You’ve seen it. The luminous post-shower greenness of a lawn; the sudden growth spurt of a plant that didn’t seem to be doing much at all; or the effervescence of new blooms on an otherwise sleepy plant. Why are these effects so evident after a good rain and absent with irrigation?
What’s the magic of rain? It’s all about what it has that tap water doesn’t, and what it doesn’t have that tap water does. And this all boils down to chemistry.
Rain water is free of minerals
Rainwater lacks the minerals usually found in irrigation water. In The Woodlands, the water that flows through an outside spigot is the same as what flows from the kitchen faucet—that is, treated water suitable for consumption. This is of course, what you want for water use in the home, but your landscape can actually suffer for it when used in excess.
Chlorine and fluoride are the first plant-offending additives in treated water. Chlorine is a necessary disinfectant and fluoride helps to prevent tooth decay. But nearly all plants are susceptible to chlorine toxicity and many are subject to fluoride toxicity as well—especially common house plants.
Another chemical component to tap water is sodium, which can help remedy the pipe-corroding effects of calcium and magnesium, also present. When a white sediment is present on the outside of containers or on the leaves of plants, it’s evidence of calcium and magnesium. Sodium, like chlorine, is toxic to plants.
Rainwater has the right stuff
Oxygen, nitrogen, and carbon dioxide. Rain highly saturated in oxygen goes right to the roots that take up this vital element. Nitrogen is what makes your lawn and plants seem to glow green after a good rain. Air is 78% nitrogen and this element in its nitrate and ammonium forms comes down in rain and is immediately absorbed by plants through their roots and leaves.
Carbon dioxide is also delivered to plants with rain. When carbon dioxide combines with other minerals in the air, it gives rainwater an acidic pH. Acidic rainwater (and we’re not talking about “acid rain” which has excessive pollutants mostly an issue in the Northeast) helps the soil release essential micronutrients such as zinc, manganese, copper, and iron that are vital to plant growth.
Something can be said about the physical properties of rain too.
Rain penetrates the soil better than irrigation. Raindrops fall at about 20 mph while irrigation droplets fall at about 5 mph. And rain falls uniformly. Both properties help water reach plants’ roots. And they do something else: they help leach salts away from the root zone of a plant where they may have accumulated over time through irrigation. This cleansing effect can have a pronounced effect on new plant growth.
The cleansing effect of rain extends to the entire surface of a plant as well. We can see how foliage glows after a rain washes away mineral deposits, dust and pollutants from leaves. This is a boon to photosynthesis. Photosynthesis is much more efficient when light reaches a plant’s leaves unobstructed by grit and grime.
The benefits of rain water over tap water used for irrigation might even motivate a person to harvest rainwater. So often, rainwater harvesting is presented only as a method for conserving water. Yet it’s more than that. By storing up rain water, you’re also creating a supply of high quality water that your plants crave.
A Drip Irrigation and Rainwater Harvesting class will be offered free by the Township later this spring.