Repot your bonsai in summer?

Repot your bonsai in summer?

posted in: Bonsai Care | 27

Can you repot a bonsai in the summer?

It’s a question I’m often asked, and the short answer is yes…sometimes.

There are many factors to consider when deciding if a bonsai can be repotted in the summer heat.  These include the purpose, species, and aftercare of the tree in question. Let’s take a look at each of these factors in detail.

Purpose

First, what is the purpose of repotting in the summer? Summer repotting should only be done if one of the following conditions is met:

1. The tree has been styled on a new angle, and repotting can be done with little disturbance to the roots. This is perhaps the most common reason we repot trees in summer at professional bonsai nurseries in Japan.  It should only be done if the tree is in good health.

2. The tree has sharply declined in health over the course of the summer.  This could be a result of a number of factors, including compaction of the soil, poor drainage, lack of oxygen penetration to the root system, fungal or pest issues, etc.

3. The tree is a tropical or subtropical species (or very rarely, a temperate species) that benefits from repotting during the hotter parts of the summer.  This includes species such as Ficus, Dwarf Star Jasmine, Gardenia, and Chojubai among many others.

Species

Next, what species can be repotted in summer?  Surprisingly, almost any.  However, there are certain circumstances that should be considered before summer repotting.

First, many temperate coniferous species can be repotted in the hottest part of the summer as they go into summer dormancy.  Many species have evolved, through the processes of natural selection, to fall into a dormancy-like state during the hottest, driest part of the summer as a defense mechanism against droughts.  Following this summer dormancy period, as trees “wake up” once again, they will often produce a flush of new foliar and root growth.

During this period of summer dormancy, some temperate coniferous species can be repotted if absolutely necessary (i.e. if one of the above mentioned purposes is identified).

Here is a list of temperate coniferous species that can be repotted during the summer:

  1. Juniperus chinensis
  2. Juniperus procumbens
  3. Juniperus rigida (repot only after the second flush of growth begins to appear following the first trimming in early June)
  4. Pinus parviflora
  5. Pinus thunbergii (only if absolutely necessary for health purposes; otherwise wait until fall if the purpose is only aesthetic)
  6. Pinus densiflora (only if absolutely necessary for health purposes; otherwise wait until fall if the purpose is only aesthetic)

Second, temperate deciduous species should only be repotted in summer if absolutely necessary for health purposes.  In the event that a temperate deciduous species, such as a Japanese Maple or Japanese Flowering Apricot, begins to wilt, drop leaves, or becomes peaked in the summer (due to the factors mentioned above), the tree should be slip-potted (meaning roots should not be trimmed or teased) immediately into a larger wooden box container.  This will allow more oxygen penetration to the root system.  Otherwise, temperate deciduous species should only be repotted in Spring.

Third, tropical, subtropical, and certain temperate broadleaf evergreens can be repotted in summer.  These species include (but are not limited to) the following:

  1. Ficus
  2. Dwarf Star Jasmine
  3. Gardenia
  4. Chojubai (repot as the leaves naturally yellow and drop in the mid to late summer heat)

 

Soil Components

As always, I recommend using a basic mix of akadama, lava rock, and pumice.  Use a 1:1:1 ratio respectively for conifers and a 2:1:1 ratio respectively for deciduous and broadleaf evergreen species.  For a comprehensive look at these components, check out my online tutorials HERE!

online-bonsai-course

Aftercare

If a tree has been repotted in the summer, the subsequent aftercare is very important in maintaining (or improving) the tree’s health.  Trees repotted in summer should be protected from direct afternoon sun for a minimum of ten days following repotting.  At Kouka-en in Japan, we use a combination of shade cloth (40% cut) and bamboo rollers (90% cut), applied from approximately 11am to 4pm daily during the summer months.

Lastly, fertilizer should be cut and foliage should be sprayed with water twice daily (once in the morning and once in the early evening) for ten days to two weeks following summer repotting.

In Closing

While repotting trees in the summer is certainly acceptable, it goes without saying that the best time to repot most species is in fact in the Spring.  However, if the above pre and post conditions are met, then by all means repot your bonsai in the hotter summer months.  Just be sure to provide the necessary aftercare to your tree!

 

 

 

27 Responses

  1. sdavis
    |

    What are rollers and what abut Pinus mugo, which seems to do best with repotting mid-June thru July?

    • Bjorn Bjorholm
      |

      There is a photo of the bamboo roller in the article. I’m not familiar with summer repotting of Mugo Pine.

      • sdavis
        |

        Thanks! Excellent article!

  2. Todd Ellis
    |

    I liked this, very helpful, and would like to share it with our bonsai club. May I have your permission to forward it to them?

    • Bjorn Bjorholm
      |

      Todd, absolutely! Feel free to share it with whomever you’d like.

