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I'm wondering if it's okay to use water that has drained through a houseplant to water another plant. Is it safe for both the other plant and the soil? What I mean is that we use at least 500ml (2 cups) of water for each pot on an average basis. A lot of the water is drained from the pot, so if someone has no trouble collecting the drained water, can it be re-used to water another houseplant?

This might seem nonsensical, but consider this when you have to water 30-40 or more plants. What one will achieve via this is conservation of water.

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Well, the correct way to conserve water is to watch your water, and make sure you don't have excess drainage. This of course takes time and experience. Till then, use measuring cups, or watering jugs that have measurements on them. Or, if watering with a hose, time yourself. A kitchen timer for example, can come in handy.

Unless you have different soils for different plants, such as citrous fruits etc., you can do this. Collect the excess water, dilute it with fresh water, just in case you have used different or excess fertilizer on a plant etc. and then re-use it.

Still, given the possibility of diseases etc., I am a bit uncomfortable with this approach.

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I haven't found any good reference sources, so this is just my personal experience. It ended up being too long for a comment, so I hope it's appropriate as an answer.

I do this frequently, with both my indoor and outdoor potted plants. Especially outdoors, I tend to be lazy, so if I've already put away the hose or watering can, and notice something overflowing, I just pour it into a nearby pot. As Srihari Yamanoor recommended, I do make sure the potting soil is exactly the same.

It didn't occur to me until just now that some plants may deposit things into the water that aren't good for a different type of plant, so maybe in order to be safe rather than sorry, you should only use the same variety. In my experience, though, it hasn't mattered. In fact, more often than not, my second plant is a different species altogether.

Again, as with Srihari Yamanoor, I have a concern for bacteria and insect growth. For that reason I never let the water from the original plant get stagnant. Although there are some plants that will re-absorb it, some won't, so I take that into consideration when water drains through the pot into the saucer below.

If I know the original plant won't use it quickly, or if I've overwatered to the point where it's spilling out of the saucer, I collect it and quickly use it to water another plant. I've never diluted with fresh water, but it sounds like a good idea.

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    Yeah, it is rare, and would probably never happen in a garden setting, but Creosote, for example, makes its own herbicide, which it releases to kill off or stunt the growth of its competitors. – Srihari Yamanoor Sep 30 '16 at 19:01
  • @SrihariYamanoor is "Creosote" the wrong word in your comment? In the UK creosote is a chemical fungicide and herbicide made from coal tar that is used for preventing wood from rotting, not something that "grows". Did you mean resin produced by Cedar trees, which has a similar chemical composition and properties? – alephzero Sep 30 '16 at 20:10
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    @alephzero, creosote is the common name for a desert bush, also called chapparal. I think the idea that it produces its own herbicide has been debunked though. – Jimmy Fix-it Sep 30 '16 at 21:30
  • Thanks. I looked it up briefly. Apparently, its ability to control its environment is not as great as previously thought because of the chemicals (there is a sign that Joshua Tree National Park has, alluding to this, and I see it every time I drive by), but because it consumes water so efficiently, other plants cannot survive. It appears, they still do produce chemicals that do make it hard for other plants to grow around it. I will try to confirm this through journals if possible. – Srihari Yamanoor Sep 30 '16 at 23:57
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Unless you are OCD about water, which might come in handy someday, watering your potted plants who depend on you completely means giving them the best chances for survival; the best soil, the best water, (learning how to water, watering with bottled, well or filtered water), how to fertilize, the proper fertilizers (less is best), light, air movement, how to assist your plant with pruning...

I would not use water that has drained out of a pot, from a saucer on a REGULAR basis to save on water. Once in a while is fine but the fertilizers, any water borne disease and salts built up from tap waters will be condensed and the type of soil, even potting soil which usually has lots of peat moss can make the water too acidic for other plants. Heavens if one is using garden soil in their pots! Pure plain old water is the only thing I will give my potted plants.

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    Good point on salt accumulation. Water in India is usually high on salt content. – Srihari Yamanoor Sep 30 '16 at 23:58

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