Using pool water for irrigation is often considered by homeowners seeking water conservation, especially when draining a pool for maintenance. This treated water contains chemicals designed to keep it sanitized and balanced. Whether it is safe for plants depends entirely on the concentration and type of these dissolved chemicals, as high levels can be detrimental to garden health. Using pool water requires understanding its chemistry and how those components interact with plant biology and soil structure.
The Primary Culprit: Chlorine Toxicity
The most immediate threat in pool water comes from chlorine, present primarily as free chlorine (hypochlorous acid and the hypochlorite ion). Chlorine is an oxidizing agent that damages the cell membranes and internal structures of microorganisms, a process equally harmful to plant tissue. Applying pool water with high levels of free chlorine causes acute chemical burn in plants.
Visible symptoms of this exposure appear rapidly, often within a day or two, and include leaf tip burn and marginal necrosis, where the edges of the leaves turn brown and die. High concentrations can also cause significant damage to delicate root hairs, reducing the plant’s ability to absorb water and nutrients. The concentration of available chlorine is the determining factor, with levels generally maintained between 2 and 4 parts per million (ppm) in a well-kept swimming pool.
Chlorine that has reacted with organic materials, such as sweat and sunscreen, is known as combined chlorine or chloramines. While chloramines are less chemically reactive toward plant tissue, their presence indicates prior chemical activity in the water. For optimal plant health, the total chlorine (the sum of free and combined chlorine) should be as close to zero as possible.
The Hidden Danger: Salt and pH Imbalances
Beyond chlorine, the salt and pH levels in pool water pose long-term risks to both plants and the surrounding soil. Saltwater pools, which use a chlorine generator, introduce high levels of sodium chloride, and repeated irrigation with this water can lead to soil sodicity. Sodium ions can displace other beneficial ions in the soil, causing clay particles to disperse and leading to a breakdown of the soil structure.
This dispersion results in compacted soil that is less permeable, restricting water infiltration and reducing the air pockets necessary for healthy root growth. Furthermore, high concentrations of salt in the water create an osmotic stress condition for plants. The high ion concentration outside the roots makes it difficult for the plant to draw in water, essentially causing a physiological drought even when the soil is moist.
The pH of pool water is often maintained in a slightly alkaline range (above 7.0) to protect pool equipment and bathers, which impacts plant nutrition. Soil pH dictates the availability of various plant nutrients, and extreme values can make essential micronutrients inaccessible to the roots. A high pH can cause nutrients like iron, manganese, and phosphate to precipitate, meaning they convert to forms the plant cannot absorb, leading to deficiency symptoms.
Safe Watering Practices and Deactivation
Using pool water safely requires deactivating or allowing chemical components to dissipate before application. For chlorine, the most practical solution is allowing the water to stand in an open container, which permits volatile free chlorine to off-gas into the atmosphere. This natural process is hastened by aeration, sunlight, and warmer temperatures, with most free chlorine dissipating within 24 to 48 hours.
A more immediate deactivation method involves chemical neutralizers, such as sodium thiosulfate or ascorbic acid (Vitamin C), which instantly convert chlorine into harmless chloride ions. These compounds are commonly available and neutralize both free chlorine and the more stable chloramines. Before watering, testing the pool water with a standard kit is advisable, ensuring the free chlorine level is near zero (ideally below 1 ppm) and the pH is within the plant-friendly range of 6.0 to 7.0.
The high sodium content from saltwater pools cannot be removed through simple waiting or chemical neutralization. If this water is used, it must be significantly diluted with fresh water to reduce salinity, or applied sparingly to well-draining soil to prevent sodium accumulation. Applying water generously (leaching) is necessary to wash accumulated salts below the root zone, preventing long-term damage to the soil structure.