From the perspective of your rubber plant (Ficus elastica), the main issue with tap water is not bacteria harmful to humans, but the chemicals used to eliminate them: primarily chlorine and, in many municipal systems, chloramines. These compounds are added as disinfectants. While they are safe for human consumption, they can accumulate in the soil of potted plants over time. For your rubber plant, this buildup can lead to a condition known as "tipturn," where the edges of the leaves turn brown and crispy. This happens because these chemicals can slightly damage the root hairs responsible for water and nutrient uptake and can cause minor chemical burns on the foliage as the plant transpires.
Another significant factor for plant health is water hardness. Tap water often contains dissolved minerals like calcium and magnesium salts. When you water your rubber plant with "hard" water, these minerals do not evaporate or get used by the plant. Instead, they remain in the soil, gradually increasing the soil's pH (making it more alkaline). Rubber plants prefer a slightly acidic to neutral pH. An alkaline soil environment can lock up essential nutrients like iron, manganese, and phosphorus, making them unavailable to the plant. This deficiency often manifests as chlorosis—yellowing leaves with green veins—stunting the plant's growth and diminishing its vibrant green appearance.
Plants in the Ficus genus, which includes your rubber plant, are notably sensitive to fluoride. This element can occur naturally in water sources but is also added to public water supplies in some regions. Similar to chlorine, fluoride can build up in the soil. Rubber plants absorbing excess fluoride will often show symptoms on their leaf tips and margins, which become brown and necrotic (dead tissue). This damage is irreversible and detracts from the plant's aesthetic value while also impairing its photosynthetic capacity.
While not a chemical factor, the temperature of tap water is a physical consideration from the plant's point of view. Water that is too cold, straight from the tap, can shock the delicate root system of a tropical plant like the Ficus elastica. This root stress can temporarily stun the roots, hindering their ability to absorb water and nutrients effectively and potentially leading to leaf drop—a common stress response in rubber plants. Consistently using very cold water can keep the plant in a state of mild stress, preventing it from thriving.
To avoid these potential issues, you can take simple steps to make tap water safer for your rubber plant. The easiest method is to fill a watering can and let it sit uncovered for 24 hours. This allows chlorine to evaporate (note: this does not remove chloramines). For dealing with chloramines, fluoride, and minerals, using filtered water, rainwater, or distilled water is a superior alternative. If using tap water is your only option, periodically leaching the soil by allowing a large amount of water to run slowly through the pot for several minutes can help flush out excess mineral salts. Always using water that is at room temperature will also prevent unnecessary root stress.
