Reverse osmosis (RO) water is created by forcing water through a fine, semipermeable membrane, effectively filtering out nearly all impurities and contaminants. This process results in highly purified water with an extremely low concentration of Total Dissolved Solids (TDS). TDS includes inorganic salts, minerals, and metals present in the water supply. While pure water seems ideal for plants, this high purity presents a challenge. Plants require more than just H₂O, and the lack of dissolved substances means RO water can be either highly beneficial or potentially detrimental, depending on its application.
The Unique Composition of RO Water
The goal of reverse osmosis filtration is to remove contaminants like lead, arsenic, fluoride, and chemical pollutants from the water supply. In doing so, the process also removes virtually all naturally occurring dissolved solids, including the minerals that plants use as micronutrients. The resulting output is water with a TDS measurement typically ranging between 5 and 20 parts per million (ppm), making it essentially “empty” water.
Two of the most significant minerals removed are calcium and magnesium, which are necessary for cell wall structure and chlorophyll production, respectively. When water is this pure, it lacks the buffering capacity found in municipal tap water, meaning its pH can fluctuate easily and may often be slightly acidic. The fundamental issue for plants is that this ultra-pure water provides no foundational mineral nutrition, forcing plants to rely solely on the soil or added fertilizer for all their elemental needs.
Using water with an extremely low concentration of dissolved solids also impacts the plant’s natural water uptake mechanism, which relies on osmosis. Plant roots naturally maintain a higher concentration of solutes than the surrounding soil water, creating a gradient that draws water into the root cells. If the water concentration in the soil solution is too low, the plant must work harder to maintain this internal gradient. The primary concern remains the complete absence of dissolved nutrients; if the soil is depleted, RO water will not replenish any minerals, which can lead to deficiencies over time.
Supplementing RO Water for General Plant Health
For the majority of houseplants and garden varieties, RO water is only suitable if it is actively supplemented with nutrients. Since the water itself provides no minerals, gardeners must treat it as a blank slate and provide a complete nutritional profile. This means that a comprehensive fertilizer containing all macro and micronutrients is necessary at every watering to ensure sustained health and growth.
One of the most effective methods to make RO water suitable for general use is to reintroduce a controlled amount of minerals. This process, known as remineralization, is often accomplished by adding a specific calcium and magnesium supplement to the water before fertilization. Some specialized cartridges can be added to the RO system to automatically dose the water, typically aiming for a TDS level of 15–30 ppm of these hardness minerals.
Alternatively, a simple and practical approach is to blend the RO water with regular tap water at a specific ratio. If the tap water is not excessively hard, a 1:1 mixture can often raise the TDS to an acceptable level while diluting unwanted contaminants like chlorine or excessive salts. Furthermore, because RO water lacks the bicarbonates that stabilize pH, the addition of a remineralizing agent helps create a buffered solution. This buffering capacity prevents the water’s pH from swinging too low, which can interfere with a plant’s ability to absorb nutrients from the soil.
Specialized Plants That Require Low-TDS Water
While most plants require supplementation, there are specific groups for which the low TDS of RO water is not just beneficial, but often a necessity. Carnivorous plants, such as Venus flytraps, sundews, and pitcher plants, have adapted to nutrient-poor environments and are highly sensitive to mineral salts. Their roots are easily damaged by the accumulation of dissolved solids from tap water, which can lead to root burn and eventual death.
For these sensitive species, RO water is the preferred irrigation method, often used without any mineral supplementation. High TDS water, particularly above 50 ppm, can cause osmotic stress in these plants, making it difficult for the roots to absorb water and nutrients. Certain delicate ornamentals, including many orchid varieties and some ferns, also show a preference for water with a lower mineral content, as they are prone to leaf tip burn from salt buildup over time.
When using RO water for these specialized plants, the objective is to prevent mineral accumulation in the growing medium. The low TDS ensures that no damaging salts or excess nutrients are deposited in the soil, which is a common problem with evaporation when using high TDS tap water. For these unique plants, the absence of dissolved solids in RO water provides an environment that closely mimics their natural, mineral-sparse habitats.