In the next few days most of the northern United States and Canada are expected to experience unseasonably cold weather. Here in the Cleveland, Ohio area, for example, it is predicted to drop to 27° tonight and 19° tomorrow night when the averages for those days are 38° and 37°.
Why should that matter for bonsai trees that are hardy to lower temperatures and do we need to provide extra protection to our trees?
I would advise that extra protection be given to trees, even if hardy to those temperatures, because there is a chance the trees have not yet acquired their full winter hardiness this early in the season.
Part of cold hardiness comes from the concentration of sugars in plant cells. This concentration increases starting in early September and reaches maximum in late November to early December. Then it holds steady until early April and begins decreasing as the plants wake up and begin to utilize that sugar for spring growth. Since plants reach their maximum concentration in late November that is when they have their maximum tolerance to cold temperatures.
The fact that it is early November means that it would be prudent to provide additional protection against the cold temperatures but care should be taken to avoid keeping the plants too warm which could delay cold hardiness.
Some good options for protection for the next couple of days include an unheated garage or shed, a cold house or cold frame, or setting on the ground and covering with mulch.
Your trees might not be ready yet for winter storage but would benefit from a little protection until temperatures return to normal in a few days.