Winter care for your bonsai

Those of you who have visited the bonsai nursery May have seen my winter “corrals” built of two by six lumber arranged in a square and filled with leaves. Under the big Dawn Redwood—Metasequoia glyptostroboides to be exact— the surface roots keep the soil about ten degrees above ambient temperature, and give good protection in our infrequent freezes. Just to be sure in case we ever reach into the single digits— which has happened at least twice singe I began in Bonsai— I use this system of mulching in the pots in raised beds. Leaf mulch has some insulating property— and bark mulch has actually very little— so you may want to hedge your bets and use a layer of pumice gravel— which drains well if rains are heavy and washes off easily in spring.
Another recourse is to house your bonsai in a small styrofoam container— be sure to poke holes in the bottom for drainage or your tree will swim through winter, and possibly freeze into the container if weather gets severe. I mulch with pumice or sand to about four inches above the root flare or soil line, keeping the tree contained in its nest for the winter months. This ensures that if the pot is not fully frostproof it will not freeze and crack from absorbing  moisture through a porous surface. Vitreous frostproof pots are high fired to around 2000’f to ensure no moisture penetrates and can crack the clay body upon freezing as water expands. Contact me for any other tips about winter care— larger trees which are hardy will weather winter fine as is. And the dormancy and steady rainfall our maritime climate provides are excellent conditions for Bonsai!