Mushrooms, Memory Markers and Grotesques can stay outside all winter.
Although hypertufa (with peat moss) and lightweight cement (without peat moss) is winter-proof, a few laws of nature do still apply.
If your piece has a cavity that can hold more than a few inches of water it would be prudent to either put it in the garage or cover it in a manner that prevents snow or rain from accumulating in the cavity. Water expands when it freezes and enough of it can cause your piece to crack or split. Thick sided but shallow alpine troughs can withstand this, some say better than conventional cement.
All mushrooms are safe to leave outside for winter, though you should keep an eye on them during spring thaw as the ground may heave enough to make them fall over. The Irish Mushroom's thin stem might snap but personally, I've not had this happen. If yours does break, please see section below.
Since cement will corrode most metals, use only steel nails (check with a magnet!) for hanging your pieces.
Are critters are knocking the caps off your mushrooms?
Did your piece get broken?
Use Elmer's Wood Glue (for outdoors) for a permanent fix. After a 24 hour set, mushroom caps will not come off again. That glue is amazing, it also does a wonderful job repairing broken cement. If you are not a DIY person, bring your piece to the studio for free gluing work.
If your piece breaks or disintegrates because of weather I need to know about this! Please please drop me an email and let me know. It will replaced or repaired - which ever is the most appropriate and best option.
My work will grow moss. If you want moss do an Internet search for moss recipe. Or you can buy Moss Milkshake™ from Wildflower Farm in Coldwater, Ontario.