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A lot of plants need well drained soil. They're said to not like getting their feet wet. The explanation given is that the roots need oxygen; they will rot and die if they're in standing water for too long.

But a lot of the same plants will root if you take a cutting and place it in water for long enough.

An example of this is kiwi vines. They require well drained soil. But their cuttings will root if you put them in a cup of water for a few weeks. I'm doing this right now (for the first time); mine have just began growing roots.

I've also grown a sweet potato in a coffee cup filled with water.

So, why does this work? Why don't cuttings drown instead of grow roots? Why do the roots continue to grow instead of immediately rotting and dying, since they don't have any oxygen?

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The oxygen explanation regarding plants in standing water is too simplistic. When you root cuttings in plain water, there's nothing else in there compared to what's in soil. Your cuttings may take up to six weeks to root, and in that time, you may top up the water with more clean water occasionally to keep the base of the cutting in the water, while it frantically tries to create enough roots for itself to survive.

The soil, though, is full of bacteria, fungal organisms, decaying matter, and various other forms of life which are not visible to the naked eye, and some which are (worms for instance). If the soil in a pot is saturated, either because it has no drainage, or is left standing in a tray of water, not only is oxygen in short supply, but other gas exchange processes are changed, the bio diversity changes, and sulphurous compounds are usually produced in the soil, which is why it starts to smell bad. The bio diversity change is because some organisms die in those conditions, others, such as fungal organisms, may thrive, so the balance of the eco system is out of whack. You could liken the results to the way the bacteria in your gut is altered by taking antibiotics, perhaps causing an overgrowth of thrush in the body. None of that happens in your rooting container of water.

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    I do notice some alge and slime trying to grow in the water with my cuttings.
    – Tim
    Sep 11, 2012 at 16:24
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    Ah yes, I nearly mentioned the algae you will get in your rooting container - if its out of sunlight, its less likely to occur in 6 weeks, but algae always occurs in clear containers with tapwater left in them after a period of time. Phosphates are usually added to tapwater to prevent algae, but obviously, tapwater's not meant to be stood for six weeks in a container. Keeping it in a bright or sunny situation, necessary for rooting really, encourages algal formation. It does not harm your rooting plant; you will have removed it long before it causes problems.
    – Bamboo
    Sep 11, 2012 at 16:40
  • You can always use an opaque container like a mug or plastic cup to avoid algae. Anyways, algae are not the villain everyone makes it out to be; it's just not tidy.
    – CloneZero
    Mar 2, 2018 at 20:49
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In addition to the conditions that Bamboo brought up regarding soaking-wet soil, maybe the cutting's predisposition for certain conditions factors in to this as well?

-I mean some plant cuttings don't like to be transplanted from one medium to another. Best example I know of being pothos.

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  • I'm not sure I understand what you're saying. Can you clarify? I know you can root Golden Pothos cuttings in water. Is that evidence for or against what you're saying, if any?
    – Brōtsyorfuzthrāx
    Jun 1, 2018 at 6:04
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    Sorry, it was a bit weird. What I mean is starting a cutting in water and then planting it in soil once it has enough roots to thrive. They seem to die no matter how much care they receive after being planted. I looked into it and apparently starting pothos in water means you have to keep it in water, according to some plant websites. -I wonder if it had some connection to the op question in light of Bamboo's answer.
    – Beeface
    Jun 2, 2018 at 20:31
  • That helps. I've rooted pothos' in water (I think in a south windowsill) before, and planted it in regular potting soil without issues. I do imagine rooting it in seed-starting mix would be faster, and potentially more effective, though, since more things root well in soil in my experience. It's possible that those who have had issues may have been putting them in containers in areas with too much light. We didn't put ours in the windowsill afterward (I think just while rooting in water). They say Golden Pothos don't like too much light.
    – Brōtsyorfuzthrāx
    Jun 5, 2018 at 3:06
  • But yeah, some plants are more sensitive to light at transplant time, which can make them harder to transplant. So, if pothos plants are like that, that would make sense that they could be difficult, even though it was a simple and easy process the way I did it. I've only done it a few times, though (maybe two).
    – Brōtsyorfuzthrāx
    Jun 5, 2018 at 3:10

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