In this instructional video, the presenter explains why most bonsai species are repotted in spring, but rosemary (Rosmarinus officinalis) and related olives must be repotted in midsummer to avoid dieback. He recounts his experience with a specific rosemary bonsai acquired from John Golding, detailing two repotting cycles: the first in July of the previous year into a shallow Thai pot that air‑prunes roots, and a second recent repot in early July to further reduce the root ball and replace the old organic substrate with an inorganic mix of akadama, lava, and pumice. The process avoided cutting roots, instead breaking up compacted organic soil with a wooden stick, and used a temporary glazed pot from Victor Harris before placing the tree in the final shallow pot. He added Oceana Root Boost to aid root establishment. The presenter discusses rosemary’s physiological traits—reduced transpiration and waxy leaves—that allow it to tolerate temporary moisture loss during repotting, contrasting this with typical deciduous trees. He emphasizes the need to complete repotting by late summer to allow recovery before autumn frosts, and notes successful growth despite high temperatures, with plans to rebuild ramification and select a final pot.
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Before I go any further, a necessary word of warning: repotting most bonsai species in high summer will kill them. Spring repotting is the norm for most bonsai species because it's the safest general time to advise people just starting out in bonsai, and it's important you get into the concept of repotting many of your trees at that time of the year and not just when you feel like it. However, as this video discusses, there are some species that really need a different timing for their repotting. A fast-growing and common herb grown in UK gardens, rosemary or Rosmarinus officinalis carries blue flowers in the early spring and like this one will feature natural deadwood features and live veins. However, rosemary are notorious for being impossible to repot in the spring without a huge risk of dieback or death of the tree. This is because repotting for rosemary must be done in the summer, from midsummer onwards, after the first flush of growth slows. Rosemary are part of a family of plants that are related to olives, A species that also needs to be summer repotted. This particular rosemary bonsai was obtained from John Golding last year. He had produced an amazing tree over decades, but it had become too big for the size and girth of the trunk. John had avoided disturbing the roots for over 20 years to avoid losing the tree during spring repotting, and so the rosemary had also been kept in an oversized pot. Last July, I repotted the tree into a smaller Thai pot, a type of pot that air prunes the roots and provides plenty of aeration to the soil. The tree lost a few of its oldest leaves but otherwise thrived. It then flowered for me for months over the winter of 2021-2022. After flowering finished in March 2022, I reduced the size of the tree. It was a pity to remove so much ramification that had been built up, but the tree needed a reduction in size and height. Rosemary don't back bud very well, and removal of all of the foliage on a branch can often lead to its loss, so I made sure I cut back to leave at least one leaf and live shoots on every branch. A few weeks ago, in early July, I repotted the rosemary a second time to reduce the root ball further and complete changing of the old organic soil into an inorganic soil mix of akadama, lava, and pumice. The exterior of the root ball had plenty of good open inorganic substrate, so my aim with this repotting was to carefully break up and remove The inner core of old compacted organic soil using a wooden stick. As with some other species such as larch, pines, and junipers, I had no intention of cutting the roots of the tree. The removal of the soil would allow the root ball to be compressed and fitted into its shallower pot, and the tree would then adjust the number of live roots it, it needed to continue to be supported. I offered the tree up to its new temporary pot from Victor Harris at Erin Pottery that has a creamy beige-green glaze, before using a shallow layer of soil on which to place the rosemary. After tying in the tree and adding some Oceana Root Boost to aid establishment of the roots, a bag filled with a very fast-draining soil mix consisting of akadama, lava pumice, With a lift grit as well. Rosemary have evolved as a species to have greatly reduced transpiration, or loss of moisture through its thin waxy leaves. This means it is able to cope with a temporary reduction in moisture uptake at the roots, whether that be a lack of moisture in the substrate or interference with the roots as we were doing here. Interference with the root ball at the For a typical thirsty deciduous tree on a hot day, where the leaves are constantly requiring moisture from the roots, would be disastrous, leading to wilting and burning of the leaves. Repotting of rosemary and olives can begin at midsummer and must be completed by the end of the summer to give plenty of time for recovery before the first frosts in the autumn. 3 weeks later and all growth is still turgid and grown successfully on the rosemary, despite several days of 35+ centigrade. The tree has now been reduced into a shallow pot more suitable for its trunk girth, as has the foliage mass. My task now will be to build up the ramification again and find a suitable final pot for the tree.
