It’s important for beginning bonsai hobbyists to learn how to care for trees at different times of the year. Those just starting out may not be aware that the best time to prune bonsai depends on the season. In this article, we’ll explore the impact that seasons have on trees’ growth cycles and why major work should be done at specific times of the year.                                                           

As I begin to write this article on a mid-February day, the temperature is 60 degrees, the sun is shining and the first tentative greenery of the crocuses in my landscaping are starting to peek through the mulch. Yes, it’s that time of year when we are thrilled to see the sun setting a bit later each day and when we relish the other signs that spring is not far away.  

For those who practice bonsai on outdoor plants in temperate climates, we’re also entering the absolute best time of year for conducting major pruning and repotting work on our bonsai. 

Bonsai can and should undergo minor trimming and shaping work during the growing season. However, hobbyists who want to perform more extensive work, such as major root pruning and repotting and rigorous top pruning and restyling need to be mindful of when they carry out those plans. It’s important to know the best time to do major pruning work on bonsai and when to avoid it. 

Before delving in to the reasons why early spring is the best time to prune bonsai, let’s provide background and context by reviewing the seasonal changes that plants undergo.

The Seasonal Cycle of Plants

Since all plants evolved living outdoors in the areas in which they originated, they naturally rotate through growth and dormant cycles related to their climate and the seasons. The majority of bonsai are kept outdoors, so in temperate climates, they go through a period of dormancy in the winter months. Their seasonal cycle looks something like the following:  

Spring

In early spring, the lengthening days and warming temperatures signal the tree to awaken from its winter dormancy.  Sap begins to flow and the buds formed during the previous year’s growing season start to swell. At the appropriate time for each species, the buds break, the tree flowers and leaves start to emerge. 

Spring is a period of energetic growth for the tree, both above and below the ground. Leaves are infused with chlorophyll, which makes them green in color. Chlorophyll fuels photosynthesis and produces energy the tree uses for living and to grow. Roots expand and lengthen in search of water to support the top growth. 

Summer

With the arrival of summer, temperatures continue to climb and rainfall happens less frequently. The increased heat and lower moisture levels make photosynthesis more difficult. The tree’s growth begins to slow and eventually stops. 

Fall

As the heat of summer gives way to more moderate temperatures, tree growth resumes, both above and below ground. 

The days shorten significantly as fall progresses, signaling trees to begin preparing for winter dormancy. Deciduous trees stop producing chlorophyll and begin to seal off the places where leaves attach to branches.  They produce a layer of cells that protects the branch in the same way a scab protects a scrape on your arm. 

As the presence of clorophyll within the leaf wanes, its underlying natural pigmentation emerges. The beautiful colors we enjoy in the fall are actually the true colors of the leaves! Eventually the leaf dies and falls off the tree. Even though the top part of the tree is becoming dormant, the roots continue to grow in the fall. This makes fall an excellent time for landscape plantings.

Winter

As winter dawns, the tree enters full dormancy. It has armed its structure with hormones that protect against dehydration and proteins that protect cells against damage from freezing. The tree rests until time to awaken and begin the cycle over again in spring.

When is the best time of year to prune bonsai?

For outdoor bonsai in temperate climates, very early spring is the best time of year to prune bonsai. This timing is particularly important if you are performing major work such as extensive restyling and root trimming and repotting. 

Aim to perform the work once the leaf buds have begun to swell, but prior to the buds breaking open and the leaves emerging. The exact timing for this will vary a bit for each tree.

There are several reasons why early spring is the best time of year to prune bonsai:

• As a tree starts to come out of winter dormancy, it is primed for rapid growth. This seasonal vigor enables helps the tree recover more quickly from severe pruning to both the roots and top.
• Pruning the top growth prior to bud break removes unwanted branches before the tree directs energy towards growing them. 
• The process of pruning and shaping top growth is easier when the branches don’t have leaves.
• Pruning the roots at a time when the tree is not yet actively growing puts less stress on the tree.
• Spring’s mild warm temperatures and frequent rainfall provide ideal living conditions. This means the tree doesn’t have to deal with environmental stress as it recovers from pruning. 

What about bonsai work in the other seasons?

Fall is the best time of year to plant landscape trees and shrubs. It can be a good time to work on bonsai as well. As the heat of summer wanes, the milder temperatures and increased rainfall promotes root growth. 

For bonsai, aim to perform the work after temperatures moderate but well prior to the onset of fall color changes, leaf drop and freezing weather. Since most bonsai live in pots, their root balls lack the insulation that trees planted in the earth enjoy. The tree will need time to recover from the work before freezing temperatures set in.

Bonsai hobbyists tend to avoid major tree work in the summer and winter. Both seasons encompass the most extreme weather conditions, which translates to higher levels of environmental stress for trees. In addition, trees’ natural growth cycles are at low points in both seasons. These factors make it more difficult for trees to recover from work performed during these times.

When is the best time to work on evergreen bonsai?

Although evergreens retain their leaves year-round, they go through the same growth cycles as deciduous trees. Therefore, early spring remains the optimal time for major structural pruning and repotting of evergreen bonsai.

When is the best time to prune tropical bonsai?

Tropical plants originated in regions close to the Earth’s equator which experience minimal seasonal variations. Grown as bonsai in temperate climates, these trees cannot survive winter conditions. They must come inside during the colder months.

Tropical bonsai are normally either kept indoors year-round, or moved outside during the warmer months. While performing major work on tropicals is theoretically possible any time of year, you might want to avoid winter. The best time to prune and repot tropical bonsai is spring into summer when the tree is actively growing.

How do I know whether to prune a bonsai?

How often to prune and repot largely depends on tree’s stage of development as a bonsai. It also depends on the growth patterns of the species, the size of the pot that houses it, and the health and vigor of the tree. 

Trees receive their first structural and root pruning when they begin bonsai training. Pruning and/or repotting will often be an annual occurrence for several years. It takes time for the tree’s top growth to achieve the envisioned style. Also, the root ball may require several rounds of reduction in order to fit into a bonsai pot sized appropriately for the tree.  

Once the tree has achieved the desired look and is properly potted it’s a matter of annual maintenance. Assess each year prior to late winter whether you need to perform root pruning in early spring. To check, carefully lift the root ball from the pot, taking care to keep the soil intact. If you see roots circling the edges, it’s time to prune and repot. You’ll also want to decide what, if any, structural pruning is needed and handle that at the same time. 

Younger, faster- growing trees might need to be root-pruned and repotted every year or two. More mature and slower-growing bonsai specimens may only need pruning and repotting every five years or more. 

In this article, we’ve explored how seasonal changes impact the growth cycles of trees. We’ve outlined why early spring is the best time to prune bonsai, both above and below the soil line. We’ve also covered how younger bonsai and those in training need pruning more often than more mature, established trees.