Distilled water is essentially pure \(\text{H}_2\text{O}\)—water that has had its impurities, minerals, and dissolved solids removed. While generally unnecessary for common houseplants, it is essential for specific, sensitive species. The decision to use it depends on your plant’s needs and the composition of your local tap water supply.
Understanding Water Purity
Distilled water is created through a process of boiling and condensation, which leaves nearly all total dissolved solids (TDS) and other non-volatile compounds behind. This results in water with a TDS measurement near zero, making it exceptionally pure. Tap water is a complex solution containing various minerals, salts, and chemical additives.
Tap water composition varies widely, but it commonly includes minerals like calcium and magnesium, salts, and sometimes trace amounts of heavy metals. These dissolved substances contribute to the water’s TDS, often ranging from 100 to over 400 parts per million (ppm).
The Specific Risks of Tap Water
The main concern with using tap water for plants is the accumulation of Total Dissolved Solids (TDS) in the soil over time. As water evaporates from the soil, the dissolved salts and minerals it carries are left behind, forming a white, crusty residue on the soil surface and the sides of pots. This mineral buildup, particularly calcium and magnesium from hard water, can create a salty environment that inhibits the plant’s ability to absorb water and nutrients.
Accumulated minerals also affect soil chemistry. High concentrations of calcium and magnesium can push the soil’s pH toward alkaline levels. Since most common houseplants prefer a slightly acidic soil pH, this alkalinity can “lock up” essential micronutrients like iron and manganese, making them unavailable to the plant’s roots, which leads to nutrient deficiencies and yellowing leaves. Municipal water also contains disinfectants like chlorine and chloramine, which can potentially harm beneficial soil microbes and damage sensitive root tissues.
When Distilled Water is Essential
For a few highly specialized plant groups, the use of water with a TDS near zero, such as distilled water, is essential. Carnivorous plants (including Venus flytraps, pitcher plants, and sundews) have evolved in naturally nutrient-poor, acidic environments like bogs. Their root systems are not adapted to filter out minerals and salts.
Even relatively low levels of dissolved solids, often exceeding 50 ppm, can become toxic to these plants, causing mineral buildup that clogs their delicate root systems. Tap water can cause progressive damage, leading to stunted growth, leaf browning, and eventual death. Certain sensitive tropical plants, such as specific orchids and delicate ferns, may also show signs of distress from high TDS water.
Addressing the Nutrient Trade-Off
Using distilled water resolves mineral toxicity and chemical buildup, but it introduces the absence of beneficial trace elements. Distilled water removes both harmful contaminants and trace amounts of micronutrients like calcium and magnesium. This lack of dissolved minerals means the water is effectively “dead” in terms of nutrition.
If you use distilled water exclusively, you must implement a rigorous and consistent fertilization schedule to prevent long-term nutrient deficiencies. The pure nature of the water gives you complete control over nutrient delivery, but you must ensure your fertilizer provides all the necessary macro- and micronutrients. For general houseplants, a practical compromise is to use tap water that has been left to sit for 24 hours to allow chlorine to dissipate, or to use filtered water that removes chlorine and sediment while retaining some beneficial minerals.