From the plant's perspective, the primary constituents in tap water that cause concern are chlorine and chloramines. Municipal water suppliers add these chemicals as disinfectants to make the water safe for human consumption. However, for your rubber plant (Ficus elastica), these additives can be problematic. Chlorine can slowly damage beneficial microbial life in the soil and, in high enough concentrations, can be absorbed by the roots, leading to leaf tip burn, marginal necrosis (browning edges), and yellowing leaves. Chloramines are even more stable and persistent, posing a similar but longer-lasting risk to the soil ecosystem.
Another significant factor for the plant is the mineral content of tap water, often referred to as "hardness." Water high in calcium, magnesium, and sodium leads to a gradual accumulation of soluble salts in the potting mix. As these salts build up, they alter the soil's chemistry, making it more difficult for the rubber plant's roots to absorb water effectively. This creates a state of physiological drought; the plant appears wilted even though the soil is moist. The visible signs of this stress are crispy, brown leaf edges and overall stunted growth. Furthermore, these minerals can be deposited on the leaves if you overhead water, leaving unsightly white spots.
Rubber plants prefer a slightly acidic to neutral soil pH. The mineral salts and chemicals in tap water can often be alkaline, causing the soil pH to slowly rise over time. An elevated pH level locks up certain essential nutrients, such as iron and manganese, making them unavailable for the plant to uptake. This frequently results in chlorosis, where the leaves develop yellowing between the veins while the veins themselves remain green. This is a clear sign from the plant that it cannot access the nutrients it needs to produce chlorophyll, directly impacting its health and vitality.
Fortunately, you can easily modify tap water to make it safer for your rubber plant, addressing its core needs. The simplest method is to let the water sit in an open container for at least 24 hours. This allows chlorine to evaporate (though it will not remove chloramines). For a more thorough approach, using filtered, distilled, or rainwater is ideal as these sources are largely free of both disinfectants and excess minerals. Additionally, a key practice is to periodically leach the soil. This involves slowly pouring a large volume of water through the potting mix and letting it drain completely, which helps flush out accumulated soluble salts and reset the soil environment for the plant's roots.
