The why of root work

[eugenev2]

I recently saw a lot of repotting videos and the one question that bothers me is the why. So for context one of the videos i saw was Nigel Sanders repotting a French lilac, video below for reference. So to my question is why, why is repeated severe root cutback needed. I realise general thought process is to cutback hard first repot for a "in development" material to setup the root structure for the future and if the tree dies its not a big loss. But why repeated hard cutbacks of 80 possibly 90%, is this really necessary? If the process takes 15 to 25 years on average, is there a reason for it, or is it a general preference thing? Could a similar result not be achieved by say a 50% reduction next repot. Ps this question comes from someone that is very hesitant to cut anything more than 50% off

 

[misfit11]

Root work is done for multiple reasons. Firstly, we repot to improve the soil substrate. Many times trees come in less than ideal soil. We change the soil out with better draining soil to keep the roots healthy and avoid root rot. Secondly, we trim the roots to improve the nebari, or surface roots. Cutting back large roots in place of smaller roots helps bring them into better scale. We also cut any crossing roots or those growing straight down. Finally, we may repot because there is a health issue. If the soil isn't draining quickly, this can mean the substrate has broken down and isn't allowing water to run through. Repotting can also be done because there may be suspicion of some sort of root pathogen (nematodes, fungal infection etc).

Repeated repottings is similar to pruning on the foliar portion of the tree. As we continuously repot over the years, we are improving the ramification and taper of the nebari, just like pruning does to the branching.

As far as the amount of roots removed during repotting is concerned, that really depends on the species of tree you're working on. In general, deciduous trees can tolerate having more removed than evergreens. A pine, for instance, will not be happy with a substantial amount of its roots being cut back and should only be half bare rooted on older trees. A Trident Maple, on the other hand, can be completely bare rooted (with a hose even), have its roots cut back to next to nothing, and it will bounce back like nothing happened.

I hope that helps

Cory

 

[dbonsaiw]

But why repeated hard cutbacks of 80 possibly 90%, is this really necessary? If the process takes 15 to 25 years on average, is there a reason for it, or is it a general preference thing? Could a similar result not be achieved by say a 50% reduction next repot.

The amount of roots removed isn't really 80%+ every time. As noted by Misfit, different trees are dealt with differently, with pines (for example) not very happy with heavy root work. Even trees that tolerate the heavy root work don't always get that. The goal is to create a nebari similar to the first pic attached. How we get there is the question. I recently repotted a Home Depot tree and, given that it was grown for landscape, the roots needed serious work to get them growing the right way. I may have taken the root reduction too far (probably not) but a serious amount needed to be removed to get this started right. I will never cut it back so much again. Rather, it will be allowed to grow out for like 2 years and then I will cut it back to basically the same diameter - over time, the fine roots will fill out.

 

[leatherback]

I am so happy I started making videos so I do not have to type out long answers anymore.. !

WHY repot?

Different repotting approaches:

 

[rockm]

All of the above is true. Soil replacement, nebari development, etc. But one of the primary reasons for repeated root reduction is simple productivity of the roots. The only portion of the root that can absorb nutrients and water are the feeder roots at the very ends of roots. The rest of the root behind it up to the tree's trunk can't absorb much of anything. In the wild, root senescence into long long roots that have to transport nutrients back to the trunk becomes physically impossible over a century or two, as does the transport of moisture up the trunk. There are physical limits to both and that is a contributing factor to old trees declining and die off.

In bonsai, the roots are pruned to induce an ever more compact and efficient root mass which exchanges old thicker roots for a new crop of fresh feeder roots at repotting time. Root masses are rejuvenated with each repotting that if done correct, pushes more feeder roots into a small space where the owner of the tree provides fertilizer and water and fresh soil on a regular schedule. There is a reason some bonsai have been alive in bonsai pots for hundreds of years--root reduction and replacement.

 

[dr.tenebris]

Thick roots simply don't do anything,
We cut them back to get the thin fine roots that help fed the tree in reduced soil,

 

[eugenev2]

Root work is done for multiple reasons. Firstly, we repot to improve the soil substrate. Many times trees come in less than ideal soil. We change the soil out with better draining soil to keep the roots healthy and avoid root rot. Secondly, we trim the roots to improve the nebari, or surface roots. Cutting back large roots in place of smaller roots helps bring them into better scale. We also cut any crossing roots or those growing straight down. Finally, we may repot because there is a health issue. If the soil isn't draining quickly, this can mean the substrate has broken down and isn't allowing water to run through. Repotting can also be done because there may be suspicion of some sort of root pathogen (nematodes, fungal infection etc).

Repeated repottings is similar to pruning on the foliar portion of the tree. As we continuously repot over the years, we are improving the ramification and taper of the nebari, just like pruning does to the branching.

As far as the amount of roots removed during repotting is concerned, that really depends on the species of tree you're working on. In general, deciduous trees can tolerate having more removed than evergreens. A pine, for instance, will not be happy with a substantial amount of its roots being cut back and should only be half bare rooted on older trees. A Trident Maple, on the other hand, can be completely bare rooted (with a hose even), have its roots cut back to next to nothing, and it will bounce back like nothing happened.

I hope that helps

Cory

Thanks for the reply @misfit11, but my question wasn't really a typical "when should i repot question" but more rather a deeper analysis or attempt at understanding at why some professionals/more experienced growers do what they do and whether my approach is misguided or simply not enough. Attaching screen shots below on the video of what i was referring to. So image named 2022_1.png was the first repot result, a bit drastic in my view, but understandable, 2024_1.png was about slight further along than halfway through the current repot, which is close to where i personally would have left it, besides some of the top roots and possibly a profile cut, but then it continues to 2024_2.png which was close to the end root mass.

 

[eugenev2]

All of the above is true. Soil replacement, nebari development, etc. But one of the primary reasons for repeated root reduction is simple productivity of the roots. The only portion of the root that can absorb nutrients and water are the feeder roots at the very ends of roots. The rest of the root behind it up to the tree's trunk can't absorb much of anything. In the wild, root senescence into long long roots that have to transport nutrients back to the trunk becomes physically impossible over a century or two, as does the transport of moisture up the trunk. There are physical limits to both and that is a contributing factor to old trees declining and die off.

In bonsai, the roots are pruned to induce an ever more compact and efficient root mass which exchanges old thicker roots for a new crop of fresh feeder roots at repotting time. Root masses are rejuvenated with each repotting that if done correct, pushes more feeder roots into a small space where the owner of the tree provides fertilizer and water and fresh soil on a regular schedule. There is a reason some bonsai have been alive in bonsai pots for hundreds of years--root reduction and replacement.

I think @rockm sort of answered my question here, i understand the compact roots is a requirement, we are working with confined space, but i did not realize that this leads to a more ramified and efficient root system. Makes sense.