  3. Andy Youtz
    |

    Awesome article. I know this will help many people. My second year in bonsai I repotted a larch in June, was a cool little tree. Lesson learned.

    Thanks for the article!

  4. Karin Rock
    |

    Great info!

  5. Vance wood
    |

    Thank you for publishing this. I have been advocating summer repotting for years, to the cat calls and accusations of being in error to the point of being stupid. I am looking forward to the possibility of you visiting our club next year.

  6. Rohit tanwar
    |

    very nice content 🙂 . I want to ask that whenever i repot a pine or a juniper can i add micorriza in the soil mix to enhance the root system and mineral absorption.

    • Bjorn Bjorholm
      |

      Absolutely!

      • Rohit Tanwar
        |

        ok thanks 🙂

  7. Gary Swiech
    |

    Thanks for the article.
    I have successfully re-potted in Summer. Zelkova can also be done this way after leaf pruning.

  8. Victor Galloza
    |

    Very good article

  9. Vance Wood
    |

    I have also done Hinoki Cypress.

  10. Bryan Johns
    |

    I just started into bonsai and was so excited I had to pot a HiNokia Cypress i bought yesterday. I spoke with a gentleman yesterday who suggested it was “risky” to do so and sent me a link to this thred. Awesome information Bjorn. Im not sure whats going to come of my first Bonsai, but i intend to be dilegent in aftercare..
    What’s more interesting is that gentleman turned out to be you father. I was at your Nursery just today and met your parents. Funny story. I had been watching content you had established on you tube for a few weeks and reached out to a local nursury and the nice lady was giving me all sorts of helpful information. She suggested I look up some of Bjorn Bjorholm’s content. I told her I knew you and had already done that… then she said “that’s my son.” Lol…I thought that was so cool. Anyway, nice people and what a small world…it would seem. Your collection is so cool. I especially liked the “rock pot” you collected.

  11. Piotr Bryja
    |

    Hi,
    “The tree has been styled on a new angle, and repotting can be done with little disturbance to the roots. ”
    Do you mean, that if the tree had been styled hardly in the spring, then if the tree is in good condition we can repot it in the summer? Am I right, or not? 🙂

    • Bjorn Bjorholm
      |

      Hi Piotr,

      Thanks for your question. What I meant is that if the tree is styled during the summer and requires an angle change, it can be repotted immediately after styling so long as the roots don’t require heavy work to get the tree situated on the new angle in the pot. If a tree is styled in the spring, by all means repot it in the spring if possible.

      • Piotr Bryja
        |

        OK, I understand, thank You 🙂

  12. Olimpio Menendez
    |

    Can Turface substitute Akadama when we repot the bonsai? In what ratio? I saw that in Japan only use Akadama. What kind of addition you add when prepare soil?
    Thank you in advance. Mr. Menendez

    • Bjorn Bjorholm
      |

      Mr. Menendez,

      Turface is a terrible substitute for akadama. It doesn’t drain well and will cause poor root growth and potential root rot. If anything you can just use pumice and lava rock and simply water and fertilize more frequently to offset the lack of akadama in the mix.

  13. Kevin
    |

    Hey Bjorn,

    I recently purchased two nursery stock trees online and they came looking very healthy, but in a poorly draining soil (they were not intended for bonsai purposes). The first tree is a Japanese white pine, and the second one I believe is some form of cypress (I live in China and I wasn’t able to find a precise translation for the Chinese name). I want to repot because of the existing soil and because I know that the roots are not exactly growing in the right way. Should I do it now, or wait until spring and leave them alone for the moment?

    Thank you!

    • Bjorn Bjorholm
      |

      Kevin,

      It seems like you’ll need to remove large amounts of the old soil, so I recommend waiting to repot until next spring. For now, take a leather punch or chopstick and aerate the soil in its current pot. That will help with drainage until you can repot next spring.

  14. Brian E
    |

    Hi Bjorn,

    Would it be ok to up-pot a mugo pine, shimpaku juniper, and Japanese black pine into a bigger pot during the summer? I plan on disturbing the rootball as little as possible. I just want to lift them out and drop them into a bigger pot. Currently the temperature Ranges from the high 90s to the low hundreds. Thanks

    • Bjorn Bjorholm
      |

      Absolutely! As long as your not really disturbing the roots, it should be no problem.

      • Brian E
        |

        Awesome. I was just worried about the maybe shocking some of the outer feeder roots. Would I need to keep them out of direct sun for a couple weeks after I up-pot them?

        • Bjorn Bjorholm
          |

          Yes, that would be best.

        • Vance Wood
          |

          You can in my experience covering a lot of years with the same material you can repot Shimpaku and Mugo Pines even with drastic root work all summer long after the summer soltice.