Your tropical bonsai are now, of course, inside your house. But tropicals inside a Wisconsin home in the winter pose a range of problems – low light intensity, low humidity, low temperature, and pest infestation are the major ones. Tropical trees have evolved where the daytime temperatures are 80, 90, 100 degrees Fahrenheit or more. In our homes they are lucky to get 70-72 degrees. Just as bad, the relative humidity in most homes is very low (unless you have a whole house humidifier). Last year Matthew G. spoke about vaporizers and humidifiers and ways to raise the relative humidity in your growing area.
But, light levels are crucial to creating bonsai with short internodes and dense foliage. In my own situation, I have utilized all the windows in my house, and installed supplemental fluoresant lights in my basement growing area. But, I still have many plants that are light starved. Light intensity drops off very quickly away from a window. In diminished light plants get ‘leggy’. The plant produces hormones to elongate the growth. “I know there is light out there somewhere – if I just grow out a little more maybe I can reach it.” Of course, this is not what we are trying to achieve with our bonsai. If you can hook-up supplemental light, your trees will be much happier.