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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 root need oxygen and will die and rot if they're in standing water for too long.

But a lot of the same plants will root if you take a cutting and put 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 coffee cup filled with water.

So, why does this work? Why don't cutting drown instead of grow roots, and why to 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 '12 at 16:24
    
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 '12 at 16:40
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