Using tap water to hydrate plants is generally acceptable for most species, especially in the short term. Municipal water is safe for human consumption, meaning it meets standards that make it largely benign for plant life. However, this convenience comes with a potential trade-off. Contents added for public health or minerals naturally present in the source water can lead to chronic issues for plants over months or years. Understanding the specific components in your local water supply is the first step in maintaining long-term plant health.
Understanding Common Tap Water Additives
Municipal water systems often rely on chemical disinfectants like chlorine and chloramine to prevent the growth of harmful bacteria. Chlorine is a highly volatile gas added for its rapid ability to kill microorganisms. While effective for sanitation, consistent exposure to chlorine can cause cellular damage to delicate plant root tissues. Chlorine is relatively easy to manage because it is volatile and tends to dissipate into the air over a short period.
A more persistent issue arises from chloramine, a disinfectant created by bonding chlorine with ammonia. This chemical compound is far more stable than chlorine alone, allowing it to remain active throughout the entire water delivery system. Chloramine does not readily evaporate, posing a longer-term threat to beneficial microbial communities in the soil. These soil microbes facilitate nutrient absorption by the plant roots. Repeatedly watering with chloramine-treated water can significantly inhibit these helpful populations, compromising the plant’s ability to take up nutrients.
The Impact of Water Hardness and Dissolved Minerals
Separate from chemical disinfectants, tap water often contains non-volatile dissolved solids, which primarily contribute to water hardness. This hardness is mainly attributed to high concentrations of positively charged ions, predominantly calcium and magnesium. These minerals do not evaporate with the water, meaning they accumulate in the soil with every watering cycle. Over time, this continuous buildup of mineral salts forms a visible white crust on the soil surface and the rims of the pots.
This salt accumulation, often measured as high salinity, can prevent the efficient absorption of water and nutrients by the plant roots. High concentrations of calcium and magnesium can also induce nutrient locking, which inhibits the uptake of other important elements like iron and potassium, leading to nutritional deficiencies. The presence of these alkaline mineral carbonates also gradually increases the soil’s pH level. Most plants prefer a slightly acidic to neutral soil pH for optimal growth, and when the soil becomes too alkaline, the availability of certain micronutrients becomes limited.
Simple Methods for Treating Tap Water Before Use
Mitigating the effects of tap water on plants does not require elaborate or costly filtration systems. For addressing volatile chlorine, the simplest technique is to draw the water into an open container, like a watering can, and let it sit uncovered for at least 24 hours before use. This exposure allows the chlorine gas to naturally off-gas or evaporate from the solution.
However, this passive method is ineffective against the more stable chloramine and persistent dissolved solids. To neutralize chloramine, a low-cost option is to use an activated carbon filter, such as those found in inexpensive pitcher filters. These carbon blocks remove both chlorine and chloramine compounds from the water. Another fast-acting option is the use of chemical neutralizers, such as powdered ascorbic acid (Vitamin C), which instantly breaks the bonds of both chlorine and chloramine.
For addressing the long-term mineral salt and chemical buildup in the soil itself, periodic flushing is recommended. This involves slowly pouring a volume of clean, treated water through the pot that is equivalent to about three to four times the volume of the container, allowing the excess salts to be washed out through the drainage holes.
Plants That Require Purified Water
While most common garden species tolerate standard tap water, a few groups of plants exhibit a distinct sensitivity to its chemicals and minerals. Certain tropical houseplants are particularly susceptible to fluoride toxicity, which is a concern because fluoride is a persistent chemical that does not dissipate like chlorine. Plants like Dracaena species, Prayer Plants (Maranta), and Calathea varieties are known to react poorly to this additive.
The tell-tale sign of this chemical sensitivity is the development of brown, scorched tips or edges on the leaves. This occurs because the plant attempts to move the accumulated toxins to the furthest point of the leaf margin. For these sensitive species, tap water is unsuitable, and growers should instead use distilled, reverse osmosis, or collected rainwater to prevent damage.
Other highly sensitive groups include carnivorous plants, such as Venus flytraps and sundews, which have evolved in nutrient-poor environments. They cannot handle the dissolved solids found in tap water. For these plants, the only acceptable watering source is pure water, as the mineral content can chemically burn their delicate root systems.