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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.

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  • I always understood that oxygen was needed by the roots, so an exchange of water or an air stone supplied it, but don't know if that is factual or not. – Evil Elf Apr 28 at 11:26
  • @EvilElf yet there are no roots on new cuttings? Even the fastest plants will need a few days? – Stephie Apr 28 at 11:44
  • Oxygen equilibrates with open water containers fairly quickly. It's much faster, seconds, if you stir, but half an hour will normally do it. -Personal experience with an oximeter. – Wayfaring Stranger Apr 28 at 17:58
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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.

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  • That’s interesting! Is there an indicator for “bad things grow” that can be something like an indicator or that a critical threshold is reached? – Stephie Apr 28 at 12:10
  • I have read that willow put so much of this into the water, people use Willow Water to root cuttings. – Evil Elf Apr 28 at 15:26
  • the only indication i have ever used of something going bad is the smell of the water - otherwise i just top up until rooted and then repot when roots have started showing up. – flowerbug May 26 at 16:24
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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

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  • Hm. As I said - the general procedure is clear (done for decades), the question is just about the water and the “why” when it comes to changing or topping up. As it stands, I am not sure whether this post answers the question? – Stephie Apr 28 at 12:07
  • There is no 'science' in respect of this other than the need for aeration, in fact, most of the time, rooting cuttings in water is not particularly recommended by experts, Schefflera varieties being a case in point - rooting those in water is absolutely not recommended, but it works. – Bamboo Apr 28 at 12:26
  • What does topping up mean in the context of the rooting water? Removing some and adding some new water? – Sam Apr 29 at 2:06
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    the water level in the container drops every 2 or 3 days, so topping up means you literally just top it back up again with fresh water. – Bamboo Apr 29 at 18:26
  • Great. I just saw that happening in my mint cuttings and I thought the water might have got spilled but it seems to have evaporated. I was thinking about your comment only. – Sam Apr 30 at 6:31

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