How cold is too cold?

Every year I bring my tree into my garage but I was wondering what everyone else does with their trees. I live in zone 6 and have been thinking about leaving them outside for the winter. How long can they stay frozen. broken pots and so on.

It depends to a large extent on what you are growing. Most evergreen conifers probably can take anything Missouri can dish out. Trees native to the Midwest also will be cold hardy.

Japanese maples, azaleas, understory trees (hornbeam, etc.) will need some protection, depending.

A well-fired pot won't break in freezing weather, Manny of those thick, glossy, porous, lightweight Chinese pots will spall like crazy and will finally vanish

This is my first winter with a few outdoor pre-bonsai. I have a pretty nice san jose juniper in a non glazed pot, and a 5 year old japanese white pine graphed on JBP stock in plastic container out on my covered porch. I used a drawer from a old dresser and filled it with peat moss to keep them insulated. I also put pantyhose (one of my wifes) over the juniper's pot to keep the moss out. Im told to watch the watering since you can't see the soil. When the snow comes ill just pile it on and let that water for me. It gets cold here somtimes -20F, will that do it?

Neil