This question is regularly asked by beginners in bonsai. The answer given is usually along the lines of ‘repotting deciduous species must be done as the buds swell and before the leaves open.’
For many years I have stuck with this repotting rule but recently have started to question its validity. Sure it works but is it entirely accurate? Some new information has indicated that this rule may be far more prescriptive than necessary. I have seen staff at commercial bonsai production nurseries potting up trees during summer. They have to, to keep up with demand – does this only apply to young ‘mallsai’ or could it mean that other bonsai might survive out of season repotting?
We are now aware that most Australian Native plants actually prefer to be root pruned and repotted during the warmer months. Why are Aussie natives so different from their exotic cousins? They all have leaves, bark, roots, sap, cambium, etc. Recently some experienced growers pointed out that growers in Melbourne have been repotting cedars (Cedrus sp) during summer with great success. One grower advocated repotting on ‘the hottest day of the year’.
All these facts have led me to question the strict rule on late winter repotting so I have started a trial to find out one way or the other.
These 2 trident maples were selected as test subjects. Note that they are relatively young, expendable plants (which may have some bearing on the results). They were both repotted on 16 Nov 2012. Note that the smaller one is growing strongly and has lots of fresh shoots. The larger tree has been a bit drier and has little active growth.
After watering in to the pots both trees were returned to the growing benches in full sun (although surrounded by other trees which gives some protection)
The leaves on this smaller one had been sheltered from direct sun by the growth above and other plants around it so the leaves were not sun hardy. The leaves on the larger one are still ok.
By Dec 4 both trees have produced healty new shoots with no apparent ill effects from out of season repotting.