A Year in Your Bonsai’s Life

Bonsai cultivation has been happening since the 6th century and along the way growers and artists have learned what does and doesn’t work when caring for trees. Most of what we know comes from hands-on experience and anecdotal evidence as opposed to careful scientific experiments. That means the techniques we use seem to work most of the time, but we don’t always know why. That’s not surprising given that bonsai is an art form. Renaissance artists couldn’t buy their paint and artist supplies off the shelf – they had to discover what worked through trial and error. Now artists can order their supplies online and similarly bonsai can leverage information learned in the nursery and commercial food business, including plant biology, plus chemistry and physics as they apply to fertilizer, growing media, etc. This article explains generally what trees experience at different times of the year and how we influence that to create bonsai. To help understand why something is happening this article includes a bit of science, but this isn’t a scientific paper and it’s not critical to understanding how to create bonsai as an art.

New Year

Deciduous trees dropped their leaves last autumn and went dormant during the cold weather. Some of the sugar produced last year was stored as starch in their roots to be used when trees wake up in the spring. Although the above ground portion appears to be unchanging, the roots are continuing to grow. Conifers slow down their growth in winter but because they retain needles they continue to produce reduced amounts of sugar that powers the tree. They do not store sugar to help them restart in the spring.

All trees continue to need water and fertilizer, just in smaller amounts than later in the year. 

Repotting of most trees is done just before new growth occurs, generally January to March.

Early Spring

In spring warmer weather and longer days of sunlight trigger trees to begin new growth. Deciduous trees convert the starch stored in their roots into sugar and send it up into the tree. This is known as “sap rising” and powers the buds that were produced last summer and fall to open as leaves. 

It has been common practice to prune the tops of trees when repotting. Often called “balancing top and roots” this practice comes from the nursery business. We now know this is a bad thing to do with conifers which do not store a lot of energy in their roots. Removing needles slows down production of sugar which is needed to grow new roots so needle removal and major pruning should be done later in the year.

When buds open into leaves or new candles appear on conifers this is called leaf flushing or sometimes the spring flush. The roots have been absorbing moisture and minerals from the soil and the inner core of the tree trunk transports these to the branches and leaves. The leaves use this plus sunshine and carbon dioxide to produce sugars. This is transported via the outer layer of the trunk to other parts of the tree the tree to power the growth of new branches and roots. Green foliage requires nitrogen so fertilizer with extra nitrogen, such as Osmocote 15-9-12, is added to the tree at this time. The amount of nitrogen applied will vary depending on the stage of development. Developing trunk and branches will consume a lot more nitrogen than refining branches. 

Spring

Trees also use energy to produce flowers for reproduction. Some plants have only flowers of single gender – only producing pollen or only using pollen to produce fruit / seed. But most plants have a mix of male and female flowers. This process requires a lot of energy so we generally remove flowers on bonsai so the tree can focus on producing more branches and leaves.

Most trees produce only one set of leaves a year, but a few species of trees can be coaxed into producing a second or even third flush. The tree often doesn’t have enough energy to produce full sized leaves so new growth will be smaller. That’s sometimes a desirable outcome so we use this in special cases. But if done incorrectly or at the wrong time in a tree’s development you may setback progress or even kill the tree. Article on our site will help you understand when to use these techniques safely.

Plants produce hormones that regulate growth. We now know that hormones control what kinds of growth occur and we can sometimes manipulate them to have the tree develop as we wish. For example, the hormone auxin is produced by the terminal bud at the growing tip of each branch. When a branch is growing in length there is no reason to produce side branches but when the tip is broken in nature, or cut by a bonsai artist, the amount of auxin is reduced and that signals normally dormant axillary buds to open and produce side branches. These buds are located on the branch where each leaf is attached and sometimes almost impossible to see before they start growing. 

When you’re developing a large trunk or main branches you want as much growth as possible to fuel expansion of those parts of the tree. But later when you are refining the smaller branches you want to minimize long growth in favor of lots of smaller growth – and that’s when you would cut off the tips of branches. Auxin also drives root growth. Reducing the amount of auxin, by cutting branch tips, slows down new root growth. That means less nutrients and water supplied to the above ground portion of the tree and so it will also slow down its growth. Pretty powerful effect from just cutting off the tips of branches.

Late spring and summer

Trees grow rapidly, producing more wood in the trunk and branches (larger diameter) and expanding its roots. They also produce buds that will remain dormant until next spring. New growth on deciduous trees is wired at this time – but only where it is needed. Usually the wire only needs to remain on for about a month before the shape has set and the wire can be removed. 

Once primary and any secondary flush of growth has hardened off the amount of nitrogen can be reduced. Replace high nitrogen fertilizer with 5-5-5. (Note: Usually we tell people to wait until autumn, but I wonder if this should be done earlier.)

Autumn

As the weather cools and and nights grow longer hormone levels change and signal the tree to conserve its energy. Deciduous trees drop leaves so less water is needed. Cells in the wood actually reduce the amount of water inside the cells so that if a freeze occurs there will be less damage, and all surplus sugar is stored in the roots as starches.After leaves drop on deciduous trees it is the easiest time to do heavy pruning and wire. Deciduous trees won’t grow during winter but will grow rapidly in the spring, so check often after January and remove before the wire marks any branches. Leaves need to have hardened off before wiring is removed or you’ll likely damage a lot of new growth. 

Winter

Conifers are less likely to be damaged by freezing weather but they reduce the amount of water, nutrients and sugar that are circulated. Because pines will hold onto needles for three or four years, that can result in a tree that is very bushy. Older foliage reduces the amount of sunlight and airflow that reaches the interior of the tree. We pull old needles to open up the tree. Extra sunlight on dormant axillary buds (?) can stimulate growth inside the tree in the next year. Fungus often infects old needles so removing them reduces the risk of fungus being carried forward to new needles. Other conifers require similar cleaning.

Do heavy pruning and wiring after clean up. Conifers are are more flexible than deciduous trees and so need to be wired for longer periods before the shape will hold. Smaller wire will be removed in spring, but heavier wire may need to remain until summer so don’t wire too tightly.

The trees continue to grow, but slower than earlier in the year. They need water and fertilizer but in smaller amounts.