27 studies, every one cited on the page where it is used. A houseplant on a windowsill is not growing. It is spending. The people who grow houseplants commercially publish the minimum light each species needs indoors. Horticultural science publishes how to convert those units into the ones plants are actually measured in. Nobody appears to have put the two together and printed the result. This book does — and the result is that four of the eight commonest houseplants in it are sold with a recommended minimum that sits at or below the level where they can pay their own running costs. A ZZ plant at its published floor is on about 0.22 moles of light per square metre per day. A lettuce is given 12 to 17. That figure is then confirmed from a completely different direction: a 2024 growth-chamber study measured three foliage species sustaining growth on exactly 0.22. Two methods, two continents, the same number. What is inside Why your eye cannot measure light, and what to use instead The four light units, and why one conversion factor is wrong for every lamp that is not the sun The published indoor minimum for eight common houseplants, converted — and which four fall below break-even How far from a window is too far, computed from the actual geometry rather than quoted What overwatering really means — a duration, not a quantity — and why four to eight hours of saturation is enough for root rot to establish The gravel-in-the-pot argument, and the 2025 experiment that overturned the correction everyone was given Why doubling a pot increases growth 43%, and why that does not simply mean use a bigger pot Salt: the dial nobody mentions, and why every threshold you read is meaningless without its extraction method Two printable worksheets: a light audit and a drying log What is honestly not in it A page listing what could not be supported — including misting, where no controlled trial was found either way, and this book says so rather than inventing one. Four further sources were located during the research and left out of the count because they could not be opened. 38 pages · 14 figures · 27 sources · PDF and EPUB. Instant download. The EPUB passes EpubCheck with zero errors and zero warnings.