From a plant's perspective, the primary concern with water is its chemical composition, specifically the concentration and type of dissolved minerals. Orchid roots, particularly those of epiphytic species, are highly specialized for absorbing moisture and nutrients from the air and rain in their natural canopy environment. They are not adapted to process high levels of dissolved salts (Total Dissolved Solids - TDS) commonly found in tap water. These salts can accumulate in the porous potting medium and on the velamen (the spongy outer layer of the roots), effectively creating a toxic barrier that prevents the root from absorbing water and nutrients, leading to dehydration and root burn.
Rainwater is, without question, the optimal water source for orchids. It closely mimics the water we would receive in our natural habitat. Its chemical profile is nearly perfect: it is soft, meaning it has a very low mineral content, and is slightly acidic. This slight acidity helps to unlock the micronutrients present in the potting medium, making them more bioavailable for my roots to absorb. Furthermore, rainwater contains dissolved atmospheric gases, such as nitrogen and oxygen, in beneficial amounts. Using rainwater allows for consistent watering and fertilizing without the dangerous risk of salt buildup that can compromise my entire root system and long-term health.
If rainwater is not available, water purified through reverse osmosis (RO) or distillation is an excellent substitute. These processes remove nearly all minerals, salts, and impurities, resulting in pure H₂O. This purity is a significant advantage as it provides a blank slate, preventing any harmful accumulation of minerals. However, a crucial point must be addressed: this water is *too* pure. It contains none of the beneficial trace elements found in rainwater. Therefore, if you use RO or distilled water, it is absolutely non-negotiable that you must use a fertilizer with every watering at a very weak strength. Watering with pure water without fertilizer will eventually leach all nutrients from my potting medium and my tissues, leading to severe nutrient deficiencies and decline.
Tap water is the most convenient but also the most problematic water source. Its quality varies drastically by location. The two main issues are its alkalinity (high pH) and hardness (high calcium and magnesium carbonate content). Watering with hard, alkaline tap water will rapidly increase the pH of my growing medium. In an alkaline environment, my roots cannot access key nutrients like iron, manganese, and phosphorus, even if they are present in the fertilizer. This leads to chlorosis (yellowing leaves) and stunted growth. Furthermore, the salts in hard water will accumulate on my roots and pot, causing the damage described earlier. If tap water is your only option, it is imperative you leach the pot thoroughly with plain water every fourth watering to flush out excess salts, and you may need to use a specialized acidic fertilizer to counteract the high pH.
