Many gardeners have noticed that grass is greener and plants look healthier after a good rain. They conclude that rainwater is better for gardens than tap water. Is that really true? Is rainwater better than tap water? Does rainwater make our gardens greener?
Let’s open that umbrella and find out.
Key Takeaways
- There is no evidence that rain makes the garden greener.
- Tap water contains just as much nitrogen as rain.
- Nitrate levels in rain are not significant enough to affect nitrates in the soil.
Are Plants Greener After a Rain?
Many believe plants are greener after rain, but it’s probably not true.
I wrote an article a few years ago about the effect of lightning on nitrogen levels and looked for evidence that plants are greener after rain. I found none. I even asked readers to let me know if they found some data – none was reported. I looked again today and found none.
Rain will wash dust off leaves which will make them look greener, but that does not mean the leaves are actually greener.
Rainy days tend to be cooler which is a benefit to plants in the middle of summer.
Plants that are under drought stress will improve after rain.
The amount of water in a rain event is huge compared to most irrigation done by gardeners. For this reason alone, plants will grow much better after rain.
Rain is certainly good for the garden, and it helps plants grow, but there is little evidence that it causes a greening effect. If you find some evidence for greening, please add it to the comments below.
Nitrogen Levels After a Rain
The claim by gardeners is that nitrogen levels in the soil increase after rain. Water floating down through the air picks up nitrogen and deposits it on and near plants. Lighting adds even more nitrogen to the process.
In agriculture, it is well understood that heavy rains reduce plant-available nitrogen and can lead to nitrogen deficiencies in crops. That is the opposite opinion of gardeners.
Four key factors affect the nitrogen level in the soil, and an estimate of the effects is as follows:
- Leaching: – 1.2 g/m2/rain event
- Rain: + 0.5 g/m2/y
- Lightning: + 0.2 g/m2/y
- Mineralization: + 1.5 g/m2/y
The data for these estimates is fully discussed in my post called Soil Nitrogen Levels After a Rain.
Leaching and mineralization have the greatest effect. The nitrate levels in rain and from lightning are insignificant compared to the other factors.
Compare that to the amount added by a single application of fertilizer to your lawn: 4.9 g/m2.
Water Makes Plants Grow Better
An increase in water in the soil will have a positive effect on plants, provided the amount is not excessive. Higher levels of water allow plants to absorb more through their roots. As they absorb water, they also absorb more nitrogen, which results in more chlorophyll, more plant growth, and healthier plants.
Watering from a tap does the same thing, but the amount of water applied to a garden from rain is much more than irrigation water. The simple act of adding rain to the garden will make plants greener and healthier.
Rainwater vs Tap Water
Some people feel that the greening effect is due to the fact that rainwater is much better for plants than tap water. This may or may not be true.
Most tap water does have a higher pH, usually between 6.5 and 8.5. Rainwater is usually acidic, around 6.5. The lower pH makes it easier for roots to access certain micronutrients. This may have an impact, but clay and organic matter have a strong buffering capacity and would probably neutralize the pH effect, especially in alkaline soil.
Remember that rain has been falling for millions of years, and such soil is still alkaline.
Nitrate in Tap Water
What is the nitrogen level in tap water? Is it really much lower than rainwater?
Nitrate is the most common form of nitrogen in both rain and tap water. Natural nitrate levels in drinking water are usually less than 3 mg/l. A safe value is 10 mg/l. About 4 percent of private wells constructed in Minnesota since 1991 have nitrate levels above 3 mg/l.
Nitrate concentrations in rainwater in the USA range from 0.5 to 8 mg/l, with an average of about 1.5 mg/l.
Based on this data, the amount of nitrate in rainwater and drinking water is about the same.
Gardening Myths About Rainwater
The following claims are made by gardeners.
Claim: Some plants like blueberries don’t do well with tap water.
Truth: Blueberries are acid-loving plants and don’t do well at higher pH. It is the pH of the soil and not the water that makes the difference.
Claim: Tap water contains too much chlorine and is bad for plants.
Truth: The chlorine added to tap water is not harmful to plants.
Claim: Letting tap water sit for a few days gets rid of chlorine.
Truth: This is true for chlorine, but not for chloramine, which is also used to chlorinate tap water.
Claim: Rainwater has a high nitrogen content that well water doesn’t have.
Truth: Rainwater has quite a low level of nitrogen. Well water and tap water contain about the same nitrogen level as rainwater.
Claim: Lightning hits the ground and charges the earth, which releases nutrients.
Truth: I doubt that is true.
Claim: Harvesting rainwater in some states is illegal.
Truth: This is a myth that makes the rounds every year, but it’s incorrect.
Claim: Rainwater is better because tap water contains fluoride.
Truth: The amount of fluoride added to tap water is quite safe for plants, but natural water in some locations can contain levels high enough to harm plants.
Claim: Rainwater contains lots of minerals.
Truth: It contains very low levels of minerals. Tap water usually has higher levels.
Claim: Snowmelt actually adds a fairly notable amount of N to plants.
Truth: It adds very little nitrogen, no more than rain. The other problem with snow is that much of it runs away as it melts because the ground is still frozen.
I also have a collection of myths about collecting rainwater.
I often wondered if rain water contains more Oxygen, and maybe that was of benefit to plants.