires patience. But it is also about being patient and keeping an eye out for potential problems. When you plant your barrel cactus outdoor, things will change. A balanced fertilizer, such as a 20-20-20, should be used every other month when the plant is actively growing. If the plant is indoors, it is better to allow it to enjoy enough sun. Having been accustomed to being indoors, they need to be slowly introduced to the bright outdoor sunlight.