The first mushrooms to appear are called "pins," for their tiny size. Mushrooms double in size in 24 hours. A bed in full production will be harvested multiple times a day. There are usually three flushes of growth, with decreasing yields for the second and third flushes. A cycle is 33 to 45 days, depending on the kind of mushroom. These baby bella or cremini are also Agaricus, just a different color form. All portobello are cremini but not all cremini are portobello. What's the difference? Once the veil on the underside of the mushroom cap tears the cap stops growing. If this happens while the cap is small the harvested mushroom is a cremini. When it grows to that yummy-for-grilling palm-size version, it's a portobello.