Why do mushrooms grow so fast?? I was doing some workouts in the front lawn of my gym a few days ago and the field was clear. Three days later (today) and I notice a ton of mushrooms scattered about! I didn't realize these things grew so fast. When I was pulling my weight sled I had to make a point to avoid them. They weren't the little ones either. They were about 4" dia and a bright white color. So what makes these guys grow so quickly?