When rooting cuttings in water, should the water be exchanged regularly?

Question

When my mom roots cuttings in water, she always insist that the water in the glass / jam jar / other container should not be exchanged, only topped up as needed. She even will place newer cuttings in the jars with older (by days or weeks) cuttings that have already started rooting. She claims that this process will actually promote root development. But she doesn’t have any kind of explanation (not that she’d bother with it¹), just a “this is how it should be done”.

On the other hand, I am thinking about the growing biofilm (algae, bacteria...) and how this would probably affect the cuttings negatively. I would assume that starting with a clean container and occasionally switching the water when it starts to get murky - just like the recommendation for cut flowers - to prevent rotting is the way to go.

So could someone please help clarify:

Which is the better or correct approach? Should the water be exchanged regularly and the container cleaned or should it best be left alone until the cuttings are ready to be planted? Why?

I am looking for a reasonably scientific or at least logical explanation, please.

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¹ She’s been successful in rooting just about everything, even unintentionally, although she has a bit of a “survival of the fittest” attitude when it comes to caring for her plants - she’s not the most diligent person with watering, fertilizing or repotting.

Answer 1

The water may work better over time, until bad things grow.

The cuttings produce auxins, the plant hormone that causes roots to grow.

This does leak out of the cuttings; an old technique for striking difficult cuttings while obeying organic rules is to place a cutting in a group of other cuttings.

But once the roots have started, I think it is fine to change the water.

Answer 2

I have propagated cuttings in water (even for plants where this procedure is not a recommended method) for years and never changed the water, just topped it up as necessary - but once the cuttings have sufficient roots, I pot them up, and then the vessel they were in is thoroughly cleaned before reusing for other cuttings. I have on occasion added one or two extra cuttings from the same plant to the same container which might have had pre-existing cuttings in it for a week or so - I cannot say they rooted faster, usually the ones that had been in there originally needed potting up before the newer additions, so it seems to confer no advantage in terms of more rapid formation of roots.

The water in the container does need to be aerated occasionally, but topping up seems to take care of that perfectly well. Otherwise, there is no advantage to new cuttings in continuing to use the old water in a dirty vessel, it's more likely a disadvantage because of possible contamination, but regardless, in the end what counts is doing what works, not what the books or science might say - your mother clearly finds her methods successful, and is likely not prepared to change her routine on the strength of 'if it ain't broke, don't fix it'.

Average time span for good roots to form is somewhere between 4-6 weeks- if the container is in sunlight, there may be excessive algal growth, so it's best to stand it in a brightly lit (daylight) spot (a windowsill is ideal) that does not receive sunlight. The water can be changed if it gets so murky you can no longer clearly see the roots during the 4-6 weeks, but I've never found it necessary. Further info here https://www.thesill.com/blogs/diy/plant-propagation-for-beginners

Answer 3

Done what your mother does for years without understanding myself until recently. Two general kinds of cuttings in my experience:

1. easy cuttings: grow great in water without funking it up and will grow roots easily -- just top off the water as it evaporates no need to change, easy breezy with lots of roots in few days/weeks.

2. hard cuttings: funk up the water in a few days and need to replace entire water every couple of days and cuttings rot easily, won't root, water smells bad, just hard to root in water.

I just got to understand that easy cuttings root without funking up the water because very good at producing the hormone "auxin" which encourages root growing and prevents bacteria and is the stuff they put in commercial growth hormone. So these easy cuttings generate their own "growth hormone" and fill their water with this auxin and if you put the hard cuttings in the same auxin filled water as the easy cuttings, then the hard cuttings will use the auxin generated by the easy cuttings and have a much much better chance of rooting and thriving without funking up the water. The easy cuttings keeps things clean in the water and BOTH cuttings do well and all you need to do is "top off" and no need to change the water because actually need that auxin filled old water to keep things fresh and growing.