Rainwater is Better for Your Garden

Have you ever noticed that after a rain, your garden seems to look especially lush, green and healthy?

No, you're not imagining things. Rainwater actually is better for plants.

It comes down to the chemistry of rain water when compared to tap water. The water that comes from a hose is tap water, too. 

Benefits of Rainwater

Rainwater absorbs a small amount of nitrogen (in the forms of nitrates and ammonium) from the air as it falls. These forms of nitrogen are easily absorbed by plants' leaves and roots. Nitrogen is one of the essential elements that plants need to grow, and this is why the plants look greener after a rain. 

Lightening, believe it our not, facilitates this process. When lightening strikes, it splits some of the nitrogen gas molecules in the atmosphere into ions which quickly combine with hydrogen or oxygen. This is how those ammonium and nitrate molecules in the atmosphere are formed, and when it rains, they get picked up in the drops. Pretty cool. 

Rainwater has more oxygen than tap water. Rain picks up oxygen from the atmosphere and the oxygen dissolves in the water itself. This dissolved oxygen is highly available to plants and it is essential for water-breathing animals and microbes in the soil. 

Rainfall brings carbon dioxide from the air down to earth. The carbon dioxide in the air combines with other minerals in the atmosphere and when the rain forms, it becomes ever so slightly acidic. The acidic solution reacts with minerals in the soil such that micronutrients can be released to be taken up by plants. 

Another benefit of the rain water is that it can pick up salts in the soil and wash them deep into the ground below the root zone of most plants. Too much of these salts at the soil's surface, which is left by tap water irrigation, is unfavorable to plant health. 

Finally, the rain water washes off the leaves, flowers, stems, and fruits on the plant above ground. Debris collects on the plants from air pollutants and tap water irrigation, and it microscopically blocks a small but significant amount of sunlight. Think of a dusty haze stuck ta clear glass window - light still gets through, but not as quite as much, and not directly. The debris inhibits photosynthesis, which the plant needs to generate energy to grow and produce the vegetables you love. 

Problems with Tap Water

Tap water is intended for human consumption and human uses. Tap water is provided by human-created water systems and must be made safe for its purposes. Accordingly, it includes chemicals that do not benefit plants and even harm some plants. 

Chlorine is added as a disinfectant, and fluoride is added to prevent cavities in teeth. These chemicals are detrimental to certain plants. 

In some areas, the tap water contains calcium and magnesium, which can build up in pipes and cause problems for the water systems. To combat the buildup, water systems will sometimes add sodium to the water. Additional sodium can be harmful to plants and can impact soil structure.  

Gardeners are usually told to water their vegetable plants at the soil level, and not to get the leaves wet when watering. I didn't that mattered very much; after all, it rains at just about any time of day, and rain gets the leaves wet. I thought that wetting the leaves with tap water from the hose wasn't really different from rain getting on the leaves.

But when you think about it, wetting leaves with tap water isn't so great because of the tap water chemicals that can accumulate on the leaves when the water evaporates. Rainwater doesn't contain these chemicals and minerals. 

All in Moderation

If your garden needs a drink, by all means, water your garden with tap water! Letting your garden dry out (while waiting for rain) is definitely more harmful than watering with tap water. 

If your community garden collects water for irrigation, you get the best of both worlds. Collected rainwater, provided it's not runoff from a dirty surface, is almost as good as a rain shower.

If you get some tap water on the leaves of your plants when watering, it's not too bad, just don't make it a habit. Strive to water at the surface of the soil as best you can. It your plants are heavily covered with dust and debris, it's OK to rinse them off with tap water, since removing the debris will benefit the plants more than the small amounts of sediment or minerals that will be left. 

Now that you know a little bit about the science behind rainwater vs. tap water, you can make better decisions. It's not a simple judgement that rainwater is "good" and tap water is "bad" - each watering method has benefits and drawbacks. Pay attention to the weather forecast and observe your garden closely. It will tell you what it needs. 

 


References: 

Begeman, John. (1998, September). Monsoon Rains Have Hidden Benefits For Plants. The University of Arizona Cochise County Master Gardeners. https://cals.arizona.edu/cochise/mg/monsoon-rains-have-hidden-benefits-plants

Redlands Daily Facts. (2017, February 13). For plants, rain has benefits that tap water simply can't deliver. The Mercury News. https://www.mercurynews.com/2017/02/13/for-plants-rain-has-benefits-that-tap-water-simply-cant-deliver-9/

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