Bonsai enthusiasts look forward to late winter, as this is the time to perform the three bonsai tasks that are the most crucial for their trees’ development.
Late winter easily qualifies as the most significant time of year for bonsai practitioners who own temperate species. It’s hands-down the best time to perform all three types of structural work necessary to develop a tree into a bonsai. Those tasks include pruning, wiring, and repotting.
Aim to complete these three operations shortly before the bonsai breaks its dormancy and starts the spring growth spurt. While the exact timing varies by location and by species, the window for performing these tasks can be relatively short. It’s important to closely monitor trees this time of year and do the work as soon as each tree shows signs that it is close to breaking dormancy.
Late Winter: The Time to Prune
Plan to prune each temperate bonsai tree as soon as you notice its buds just beginning to swell. The bare branches of deciduous species this time of year make styling decisions easier by showing off the tree’s basic structure.
You’ll need to check your trees frequently and make time to prune any that show signs of coming out of dormancy. As I write this article, it’s early February in Ohio. I just checked my trees and found several that need these late winter bonsai tasks performed right away. It’s time to get busy!
Types of Pruning
There are three types of pruning used in bonsai, each with a varying level of intrusiveness. The type used depends largely upon the tree’s stage of development.
Maintenance pruning is the lightest and least invasive type of pruning. Frequently, it is the only type of pruning performed on mature and maturing bonsai specimens. It involves removing new growth from the previous year that interferes with the intended shape of the bonsai tree.
Think of it as an annual housecleaning that restores order to the look of the bonsai. Getting rid of extraneous growth ahead of spring helps to properly direct the tree’s energy and development in the upcoming season. Repeated each year, maintenance pruning keeps a bonsai on the path to refinement and maturity.
Replacement pruning is more invasive and is used to change or establish the structure of the bonsai. It can involve shortening branches and/or lowering the tree’s height. Height reduction is accomplished by removing the tree’s leader and establishing a new one using a secondary branch. This type of pruning makes the tree’s structure more compact and also serves as an important step in developing the bonsai.
Structural pruning is the most invasive form of trimming used on bonsai. Also known as hard pruning, it is most most suited for trees in the initial stages of bonsai training. Field-collected stock, larger nursery plants, and trees and shrubs repurposed from landscaping commonly undergo structural pruning.
Structural pruning usually involves reducing the height of the plant – sometimes quite dramatically. The hobbyist makes a tapered cut just above a branch or bud that will become the tree’s new leader. In addition, branches are severely pruned or removed entirely. Sometimes, bonsai hobbyists will reduce this type of stock to nothing more than a short, interesting trunk. The trunk will sprout new branches and begin to fill out again. Over time, the new growth is developed, yielding a nice bonsai specimen.
When pruning, be sure to use cut paste to cover the wounds made by medium and larger cuts. This keeps protects the wound while it heals, preventing infection.
To learn more about bonsai pruning, check out our article about the best time to prune.
The second of the three late winter bonsai tasks is wiring. While it isn’t critical to wire only during winter, it’s still the best time for temperate bonsai, particularly because wiring goes hand-in-hand with pruning.
For deciduous trees, the lack of foliage makes it much easier to assess trunk movement and taper, apply wire and shape branches. For evergreen species, wiring can be time-consuming, so winter works well since there are fewer competing bonsai priorities.
It’s fine to wire trees anytime during the winter months, although it works best if wiring comes after pruning. For trees that just need a light maintenance pruning, trim and wire them earlier so there is less to do when the pruning/repotting rush hits.
Our article about the basics of wiring for bonsai provides additional information about wiring and applying it to trees.
Third in the list of late winter bonsai tasks is repotting. The ideal time of year for repotting most temperate bonsai is, again, just before the tree awakens from dormancy. Because the tree is “asleep” and not actively functioning, repotting at this time reduces stress to its systems. Newly trimmed roots are vulnerable to frost damage, so performing the work at the end of winter helps ensure the roots stay healthy.
For a mature bonsai living in a complementary and properly sized pot, the repotting process is usually minimally invasive. Every few years, the tree comes out of its pot for a light trimming around the edges of the rootball. It usually goes back into the same pot, along with a dose of fresh soil to fill in the gaps.
In contrast, a bonsai in the early to middle stages of training needs more frequent repotting. Shorter intervals are necessary in order to develop a fine, shallow root system. Every two years is a typical repotting schedule for trees in training.
A major repotting of deciduous species often involves “barerooting” the tree by removing all the soil around the roots. Thick, misshapen and long roots are cut back or removed entirely to encourage finer roots to grow.
Once buds on deciduous trees start to open, the time for repotting has passed. A second, but less optimal, opportunity to repot will come later in the year after the tree’s growth has slowed and the leaves have matured.
In contrast, evergreen species such as pines and junipers offer a longer repotting time frame. Repot pines until needles start to emerge from the new candles and junipers until new growth appears.
Pines and some other evergreens should not be bare-rooted during the repotting process. These trees are highly dependent on the mycorrhizae that populate the soil around their roots.
Mycorrhizae are fungi that coat roots and/or form a network between the roots of a tree. Visible as fine, white filaments in the soil, the mycorrhizae form a crucial symbiotic relationship with the tree. They absorb water and nutrients from the soil and direct it to the tree, while the tree supplies the fungi with sugar. Therefore, leaving some of the soil around the root ball preserves a portion of the mycorrhizae, which can then repopulate after repotting.
Containers for Repotting
Trees in early stages of bonsai training often live in oversized bonsai pots, large containers, or even in the ground. This practice allows roots to grow more freely. This is especially true for field-collected specimens or those repurposed from landscaping.
With each major root pruning, the tree can be repotted into successively smaller pots or containers. Within a few years, the root system development and the top growth training come together to yield a respectable bonsai in a properly sized pot.
Freezes can damage a bonsai’s newly pruned root system. Given that late winter is the time to repot, these trees need protection if the weather turns and temperatures dip. Keep a keen eye on the forecast and, if necessary, move them to an unheated garage, greenhouse or other shelter.
Performing late winter bonsai tasks such as pruning, wiring and repotting allows trees time to recover prior to coming out of dormancy. It also helps direct their vigorous spring growth towards making a better bonsai specimen.
Temperate bonsai experience a window of time in late winter that is ideal for performing the operations most crucial for their development. Hobbyists need to monitor their trees during this stage of the season in order to properly time these three late winter bonsai tasks of pruning, wiring and repotting.