We’ve all noticed that after a rain, plants perk up. They look greener and brighter. They suddenly put out new growth and flowers, even though they were regularly watered with tap water. Discover the amazing ways that rain water is better for your plants than tap water, and why desert plants especially love the rain.
It’s not your imagination — plants really do look better and seem more alive after a rain. When I delved into the benefits of rain water over tap water, what I found turned out to be more interesting and complex than I would have imagined! (Especially reason #9. After learning this, I’ll never look at a rain drop in the same way again.) š²
#1 Rain Water Provides Bioavailable Nitrogen
Let’s start with the most well-known benefit of rain water — it provides nitrogen to plants, which makes them look greener.
Nitrogen makes up about 78% of the atmosphere, but plants can’t absorb it from the air. Fortuitously, rain water contains nitrate (nitrogen + oxygen), which is the most bioavailable form of nitrogen for plants.
Nitrogen is one of the three key macronutrients that plants need and, in the desert southwest, it’s the most common nutrient deficiency. Lack of nitrogen turns leaves yellow-green, which explains why a blast of nitrogen might green them up.
While you don’t need lightning to turn atmospheric nitrogen into nitrate (NO3), it helps. Nitrogen gas is chemically stable, but lightning provides the energy needed to combine it with oxygen. The resulting nitrate dissolves readily in rainwater and is delivered to your plants.
#2 Rain Water Soaks All the Roots
Most desert gardeners use an irrigation system that places an emitter or two somewhere near each plant. While this can give a plant enough water to survive, only a small portion of a plant’s root system gets water this way. While you can’t know exactly how far your plants’ roots extend, here are a few amazing examples.
Most cactus have root systems that are at least as wide as the plant is tall and, in some cases, much wider. Saguaro roots can extend as far as 100 feet!
According to the University of Arizona, aĀ treeās root systemĀ generally extends 1.5 to 4 times beyond the width of the tree canopy.
So every time it rains, you can feel good knowing that all the roots of all your plants are getting water.
#3 Rain Provides More Water Than You Think
When it rains, your plants may be getting much more water than you realize due to run off. One inch of rain results in 600 gallons of water flowing off 1,000 square feet of roof. That’s a lot of extra water flowing off your house and potentially onto your plants.
Rain also runs off other hard surfaces — sidewalks, patios, and driveways — and hopefully some of that makes its way to your plants.
#4 Rain Water Has a Lower pH
Desert soils are notoriously alkaline (i.e., high pH). They are loaded with calcium carbonate (CaCO3) which causes the pH to be high, usually aroundĀ 8 to 8.5. And when the soil is alkaline, it binds with some minerals, making them unavailable to plants. Watering with hard tap water, which is also alkaline, further increases soil alkalinity.
Rain water, however, is naturally slightly acidic.(Carbon dioxide in the air reacts with water to form carbonic acid.) This is another reason your plants look better and are happier when it rains. Rain water, at least temporarily, lowers pH and frees up some of these bound minerals.
#5 Rain Washes Away Soil Salts
Unlike tap water, rain water has no chlorine, fluoride, or other added chemicals your plants may not like. Additionally, it is low in minerals, which is actually a good thing for desert plants.
If youāve gardened in the southwest for a while, you’ve probably heard that your soil will accumulate āsaltsā (i.e., minerals) from our hard water and should beĀ occasionally flushed. This is true for both potted plants and plants in the ground.
But flushing them with the same tap water that created the problem is not ideal. But rain water flushes plants with relatively pure rain water.
I have a rain barrel that doesn’t meet all my watering needs, but I prioritize that water for my container plants — both indoor and outdoors — to prevent mineral build up in my pots.
#6 Rain Water Cleans Your Plants
Plants have pores called stomata that open and close to release water vapor as part of a process called transpiration. Transpiration helps to keep plants cool during hot weather.
Clogged stomata can reduce a plant’s ability to transpire and cool itself. You can and should clean the dust off your plants with a shower from your hose occasionally to improve transpiration.
But rain water works even better than hosing them off, since tap water can leave mineral deposits on leaves.
#7 Rain Water is Cool
Plants don’t appreciate being watered with hot water, but sometimes it can’t be avoided. There is no such thing as “cold” tap water in the summer here in Tucson. Sometimes the water coming out of the cold tap is hotter than the water coming out of the hot tap!
But the temperature of rain water is cooler than air temperature. And if your plants could talk, I’m sure a cool rain shower would have them saying “ahhh.”
And of course, summer storms are usually accompanied by dramatic temperature drops, which are enjoyed by plants and people alike.
#8 Rain Increases Humidity
Have you ever noticed that some plants don’t bloom or leaf out after being watered, but suddenly come to life after a rain? This phenomena can be due in part to an increase in humidity. After a soaking rain, the humidity can stay higher for several days.
Even desert natives appreciate some humidity. Texas rangers, for example, typically bloom after a rain but not after watering. They sometimes bloom before a rain due to the pre-storm increase in humidity.
#9 Rain Enhances Your Soil’s Microbiome
Just as rain water is better for your plants, it’s also good for essential microbes that live in the soil. After a rain, soil microbes multiply rapidly.
(Compare this to tap water. Chlorine is added to tap water to kill microbes, and it can also kill beneficial microbes in your soil.)
Also, rain water replenishes soil with new microbes. Scientists have identified thousands of species of bacteria, fungi, and other microbes in clouds. So when it rains, it’s literally raining down microbes onto your soil!
Here’s an image of a microscopic nematode found in rain water.
I find this fascinating!
You can see more stunning images of amoeba, algae, and other microbes found in rain water at CanadianNaturePhotographer.com.
You might never look at a rain drop in the same way again.
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Author Bio
Deane Alban is the creator of Southwest Gardener. She is a science writer with a bachelor’s degree in botany from the University of South Florida. Gardening is her lifelong passion. She’s been gardening in Tucson for over 15 years.
