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How is Root Pruning Possible?

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We all know that we can root prune in the spring (when buds swell). What is it about spring that makes it possible to rip all the hair roots off and have no (lasting) deleterious effects? I would like to understand why.

I've also noticed that it seems to be possible to root prune Japanese maples after bud break, to about the point of hardened leaves and still not have any serious trouble. But shortly thereafter, messing with the roots is pretty much guaranteed to be fatal to the tree. It seems to make sense that maples (and possibly all deciduous trees) have enough stored water, carbohydrates, and phytohormones to not need roots in spring until sometime after the leaves are out?
On the other hand, there are conifers, mugo pines in particular, that seem to best be root pruned in August/September (northern hemisphere) instead of spring. Spring is okay, just Aug/Sep makes for stronger mugos and appears to contradict the 'all needs are in store' idea (above). Why would it be that late summer is also a good time to 'rip your trees roots off'?

Root pruning does not involve ripping hair roots.Roots are pruned and usually taking about a third of the root ball I take this from each side and from underneath the root ball, making a slight concave on the underside. This allows the tree to grow further.
The reason for spring re potting which includes root pruning of course, is that the trees are coming into growth for the year and it is less stressful on the tree.

Dan W.:
I believe one reason late summer/early fall works for many... if not all conifers, is because they transition into a root growth phase this time of year. So, similar to spring there is a new push of roots. This is my understanding, someone pleas corect me if I'm wrong.  :)

I think the main reason for why repotting is most successful in the spring is the plant is not respirating--and therefore not transpiring water-- nearly as much as say, july. If the tree is not losing as much water through its leaves(for deciduous trees, not at all because it doesnt have leaves yet) then the roots dont have to supply as much water.  Cooler temps, humidity etc. help with this process during the transition from winter to spring.

Thanks for your thoughts thus far, everybody.

I should have been more careful in the way I said things. When I referred to ‘hair roots’ I meant the extensions of single epidermal cells behind the growing root tip – these are the primary adsorption surface of roots. I’m of the mind that these are inherently destroyed when removing the soil. I too often refer to fine fibrous roots as ‘hair roots’ as you assumed I meant. Nevertheless, I think that when we root prune, we cut off the root tips but they are regenerated from the exposed root cambium.

Your point about transpiration is a good one. The sudden sensitivity of JMs after leaf hardening that I noted could be exactly that transpiration exceeds supply. This should cause the leaf’s stomata to close to save water, but … etc., etc., and down the drain. This much makes sense to me.

Another good idea: spring repot is before a root growth period; late summer / early fall is before another phase of root growth. In other words, there is a period of very little root growth in between! When repotting in spring, we are root pruning in advance of a surge in root growth that lags behind bud break. Messing with the roots at the later stages of this first phase is deadly because there is little or no capacity to replace them at that time. So, then there is a pause and somewhere around September (depending on specie and local conditions, I would suppose) we again are at a point just before another surge of root growth and that is why it again is safe to root prune. Hmmm …

BTW, when I was oh so young I had no patience and no interest in horticulture. Now that I’m not so young, I find trees almost endlessly fascinating. It just stuns me how my wrong my (lifelong) beliefs about how trees were. Trees don’t just wake up in the spring, grow, and go back to sleep in the fall. IMHO, bonsai is manipulating the natural processes of trees to create art. I have the best intentions of getting to the art part!

Thanks again.


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