Temperature: a range, and a floor
Tea grows actively in roughly 18–30°C and slows sharply outside it. Growth largely stops below about 13°C, which is what produces winter dormancy in the cooler districts and what makes a first flush possible at all. The binding constraint is cold, not heat. Mature bushes tolerate brief light frost and many cultivars are selected for it, but a hard sustained freeze kills. That single fact sets the poleward edge of the map, and it is why the marginal growers — Georgia, the Black Sea coast, Cornwall, Washington State — are all in places moderated by water. At the equatorial end there is no dormancy at all, so plucking runs year-round. Kenyan tea has no flushes in the Darjeeling sense; it has a rainy season and a dry one.
Water: a lot of it, and spread out
Tea wants around 1,500 mm of rain a year as a working minimum and thrives on considerably more, and the distribution matters as much as the total. A long dry season forces irrigation or stops the crop. Most of the world's tea is rainfed rather than irrigated, which is why monsoon Asia and equatorial East Africa dominate the map and why weather in a given year moves both quality and supply. High humidity and frequent mist are separately useful: they reduce direct light and moderate temperature, which is part of why so many prized districts are cloudy ones.
Soil and slope
Acidic — roughly pH 4.5 to 5.5 — and free-draining. Tea is unusual among crops in wanting soil this acidic, and it will not tolerate waterlogging: the plant is a hillside species and roots rot in standing water. Slope is therefore a requirement rather than a picturesque accident. Terraced and contour-planted hillsides drain, and the same slope that drains is what makes mechanical harvesting difficult, which loops back into the labour question. Depth matters too. Tea roots deeply and a bush is a fifty-year investment or more, so shallow soil limits a field's working life rather than just its yield.
Labour, which is the constraint nobody lists
TeaHQ's own reading of well-attested general knowledge, with no single source behind it.
A tea field is picked repeatedly through the season — every seven to fourteen days in the growing period — and the standard picking unit is a bud and one or two leaves taken by hand. That is enormously labour-intensive, and it is why tea is grown where labour is available at a cost the crop can support. Where labour is expensive, the answer is machine harvesting, which works on flat or gently sloping ground and takes a coarser, less uniform leaf. Japan is the clearest case: a high-wage country that mechanised heavily and designs its fields around the machine. The map of world tea is therefore not only a climate map. It is a climate map intersected with a labour map, and the historical version of that intersection is what the plantation systems of the nineteenth century were built to supply.
Shade, and a common confusion
Tea is understorey in origin and tolerates shade well, and many gardens plant shade trees — silver oak in south India, various legumes elsewhere — for temperature moderation, wind protection and, where the tree fixes nitrogen, soil benefit. That is a different thing from the deliberate pre-harvest shading of Japanese tencha and gyokuro, which is a short, intense covering applied for a few weeks to change the leaf's composition. Both are called shading and only one of them is a processing decision.