Bonsai Care: Tips & Care for winter

Honestly, I have to admit that I am writing this post for everyone else except me, because living in a tropical climate like Miami I do not have to worry about any special care of my bonsais during winter.

So this discussion is for care of temperate trees where winters are harsh. 

It is not necessary to be scared about the onset of winter and your bonsai care. Most temperate climate trees grown as bonsai are capable of handling the severe weather as long as a few simple conditions are met. 

Winter winds combined with temperatures below the mid twenties Fahrenheit (or below -5 Celsius) will freeze dry almost any bonsai. This is the biggest enemy.

Attempting to keep temperate plants warm by placing them in the sun is a mistake. The added warmth from exposure to too much direct sun can cause a tree to wake up from dormancy too early. If this is followed by low temperatures, the water in the pot may freeze within the cambium layer and kill the tree.

Usually it is not necessary to take trees out of their pots. Good pots are capable of handling the temperature changes. (One exception to this rule would be the Trident Maple). This particular tree should be removed from its pot and the soil ball buried in the ground or a large box. If it is wintered in the pot the roots will turn to mush.

All other temperate bonsai can be kept in a sheltered area out of direct sun and wind. The pots should be mulched or buried to the top of the rim in the earth. Burlap barriers work well as a wind break. You can also make a wall using bales of straw.

If there is not much snow or the weather warms it is possible you will have to water your trees from time to time. 

Rule of Thumb: If the soil ball is frozen, you do not need to water. If the soil ball thaws and starts to dry you must water.

Winter's low temperatures can be dangerous to these plants, but the winter period is vital to the development of many bonsai.

The cold winter is a necessary time for many trees, since they must enter a dormant period where no growth occurs. This stimulates growth after dormancy has ended.

Besides monitoring the temperature, very little maintenance is required during the winter. Water the bonsai occasionally to keep the soil from drying out. Temperature must be monitored for all bonsai during the winter. Temperatures below 15 degrees Fahrenheit will be fatal to most bonsai. When temperatures fall below 15 degrees, place outside bonsai in an unheated room or shed to protect them. When temperatures are consistently above 40 degrees, the dormancy period ends.

Winter Tips:

WATERING - The need for water will diminish this season since the weather will be cooler. Therefore, water only as needed in order to prevent root rot. If you need to water your plants, try to water them early in the day  it is a good idea to water your plants early in the day so the leaves will be dry when going into the night hours thus decreasing the chance of any leave fungus.. If the temperature drops to freezing or below, the water soaked bonsai soil could possibly expand and break the container.

FEEDING – Because of shorter days and cooler temperatures, plants will be doing little to no growing. You will probably not even have to fertilize your plants at this time of the year. If you feel you still need to fertilize, remember to use a fertilizer with no nitrogen in it.

WIRING – A good time to wire is before new leaves begin to emerge or before new growth gets too thick to see the branches. Wiring or removing wire from a deciduous tree is a much easier job when there are no leaves to get in the way and the structure of the tree can be seen more easily. Therefore, if all the leaves on your deciduous trees have fallen off, remove and replace the wire as necessary. Continue to keep a close eye on the wire on your other trees as well because the branches will continue to thicken. Leaving the wire on the plant for too long will disfigure the branches; therefore, remove the wire if the branch remains in the desired position or if the wire is cutting into the cambium.

PESTS AND DISEASES – With the colder temperatures, the activity of harmful insects will slow down or become non-existent. If it gets cold enough, the cold temperatures will actually kill insects, however continue to check for pests and diseases and treat accordingly with soaps and oils if needed.  Use environmentally friendly treatments such as the encouragement of beneficial insects, hand picking larger harmful insects, and/or the use of soaps and oils. Always remember to spray a small branch first in order to make sure there will be no phytotoxic reaction to the plant from the soap and oil. 

TRIMMING – Deciduous trees such as maples, crepe myrtles, cypress and elms should have lost their leaves by now. They need a complete rest during the winter months. If not, they will become weaker expending too much energy maintaining older leaves that should have already fallen off. Therefore, remove all the remaining foliage on deciduous trees. Continue to trim other evergreen plants as necessary to maintain the desired shape, but this should become almost non-existent because of the lower levels of nitrogen, shorter days, and cold temperatures.

PRUNING – Avoid heavy pruning if possible. Pruning will cause excessive, tender new growth, which will get damaged in cold weather. Even the new growth on cold hardy plants such as hollies and boxwoods would be damaged. It would be a waste of energy for the plant to lose all the new growth.

LIGHT – Plants that have been protected from the harsh summer sun such as maples and azalea plants can now be exposed to more sunlight. Rotating the trees on a weekly basis will prevent the trees from becoming one sided.

REPOTTING -  Depending on how many plants are in your collection, repotting can begin towards the end of January. Begin repotting your evergreen material such as hollies, boxwoods, junipers, and azaleas. It is recommended to repot deciduous plants when the growing buds begin to swell which is usually sometime in February. Make sure all the older leaves have fallen off or have been removed. Avoid repotting all tropical plant. 

TEMPERATURES - The colder temperatures will continue this month but it will probably fluctuate from mild to cold so be prepared to protect you tropical plants. Place all of your tropical plants on one bench so you will not forget any. Also, if you have to move your plants back and forth from a protected location back outside, you may want to leave them on a cart or a tray so it is an easier chore. Some examples of tropical plants are ficus, buttonwood, bougainvillea, and Fukien Tea.