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Friday - September 18, 2009

From: Austin, TX
Region: Southwest
Topic: Rainwater Harvesting
Title: Rainwater vs. city water in Austin
Answered by: Barbara Medford


Dear Mr. Smarty Plants, My native plants respond better to being watered with 'city' water from a sprinkler than 'city' water applied with a hose at the base of the plant. And they respond even better to rain water. Can you explain: 1. why they respond better to city water from the sprinkler? Is it possible that the chemicals in the water dissipate when sprinkled? 2. why they respond best to rain water? Thanks!


Since we are gardeners, not chemical hydrologists, dealing with the chemical characteristics of water, all we can do is look for research on the subject. Unfortunately, mostly what we found was unsubstantiated opinions, usually in forums of some sort online.  

Question 1. Why do plants respond better to sprinkler water than to ground watering? Ordinarily we recommend the ground or drip watering as opposed to sprinkler dispersal. One reason for that is that overhead watering can sometimes cause fungal diseases or, if done during the heat of the day, can even cause scalding of the leaves. Of course, right now in Austin you can't even water during the heat of the day, so that's a non-issue. We could find no research on whether chemicals in the water dissipate when it is sprinkled. One possibility that occurs to us (and we're talking personal opinion here) is that if there is a lot of clay in your soil and not very good drainage, the water going directly into the soil may not be draining properly, and water could even be standing on the roots. On the other hand, even water hitting leaves from a sprinkler has to go down to the soil, and thence to the roots, to be absorbed by the plant. Then, it is pulled back up through the plant's circulatory system to the leaves, and lost again as water vapor through transpiration. It's difficult to see how overhead watering hitting the leaves could make any difference from water hitting the soil.

Question 2. Why do plants respond better to rainwater than city water?  We have all heard that plants do better in rainwater because the lightning releases nitrogen. True or False? Don't know. Another fact that we hear is that rain picks up air pollution, and the problem of "acid rain" from industrial sources is widely reported. On the flip side, we found opinions that the rain washed the air, and after a few minutes you were getting clean water from the rain. Our question? And just where did those chemicals go when the rain washed them out of the air? Don't know.  According to the Austin Water Utility, Austin's water supply is considered moderately "hard." "Hard" water has a high mineral content, mainly calcium and magnesium but can also include iron, aluminum and manganese. None of those do any harm in drinking water, and are unlikely to affect plants, as the same trace minerals are in the soil and are used by the plants. 

In summary, we don't know the answers to your questions, and can't even refer you to research making a firm conclusion either way. In answer to the question, Is rain water better than city water? Yes, hands down. Why? It's free, and some of it, hopefully, will go into our aquifers and help our drying lakes. We would suggest that you follow your own observations and use the watering method that appears best to you, and hope for rain. 


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