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After the 2 weeks are up, lift up the newspaper. Look for tiny, white heads coming out of the peat moss. If they’re there, leave the newspaper off. You may see white streaks in the soil, which means the mushrooms are taking root. Leave the newspaper in place and continue misting for another week. Mist the peat moss every day so water continues to drip into the bed. After about 10 days, the mushrooms will be fully-grown, although you can harvest the mushrooms sooner if you wish. Portobellos reach their peak when the mushroom caps are about 4 to 6 cm (1.6 to 2.4 in) in diameter. Dig them out of the compost by hand, then clean them off with a damp paper towel and store them in a brown paper bag in the refrigerator. Harvesting the mushrooms early means you’ll be eating brown criminis instead of portobellos. Wait until the edges of the caps are flat instead of curled. Keep your tray with compost intact for now. Continue moistening the compost daily, then add a new layer of newspaper once white streaks form again. By following the same steps, you will usually get 2 or 3 batches of mushrooms out of 1 tray.
Remove the newspaper in 2 weeks if the mushrooms are growing. Continue misting the mushrooms as they grow. Dig out the portobellos when the caps have fully unfurled. Repeat moistening the compost until new mushrooms form.