Orchids

Most houseplant orchids (Phalaenopsis, Dendrobium, Oncidium) are epiphytes: they grow on tree branches in tropical forests, absorbing water and nutrients from rain, humidity, and decomposing organic matter. Their roots are exposed to the air, covered in a spongy layer called velamen that soaks up moisture and whatever is dissolved in it. In nature, that means rainwater with almost no mineral content. In your kitchen, it means whatever is in the tap.

The tolerance range for orchids is wider than for carnivorous plants, but the principle is the same: minerals accumulate. Epiphytic orchid roots sit in bark, sphagnum, or mounted on cork with no soil to buffer mineral buildup. Each watering deposits a thin layer of salts. Over weeks and months, these deposits appear as white crust on the pot rim and growing medium, and the roots begin to brown and desiccate from salt burn. The St. Augustine Orchid Society and most orchid growers recommend keeping the total dissolved solids of your watering solution (water plus fertiliser) under 175 ppm. That means your base water needs to be well under 100 ppm to leave room for the nutrients you are deliberately adding.

This profile targets a TDS of around 62 ppm: low enough to serve as a clean starting point for fertilisation, mineralised enough that you are not stripping the medium of all buffering capacity. Calcium at 15 ppm and magnesium at 4 ppm provide the base macronutrients that orchid fertilisers often supplement. Phalaenopsis are the most commonly grown and the most tolerant of this group. Dendrobium and Oncidium are slightly more sensitive to salt accumulation. If you are growing mounted orchids or watering by dunking, salt buildup happens faster because there is less flushing, so starting with cleaner water matters more.