Municipal tap water is treated with various chemicals to eliminate pathogens and ensure it is safe for human consumption. While this treatment makes the water potable, the additives can pose challenges for sensitive houseplants and garden soil. Allowing water to sit before use helps neutralize or remove these compounds, making the water more hospitable to plant roots and beneficial soil life. This preparation step promotes better plant health.
Understanding the Harmful Chemicals in Tap Water
The primary concern in municipal water systems is the presence of disinfecting agents: free chlorine and chloramine. Free chlorine is a highly reactive chemical added to kill bacteria and viruses. This oxidizing property can damage plant cell walls and burn delicate leaf tips, particularly on sensitive species.
A more pervasive issue is the effect of these sanitizers on the soil’s microbial ecosystem. Both chlorine and chloramine, a stable compound formed by combining chlorine and ammonia, can kill the beneficial bacteria and fungi that help plants absorb nutrients. Chloramine is a persistent disinfectant, remaining active longer in the water supply and being harder to remove than free chlorine. Beyond sanitizers, water hardness—the concentration of dissolved minerals like calcium and magnesium—presents long-term problems for plant growth.
How Long to Let Water Sit for Chlorine Removal
The practice of letting water stand targets free chlorine, which is a volatile gas at room temperature. When exposed to the air, dissolved chlorine molecules naturally escape from the water in a process called off-gassing. Most recommendations suggest letting a container of water sit undisturbed for a minimum of 24 hours to effectively reduce free chlorine to non-harmful levels.
The speed of this evaporation depends highly on environmental factors, particularly the surface area exposed to the air. Using a wide-mouthed bucket or watering can allows the chlorine to dissipate faster than a narrow-necked bottle. Increasing the water temperature also accelerates molecular movement, encouraging off-gassing. While a full day is a good standard, water with a high initial chlorine concentration may benefit from up to 48 hours of sitting time for complete removal. This method is simple and effective, but it addresses only volatile chlorine and does not remove other common water contaminants.
Addressing Chloramine, Hardness, and Other Tap Water Concerns
The simple act of sitting water is largely ineffective against chloramine due to its chemical stability. Utilities use chloramine for long-distance water preservation because it does not readily off-gas. For water treated with chloramine, alternative methods are necessary, such as using a carbon filter, particularly one containing catalytic carbon designed to break down the chloramine bond.
Chemical Neutralizers
An effective and immediate solution is the use of chemical neutralizers, often containing sodium thiosulfate or ascorbic acid (Vitamin C). These substances instantly break the chemical bond of both chlorine and chloramine, making the water safe for plants immediately.
Addressing Water Hardness
Sitting water does not remove the dissolved calcium and magnesium ions that cause water hardness. Over time, these minerals accumulate in the soil, potentially raising the pH and causing nutrient lockout. Nutrient lockout prevents the plant from absorbing essential elements like iron. Solutions for excessive hardness include using reverse osmosis filtration or periodically flushing the soil with distilled or rainwater to wash away the mineral buildup.