mancorn
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The top terminal shot, sends out hormones to restrict the branches below it from growing. But the hormone only goes down the main stalk so far. As the plant grows this hormone no longer works to restrict the growth on the lowest branches and they start growing. This gives you the Christmas tree shape as the main shot moves further and further from the lower branches. When you top, it removes this hormone, so then the next two branches take off, since they're not being restricted (like they would normally be). Top again and plant will become much rounder.