rockm

What Dav has said is key to overwintering. Mulch is used as much to keep roots cold in the early spring and late winter thaws as much as it is used to keep the worst winter chill out. Early bud break is a real problem if you have nowhere to put trees that have leafed out.

Storing trees in the shade is a must. Late winter thaws followed by early spring hard freezes will kill exposed bonsai, or bonsai that have inadequate root protection. Once trees leaf out, their roots lose their winter protections. If they have leafed out or have extended buds where leaf edges are easily visible, leaving them out to freeze can mean disaster.

Late winter and early spring are the most dangerous time for bonsai stored outside. Even more dangerous than deep winter freezes.