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Why is Rainwater the Best Water For Plants?
With a video about the best water for plants

Rainwater On Green Plants In A Garden.

Have you ever noticed that your plants seem to perk up better after a nice rain?  Rainwater appears to have qualities that aren’t available from any other type of water.  So, what makes rainwater so different?  We’ll delve into this mystery and discover the benefits of rainwater that make it the best water for plants.

Video: The Best Water for Plants

How Can Water Cause Problems in the Garden?

Water quality has a significant effect on plant health.  Some gardeners use their tap water every year without any plant problems.  Other gardeners end up with yellowed leaves and unhealthy plants.

Municipal water from the tap might cause problems.  But what happens if you use hard water or small amounts of chemically softened water?  Also, is there any advantage to using filtered water or distilled water?

Since so many things can affect water quality, we have articles to help you.  Here are the links:

If your water doesn’t cause problems in the garden, keep using it.  Watering with rainwater could be the easiest solution for gardeners with water issues.

The Water Cycle:  How Rain Forms

Water goes through a never-ending process called the “hydrologic cycle.”  The less-scientific name that we normal people use is the “water cycle.”

The National Oceanic and Atmospheric Administration (NOAA) lists nine steps in the water cycle.  Throughout the cycle, water is present in the atmosphere as a vapor or as a liquid or solid on the earth.

In this article, we’ll deal with the first three steps in the water cycle.  Those steps are the ones that create the benefits of rainwater.

Water in the Atmosphere

  • Evaporation.  This is when water from any source turns into vapor and returns to the atmosphere.  Water can even evaporate from raindrops as they’re falling.
  • Condensation.  Water vapor cools and reforms into tiny water droplets.  Droplets can form around airborne particles or ions caused by lightning.  The water droplets become visible as dew, fog, or clouds.
  • Precipitation.  This happens when water droplets are large enough for gravity to pull them back to the ground.  Precipitation includes dew and fog, but it most often refers to rainwater.

Water on the Ground

These steps describe what happens to water after it reaches the ground.  Water continuously returns to the atmosphere by evaporation during any of these steps.

  • Interception.  This is anything that stops the flow of water after precipitation.  It could be puddles, snow, ice, rainwater on vegetation, or rain storage systems.
  • Infiltration.  Soil absorbs the water that reaches it.  The water absorbed depends on the soil porosity, and excess water becomes runoff.

Infiltration combined with percolation gets water to all plant roots.

  • Percolation.  This step is the movement of water through the soil.  Gravity pulls the water lower until it reaches a layer where it can move horizontally.  This is the step that forms aquifers and springs.
  • Transpiration.  Plant leaves have tiny openings called stomata.  When open, they release water vapor and oxygen, and absorb carbon dioxide.  Releasing water vapor cools the plants.
  • Runoff.  Any groundwater on or below the surface that moves to lakes, ponds, oceans, streams, and rivers.
  • Storage.  NOAA states that water storage areas are the atmosphere, the earth’s surface, and below ground.

The Benefits of Rainwater for Plants

Looking at the steps in the water cycle doesn’t answer the question, “Why is rainwater better for plants?”  The secrets to rainwater lie in the first three steps of the water cycle.

Water Vapor Absorbs Oxygen

  • After water evaporates, the vapor absorbs oxygen from the air.  A significant amount of that oxygen remains in rainwater as it falls.
  • Plants absorb oxygen from the rainwater, which benefits their growth.
  • Oxygen also helps to protect the plants from root rot.

Lightning Performs Chemistry on Rainwater

Even when you don’t see lightning, electrical charges move through the atmosphere.  These charges can get to high voltages, but they aren’t always high enough to cause lightning.

Once lightning discharges, many remarkable things happen.  One of them leads to the formation of nitrates.

  • Nitrogen and oxygen like to combine two atoms into one molecule, so they normally exist as N2 and O2.
  • Lightning releases so much energy that it breaks the nitrogen and oxygen bonds.  Some of these loose atoms combine with each other so that N2 + O2  –>  2NO.
  • The 2NO can combine with O2 to form 2NO + O2  –>  2NO2.  These NOx substances are nitrogen oxides.
  • The NOx formed by lightning combines with water to form 2NO2 + H2O  –>  HNO2 + HNO3.  These substances are nitrous and nitric acids.
  • These acids react with metal carbonates in the soil to form nitrates.  For example, with calcium carbonate, CaCO3 + 2HNO3   –>  Ca(NO3)2 + H2O + CO2.

The result is that lightning forms acids that rainwater carries down to the soil.  These acids then combine with carbonates to form nitrates.  Nitrates are valuable nutrients for growing healthy plants.

Legumes like beans, peas, and clover provide more nitrates than lightning and rainwater.  Plants always use nitrates, so every little bit helps.

Rainwater Has the Right pH Level

  • Rainwater has pH levels of 5.6 – 6.0, which is slightly acidic.
  • This is the ideal pH needed to maximize the absorption of micro- and macro-nutrients that plants need to thrive.
  • Carbon dioxide and rainwater form a weak acid called carbonic acid.

Rainwater is Free of Chemicals

  • This isn’t the case in areas that have significant air pollution. Rain cleans the air, so it will contain any chemicals that are floating in the air.
  • Rainwater won’t contain water treatment chemicals like chlorine and fluoride, or chloramine.

Rain Cleans Plants and Soil

  • Falling rain washes dust, minerals, and pollutants from leaves.  That gives them better sun exposure.
  • Rainwater washes salts from the soil.  Potted plants often have this problem since they don’t have rain exposure.  Plants won’t thrive when exposed to salts, so rainwater is a great help for improving the soil at the root zone.
  • Potted plants build up salts in the soil over time.  Rainwater is far better than distilled water for leaching out the salts.

Falling Rain is Naturally Soft Water

  • Since it doesn’t have any dissolved calcium or magnesium, rainwater is naturally soft.  That makes it soft water that won’t harm plants.  Chemically softened water, on the other hand, is harmful to plants.

Rainwater Harvesting Keeps a Supply Available

  • Collection systems make it easy to harvest rainwater.  That way it’s available for future use and for indoor plants.  But check first since some misdirected local governments don’t allow rainwater collection.
  • If you collect rainwater, it can pick up more organic matter as it travels to the rain barrels.  This could include bird droppings and leaf or twig waste; it’s beneficial for plant growth.
  • Rainwater might contain pollutants or bacteria that it picked up from the atmosphere.
  • Keep stored rainwater covered to prevent mosquitoes.


Nature blasts the proof everywhere we look – the benefits of rainwater make it the best water for plants.  Rain falls on all the plants that human hands never touch.  If a plant has a water problem, the water probably came from a treatment system.

Using rain water regularly will lessen any water problems with potted plants and gardens.

Bob Styer

As a child, I hated gardening. That was mainly because Dad expected us to work in the garden every so often even though we thought play was more important. Over the years, though, I've developed a real appreciation for growing things. Whether you're growing plants for food or to enjoy their beauty, gardening can make your life better. Seize the moment!

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