When is the right time to stow away bonsai for winter storage? There are very different opinions about that. Some put them into a safe storage place before the first low freezing nights are present, where others wait for the first frosts to appear. I am the one for the last option, because i find it important to help the trees go into their normal dormancy. So one or two lightly freezing (- 1 / 30 f) nights do the trees good and not bad.
It also takes the most vulnerable bugs out of business letting the cold get to them, when they hide in bark cracks or under the top soil.
This goes with native trees, but take care if you have imported trees that are less frost tolerant. Native trees (from Northern Europe) cope with freezing well, but are as bonsai more exposed with roots in a pot. Though roots cope cold well, they do not cope as well as branches. Not growing freely in the ground and being able to extend their roots in depth reaching a warmer soil area, it is important to store bonsai protected. This can be in an unheated glasshouse or something similar. Just keep them from frosty winds, and secure the roots from severe cold. Placing your bonsai in boxes filled with an isolation material is an option. Not covering them though so you can´t check watering status. Isolation material arranged under the pots and around them is a good solution for heavy winters if stored in a cold house where freezing is present.