Tip of the Week  #26

Mike,
Wintering bonsai is confusing, like most unfamiliar things in life. As with most other things, as you learn how they "work" the confusion subsides as your knowledge of the subject increases. All bonsai should be outdoors from spring to fall if possible. They'll do better there, and in most cases you'll be at work all day, so why keep them inside if you won't be there to see them anyway? If there's no outdoors place available to you, do the best you can to provide them with good sun and air circulation indoors, then read "Tip of The Week #19" for information about wintering.

Trees can be broken into 3 groups.First, there's Temperate trees which need temperatures near or below freezing in winter. Second there's Sub-Tropical bonsai which cannot survive freezing in winter.

Within these groups, there are trees that like different temperature ranges. Some Temperate Trees such as Japanese White Pine do best when kept between 20 & 40 degrees in winter. Some, such as Chinese Elm do best when kept between 30 & 50 in winter. Some Sub Tropicals such as Serissa can take a light frost (no lower than 30, and for a few hours only) but would prefer to go no lower than 50. Some, such as Buttonwood should never go below 50, and would prefer to always be in the 70's. Most trees can take temperatures a little above or below their preferred range, but they won't like it quite as much. There is information on the internet about winter care for just about any species of tree. Just put the name into your favorite search engine, and sort through the results until you find a page that answers your questions. If you're still not confused, try this! There are trees such as Dwarf Japanese Juniper or Kingsville Boxwood which can be buried beneath 2 feet of snow all winter, or kept on a windowsill that never goes below 50 degrees. Many others have a much narrower "winter comfort range."
A sub tropical tree like the Natal Plum cannot be kept outside during the winter in Pennsylvania, but can be kept outside in an area where the temperature never goes below 50. In other words, your climate determines how long a particular bonsai can stay outdoors. Your desire to have one does not.
If all that's clear, consider another factor about wintering bonsai outdoors in cold climates. If you plant a tree that's native to your climate in the ground in spring, the roots will have grown below the frost line by winter. The frost line is the depth to which the ground will freeze in the coldest winter your area is likely to see. No matter how cold it gets, some roots will be in moist ground, and be able to bring water to the parts of the tree that have lost moisture from the wind. A bonsai in one to three inches of soil has no such luxury. Once the soil in the pot freezes, there's no moisture available. If it stays below freezing for a few days & nights in a row, the tree could be irreparably damaged For that reason, it's important to protect the bonsai in cold climate. Ways of doing that are found in articles #17, 18 & 19 in this section.
The main thing is to come to terms with the fact that you're not going to learn everything there is to know about The Art of Bonsai in general or wintering in particular today, or this year, or in this lifetime. I've been at this hobby since 1964, and I still learn new things almost every day. About 30 years ago I had the honor to work as a volunteer with Frank Okamura, the curator of the Bonsai collection at the Brooklyn Botanic Garden, and one of the most respected Bonsai Masters in the world. We were discussing bonsai care and I referred to him as "an expert." He looked at me and said "there are no experts", meaning that no one learns everything there is to know about trees. This from a man who had spent years in Japan studying under the greatest bonsai minds on the planet, then over 40 years in charge of one of the greatest Bonsai collections. No matter how much you learn, there's tons more you will never know. The idea is to know a little more today than you knew yesterday. Learn as you go. Learn the basics first. Read bonsai books, watch bonsai videos. Join a bonsai club or society. If there's none in your town, start one. If you get confused about something, ask someone whose knowledge you respect for help. If they know the answer, you will have learned a little more. If they don't, you'll feel a little better about not knowing yourself.