Mushrooms thrive under very different conditions than most plants, but this shouldn't keep you from trying your hand at growing them; they're actually quite easy to care for. If you've got some dark spots in your greenhouse, you can easily grow mushrooms using straw and some mushroom spawn, which you can get at many garden supply stores with vegetable growing supplies. With the right equipment and proper growing conditions, you can be harvesting and eating your own mushrooms within a couple of weeks.
Fill a large stockpot with water and bring it to a boil. Place the straw in the boiling water and continue boiling the water until the straw is quite soft and almost mushy.
Remove the straw from the water and allow it to cool down so you can handle it. Mix the straw with your mushroom spawn and place it on your plastic sheeting.
Pack the straw very tightly and wrap the plastic around it to form a solid log. Use a knife to make several ventilation holes on the top of the log. Place the straw log in a dark, warm, dry place in your greenhouse.
Check the straw log regularly, but do not water it until you see a white substance forming on the straw. When you see a white substance forming, move the log to a cool place in your greenhouse and water it regularly to help the mushrooms grow. You should see edible mushrooms after about one week.