Does letting tap water sit out improve it for watering plants? Understanding the science behind aging tap water for gardening.

Context

Many gardening enthusiasts recommend letting tap water sit before using it to water plants. The primary reason cited is the dissipation of chlorine or other treatment chemicals. This question explores the scientific basis for this recommendation, considering factors like CO2 dissolution, chemical breakdown due to air and sunlight, and the overall impact on plant health.

Simple Answer

• Chlorine evaporates from the water
• Harmful chemicals break down
• Water becomes less harsh on roots
• Carbon dioxide dissolves, making water slightly acidic
• Algae and beneficial microbes can grow

Detailed Answer

The practice of letting tap water sit before watering plants stems from concerns about chemicals added during water treatment. Chlorine, a common disinfectant, is often used to kill harmful bacteria in municipal water supplies. While beneficial for human consumption, chlorine can be detrimental to plant health, particularly sensitive species. Letting tap water stand allows the chlorine to dissipate through evaporation. The amount of time required for complete dissipation depends on factors like temperature, surface area, and initial chlorine concentration. Warmer temperatures and wider containers facilitate faster evaporation. Some water treatment facilities use chloramine instead of chlorine. Chloramine is more stable and does not readily dissipate through evaporation. It is essential to check with your local water authority to determine which disinfectant is used in your water supply.

Beyond chlorine dissipation, other chemical processes occur when tap water is left standing. Some treatment chemicals may break down due to exposure to air and sunlight. For instance, volatile organic compounds (VOCs) can evaporate, reducing their potential harm to plants. The decomposition of these compounds can alter the water's chemical composition, potentially making it less toxic to plant roots. However, it is important to note that the extent of these chemical changes varies depending on the specific chemicals present in the water and environmental conditions. The process may also depend on the pH, with acidic conditions possibly catalysing some breakdown of chemicals. In addition, there can be a chemical reaction from the pipe used such as copper pipes having a different effect compared to PVC pipes.

Another factor to consider is the potential for increased CO2 dissolution. When tap water is exposed to air, it absorbs carbon dioxide from the atmosphere. This process can slightly acidify the water. Some plants prefer slightly acidic soil conditions, and using water with increased CO2 content may benefit their growth. However, the change in pH is typically minimal and may not significantly impact most plants. The pH of the soil is a key factor for plant growth and impacts the nutrient availability. Plants have a specific pH range that they thrive in, so understanding this is important before watering. Therefore, while this factor is relevant, it will likely play a minimal part in being healthier for watering plants. Some plants may also prefer alkaline water.

The sanitizing effect of tap water diminishes as it sits. While initially inhibiting microbial growth, the water gradually loses its disinfectant properties. This allows algae and other microorganisms to colonize the water. In some cases, the presence of beneficial microbes can improve water quality and promote plant growth. However, uncontrolled microbial growth can also lead to problems, such as foul odors or the spread of plant diseases. The growth of microbes can vary depending on nutrient availability, sunlight, and temperature. Therefore, one would need to be careful leaving the water for too long. It is important to use clean containers to prevent contamination and to monitor the water for any signs of spoilage.

In conclusion, letting tap water stand can offer some benefits for watering plants, primarily through chlorine dissipation and potential breakdown of other treatment chemicals. However, the extent of these benefits depends on the specific chemicals present in the water, environmental conditions, and plant species. In addition, there is a low risk of algae/bacterial growth in the water if left standing for too long. It is always recommended to consult with local water authorities to understand the composition of your tap water and to research the specific needs of your plants. The effect may be negligible for certain plants and can depend on the water chemistry. In the case that your plant is sensitive or have water with many additives, there is an option to use filtered water or buy water from the store.