The autumn months are a time of transition for bonsai and their owners. In this article, we outline five essential fall tasks hobbyists should perform during this season to get their trees ready for the winter ahead.

Fall is many folks’ favorite season, and mine as well. I look forward to the crisp nights, clear, haze-free days, and of course, the cavalcade of colors that blanket the landscape this time of year. 

When it comes to fall bonsai tasks, autumn is all about helping your trees wind up the growing season and get prepared for winter. Whether a tree will be spending the coldest months out in the elements or snugly indoors, most of the fall tasks we cover below still apply.

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1. Fertilize with a low- or no-nitrogen food.

Fertilizer has three components – nitrogen, phosphorus, and potassium – which affect different aspects of plant health. As trees prepare to enter dormancy in the fall, they don’t need the growth-stimulating effects of nitrogen, but they still benefit from the other two elements. Phosphorus is important for balancing tree growth and strengthening the root system. Potassium helps regulate the plant’s respiratory process and protects it from the stress of winter weather.

Using a fertilizer like this one that is formulated without nitrogen throughout the fall season will help trees store energy to stay healthy during winter and enter spring well-prepared for the next growth cycle.

To learn more about fertilizer and the role of its three components, check out our article about feeding and fertilizing bonsai.

2. Clean up trees. 

Second in the list of fall tasks is to inspect and clean up each tree in preparation for winter storage. While fall is not the best time to heavily prune trees, it’s a good idea to snip off any dead branches. The same goes for dead foliage on evergreens.

Closely examine the wiring on trees. Branches on some species, such as pines, tend to thicken in autumn. If wires are starting to dig into the bark, remove them and plan to rewire in the spring. Leaving wires on too long results in ugly scarring that can permanently disfigure a tree. It’s always best to remove a snug wire before it becomes a problem. 

Just prior to moving trees to their winter storage areas, pull any weeds and remove leftover dead leaves or needles from the trees and pot. If any trees have developed an overly thick layer of moss, remove the excess. Very thick moss can cause the pot to retain too much moisture over the winter. It’s also a good idea to top off the soil to provide a bit of extra protection for the roots.

3. Treat for insects and disease. 

Carefully inspect each tree for pests and infections, and treat accordingly. A general insecticide and fungicide applied to trees prior to winter storage will help prevent problems from developing over winter. 

4. Bring tropical bonsai indoors.

Tropical plants flourish outdoors when the weather is warm, but need to come inside once nights turn cool. As a general rule, tropicals need to come indoors when nighttime temperatures drop into the mid-50s. However, it’s important to know the temperature range for all your trees, and protect them accordingly as late season weather sets in. 

Once indoors, tropicals need to reside in a sunny spot by a non-drafty window. Alternatively, a full-spectrum grow light such as this one can help keep them healthy until they move outdoors again in spring.

5. Plan appropriate winter quarters for temperate trees.

Temperate bonsai suited to the local climate need to stay outdoors during the winter. Cold weather induces a natural dormancy that the tree needs to experience in order to stay healthy. However, with their small pots and shallow root systems, these bonsai require refuge from extreme cold and the drying effects of winter winds.

The further north you live, the higher the degree of winter protection you might need to provide for your trees. Knowing the requirements of each species will make fall tasks easier and help you choose from among the several common methods hobbyists employ to overwinter their trees:

Open to the outdoors. 

Find a wind-protected location near a fence or along the foundation of the house to help shelter your winter-hardy outdoor bonsai. Place trees close together on the ground. Many hobbyists heap a layer of mulch around and between the pots for added protection. Some prefer to remove bonsai from pots and bury the root ball in the ground. Some even bury the tree and the pot for winter. 

Cold frames and similar structures. 

In cooler climates with harsh winters, bonsai owners often employ a cold frame to keep their trees well protected. A cold frame is a bottomless box structure topped with a clear cover. It can sit on top the ground or be partially buried, and acts much like a miniature greenhouse. 

Cold frames protect plants quite well, but have the downside of requiring that the temperature within the frame be closely monitored. They heat up quickly when exposed to sunlight, and the clear cover must be raised and lowered to the degree necessary to maintain the desired temperature range. Otherwise, the trees stored in the cold frame can come out of dormancy too soon. 

One alternative to the more permanent structure of a cold frame is a temporary greenhouse that is assembled for winter and taken down in the spring. Inexpensive plastic greenhouses such as this one can be zipped up when frigid cold snaps are forecast, but left open during milder winter conditions.

Unheated protected spaces.

 An unheated garage or shed can also be a good choice for overwintering hardy bonsai. Surprisingly, once a tree has entered dormancy, it has no need for light until it breaks out of dormancy in the spring. So, even a garage or shed without windows works perfectly fine for winter storage. 

Hobbyists often acquire temperate tree species are not hardy in the local climate zone. These trees are not tropicals so they need a period of winter dormancy, but they wouldn’t survive if left outside over winter. Depending on the species and the local climate, an unheated garage might offer enough protection, or they may need to spend the winter in a cooler room inside the house. Again, it’s important to know the requirements of each species of tree in order to provide the optimum winter conditions. 

Keep in mind that trees need moisture, even during the winter. Occasional watering is essential, particularly when bonsai overwinter in a protected space that doesn’t receive moisture from rain and snowfall. 

In addition to the fall tasks we’ve covered, it’s important to know what not to do. As we prep for winter, avoid activities such as repotting, styling and heavy pruning. Doing them now will unnecessarily stress trees as they enter a vulnerable period. These tasks are best done in early spring when the tree is starting to emerge from dormancy and is poised for another season of growth. 

We covered five essential tasks that bonsai hobbyists should perform during the fall months. From fertilizing, pest control and clean-up to settling on appropriate winter quarters for each tree, these activites will help keep bonsai healthy.