Have a question about taking care of your bonsai? Our senior members answer your questions - simply email us at midwest.bonsai@yahoo.com. Questions and answers edited for clarity.
In the late fall we received a question from a Chicago Botanic Garden patron who was planning-ahead to taking care of their first bonsai, a Juniper tree, in the winter. After researching online, they asked questions about watering, fertilizing the tree, and where to keep the tree over the winter.
Answer:
Welcome to the Bonsai community.
It sounds like you've gotten some good advice so far. You are correct to water whenever the soil feels dry right up until temperatures stay below freezing, at that point the plant will dramatically slow down its water intake. Watch the temperatures outside for long periods above freezing where the plant may begin taking up water again and could need a watering.
Outside in a cooler as you described is a fine way to keep your tree over winter. The only things I would add are to bury the pot fully in the straw, up to the lowest branch of the tree to reduce possible evaporation from the soil, and to ensure the cooler has an open drain so it will not fill with water and potentially drown the tree. Junipers will actually benefit from receiving some light over the cold months, so a spot with some sun would be best, and by all means, let the rain and snow fall naturally on it.
As for fertilization, once a month with miracle grow mixed for houseplants will do fine. There's no need to fertilize over the winter, however, as the plant will not take up any nutrients during dormancy. April is probably the best time to start feeding again.