At this time of year I get a lot of people asking when they should begin re-potting their bonsai. Unfortunately I can’t give a straight answer to a question like that because, as with most things concerning bonsai, it depends upon many factors. However there is a simple answer….
When to Re-pot Bonsai
When you see signs of life.
The notion of re-potting bonsai in the dormant period is a carry over from the old horticulturalists BARE ROOT SEASON. November to April. Now if you are planting a hedge, and your garden is not under water, this will work just fine and has done for generations. However bonsai trees are not planted in the ground and very often they are a little more exotic varieties from foreign climes. assuming that you can treat a variety from southern Japan the same as one from the dark cold and wet shores of Blighty, upon careful consideration, is simply impossible. However conventional wisdom based upon the ‘bare root’ season suggests that early spring is the time to re-pot all bonsai. The authors of those old books have a lot to answer for!
Simply suggesting that early spring re-potting suits all trees is like saying everyone should drive the same car, live in the same house or eat the same food irrespective of whether you are a humanoid biped, a dog or a giraffe. That sounds stupid right? So does doing all your re-potting in early spring, that’s as stupid as suggesting we should all live on cake! Go outside and take a look at your bonsai benches. You will most likely see trees local to your immediate area as well as those from Japan, China, Korea and any number of foreign shores. Also the plant varieties you have there could have originated anywhere, we have been very efficient at moving plant species around the globe over the last 200 years.
Another issue is how you define ‘early spring’. In my early days at local bonsai clubs I was told it was March. From decades of experience I can say that early spring varies from year to year and place to place. For our purposes we can say that early spring has arrived once plant growth begins in earnest. I live by the coast and our winter lows are no more than -2 Celsius. Go 6 miles in land and the temperatures are typically 5-7 degrees lower. If you live in the north, spring is much later than it is in the south. If you live at altitude spring will be later than if you live on a south facing slope at sea level. Everything we do in bonsai should be dictated by our plants. Get the timing right and you can experience dramatically better results than if you get it wrong.
Phenology is the study of the timing of biological events and their relationships to climate and one another. Such events include bird migration, animal hibernation and the emergence of insects as well as the germination and flowering of plants. Our great grandparents were much more closely in tune with these events. Back then a lot of what went on in our country was dictated by the farming year. School holidays for kids were based on when extra help was needed in the fields and gardens. Nowadays most folk get their information about climate and the weather from TV. If you believe what you see on TV, especially the BBC, it might be time to consider your sanity.
Re-potting bonsai at the wrong time can cause a great deal of stress and is a very common cause of failure. With the correct aftercare it’s possible to do almost anything at any time but few folk have the facilities or skill to get away with such actions. Choosing the best time ensures we are working with the plant and giving it the best chance of growing strongly. Re-potting bonsai is a strange operation. Trees growing in pots become stressed over time as the pore space in their soil decreases beyond a critical point. Some trees are happier than others in a pot bound condition. For instance Japanese five needle pine (Pinus pentaphylla) NEEDS to be pot bound in order to do well and develop great bonsai characteristics. On the other hand Chinese elm will suffer very badly indeed if not re-potted every one or two years. So we take a tree that is already a little under stress and increase the stress more by re-potting in order to reduce the stress levels of the plant. But, strangely, it works because trees are tough and WANT to grow and thrive. Re-potting bonsai too early means cut roots in wet soil in cold weather that leads to rot and fungal infection and excess die back of the roots left behind after pruning. The soils we tend to use are also very open and mean cold can get to the roots much easier causing further damage. Also little has ever been said about the actions of beneficial fungi and bacteria within soil. Often these helpful allies will be severely compromised after re-potting.
So, once you have decided to re-pot one of your bonsai how DO you know when is the best time?
In general, and with few exceptions deciduous trees should be re-potted once the buds begin to crack open and show green (or red in some species). The pictures below hopefully illustrate when this point has been reached.
For evergreens the subject is a more complex one but again, as a general rule once you see signs of genuine growth you can jump in.
Living in the UK spring can be elusive. Most bonsai folk are very keen to see their trees growing. Around February we emerge into the great outdoors hoping to see signs of life. However wishful thinking and a magnifying glass won’t bring the spring any closer. It’s not unusual to see warm sunny days at 12 Celsius in February where I live but then it’s not unknown to see snow at easter or frost in May. The fact that your tree is showing signs of life does not mean spring is here. Plants are dictated to by the weather and in particular prevailing temperatures. So, don’t be too keen to get out there, spring is always further away than you think. Also remember that a cold snap can do immense damage and January or February in the UK, even March, can throw up some very cold weather. Those periods can also proffer warm spells lulling us into a false sense that the spring we all long for is closer than it actually is. Out here on the east coast where the weather is mild and, by UK standards, dry we do not begin re-potting until the end of March at the earliest even though our average is close to 450 re-pots every year. Remember the old folk lore of 12 degrees for 12 days before planting.
Just one final tip I can pass on. If you use a damp, NOT WET, soil in which to re-pot your trees it’s not necessary to water in the tree after completion of the work. That’s another of those old ‘bare root’ ideas. Puddling in a hedgerow works fine but puddling in a bonsai does not. Use a slightly damp soil mix, pack it into your pot and trees roots in the normal way and assuming you can keep the plant under cover only give a light sprinkling of water once the soil becomes dry on the surface. Over the following weeks try to maintain the soil just damp but not wet. Once the tree has leafed up treat as normal. Excess wet after re-potting is the biggest cause of failure. Think of it like this, would you want your bare feet in that soil mix? A wet claggy soil is cold and roots will not grow in it. An open soil that’s just damp is a considerably warmer and nicer place to be and will encourage the formation of new roots and fast healing. That means earlier growth, a longer growing season with more development opportunities and better quality bonsai trees and that’s what we are all here for right?