General
The Chattanooga area plant hardiness zone is 7a/7b. The mountain tops get snow a couple of times a year. It is important to shelter bonsai trees since it may get to near 10degrees F.
Indoor bonsai generally don’t like temperatures below 40degrees F. They stay inside from about October to April.
Outdoor trees stay outdoors year round. Deciduous trees need to have their roots protected below freezing. Conifers need to have their roots protected in extreme cold (< 20degrees F).
Deciduous trees must go dormant each winter. This means they need to be below 39degrees F most of the winter. Don’t bring them in the house or a greenhouse for extended periods in the winter. Dormant trees can go without light during winter. Their water requirements are also greatly reduced.
Pines slow down but don’t go fully dormant in winter. They still need light and water. Other conifers (junipers) may go dormant. Your junipers may also change color and look gold/brown. This is normal.
Don’t water your trees if temps will hit freezing in a few hours. This may damage (crack) the pots. The biggest cause for dry out is wind. Monitor especially on windy days. Keep the trees shielded from wind as much as possible.
Protection
Only use a greenhouse on trees that won’t go dormant (tropicals).
One of the best ways to insulate trees through the winter is a cold frame. This will protect trees even in sub-zero temperatures. Remember the purpose of a bonsai cold frame is protection not getting an early start like they are used for vegetables. Don’t get them too warm too early in the season.
If you want to take on a little more risk (common in the south) you can find a shady place that is shielded from the wind and heavily mulch your trees (up to the first branch). An excellent material for the mulch is prepackaged cedar pet bedding that you can get at DIY and pet stores. A bale is highly compressed and goes a long way. They are about $10 a bale. The cedar bedding also discourages pests and insects.
The mulch holds in moisture reducing the need to water. If you see the mulch drying out after about an inch it’s time to water.
If you get early leaves you will be need to learn the bonsai dance. You will need to move your trees to a garage or other shelter on freezing nights and bring them back out when it warms up. You can also cover them with drop clothes. If you forget the leaves will frost and die which weakens the tree.
Additional winter care information is found here. If you have questions; ask your club.