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.
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.
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:
- Juniperus procumbens
- Juniperus rigida (repot only after the second flush of growth begins to appear following the first trimming in early June)
- Pinus parviflora
- Pinus thunbergii (only if absolutely necessary for health purposes; otherwise wait until fall if the purpose is only aesthetic)
- 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:
- Dwarf Star Jasmine
- Chojubai (repot as the leaves naturally yellow and drop in the mid to late summer heat)
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!
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.
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!