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Is it better to water your plants with rain water collected in water butts? I mean is that better than water straight from the tap?

And is it just a matter of temperature (presumably plants don't like to be soaked with very cold water (as mains tap water is)? Or are there other factors?

I'm interested in the effects on plants, rather than the separate issue of the cost to the planet of using mains tap water.

See also Does water temperature affect plant growth?.

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Apart from the chemical differences, I'd say rain water is a much better choice in drier parts of the world, given the scarcity of water there. Mains water typically comes from river systems which can't always bear the over-extraction of flows and the whole ecosystem suffers. Rain water on the other hand is simply being flowed past your plants' roots prior to resuming its former natural subterranean course (esp. if it was going to flow into the storm water system instead), so it barely impacts the local ecosystem at all. –  Lisa Dec 12 '11 at 23:44

6 Answers 6

up vote 13 down vote accepted

Your tap water will have particular properties depending on where you live, the source of your water, and whether you have some sort of water softener or filtering system. My tap water, for example, is from the public system and is considered 'hard' because it has a high mineral (calcium and/or magnesium) content. If I use tap water consistently on certain plants, I can see white rings of calcium deposits sitting on the soil surface; this can change the chemical properties - such as the pH - of the soil which can affect plant growth. Softened water has higher levels of salt than plants prefer, and these salts can build up in the soil. I've never seen the chlorine in public water causing any ill effects on my plants, but if you have sensitive house-plants you can let chlorine-treated water sit out for 24 hours to let the chlorine evaporate. As long as you live in an area that isn't subject to extremely acidic rain, rainwater is preferable because it's missing things like excessive minerals or disinfectant chemicals.

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If your plant is not special plant such as carnivorous plant, any water from the tap is okay.

The fact is tap water may contain different things such as chlorine, and a lot of different minerals. My plant is drinking water with chlorine day to day, but they are thriving.

In the end, rain water maybe good, but who knows what is inside the water because the contamination from human activities such as car driving and industrial plant releasing exhaust fume from the chimney. Check this link, and this too. You may want to have your own judgement.

I think rain water and tap water are both okay in some extent, on the other hand, distilled water is not really that good because most of the minerals are lost. I prefer tap water ,it is cleaner because chlorine kills bacteria.

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Rain water is always the better choice. City water can contain elements that could eventually harm the plant. When the rainwater is unavailable the tap water will be just fine.

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"In most cases". The additional elements are generally not a problem (look how many of us are forced to water the garden with tap water), but some plants can be very sensitive to it. Carnivorous plants should always be watered with rain water or deionised/distilled water. Many orchids will be the same. –  winwaed Jun 18 '11 at 13:51
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Just beware of rooftop contamination, if that's how you're collecting the rainwater. –  bstpierre Jun 18 '11 at 15:31

Sweet peas, garden peas and broad beans prefer rain water. Although they still grow when watered with tap water the results you get from watering with rain water are vastly improved. Chlorine and other substances in mains water inhibit the uptake of nutrients and thus reduce plant growth and health.

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Rain water is far superior. Water from our public utilities and even water derived from deep water wells lack many things. We as a nation have been using chemical pesticides since the mid 1940's and studies have shown that it's not so much what's "in" the water but what is now "gone" from the water. Our current water systems do not possess the full nutrient profile it once did. Our overuse of phosphates have helped to create soil (hence water) that is out of balance. It is not good for plants or us.

Rain however is readily absorbed by the soil and contains many more nutrients it gains from the atmosphere. the rain falling on the great plains say, may have dust particles that originated in the atmosphere from the Sahara. Also consider the barometric pressure during a rain storm. The barometric pressure of the soil and atmosphere are lined up perfectly to allow for the rain to be utilized to have best results.

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Do you know of any research that supports this? –  kevinsky Jul 3 '13 at 12:18

Rain water has a neutral pH. It's usually softer than tap water and soaks in the ground deeper than a hose watering; people don't water long enough to equal the soak of a good rain. There are particles picked up in the atmosphere during a rain which offer nutrients to the soil that tap water does not have.

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