Stall Shading and Wilting: How Much Selling Time Shade Buys for Leafy Produce, and When It Pays

Leafy produce displayed at an open market loses water continuously, and once cumulative loss passes a visible wilting point it becomes difficult or impossible to sell at full price. This paper models that process for a single stated case — a 200 g lettuce head, with unsaleability set at a loss of 5.0% of fresh mass — under open sun and under shade cloth of three transmissions. Water loss is driven by the vapour pressure difference between the leaf and the air, with leaf temperature rising with the fraction of sun that reaches it. On a typical day defined here as 30°C air temperature and 45% relative humidity, the modelled time to wilting is 2.756 hours in open sun, 3.287 hours under cloth transmitting 0.7 of the sun, 3.727 hours under cloth transmitting 0.5, and 4.259 hours under cloth transmitting 0.3. Medium shade, at transmission 0.5, therefore buys 0.971 hours, a gain of 35.2%. On a hot day at 35°C and 35% relative humidity the times are shorter for every treatment and the absolute gain from medium shade falls to 0.568 hours, even though wilting arrives sooner and matters more. On a mild day at 25°C and 60% relative humidity the open-sun time is already 4.287 hours and medium shade extends it to 6.275 hours. An economic calculation for a 6.0-hour market, 20 kg of stock priced at $6.00 per kilogram, and shade costing $0.55 per day shows shade paying by a wide margin whenever the market lasts longer than the open-sun wilting time. The break-even stock value for medium shade is only $3.40 on the typical day and $5.81 on the hot day. The originated rule is therefore conditional, not universal: first compare market length with open-sun wilting time; if the market ends before that time, shade buys no saleable produce in this model, and if it runs beyond it, even a small leafy stock justifies medium shade, with heavier shade reserved for long markets on hot days where the extra hour is actually used.