What the plant actually requires
The plant’s requirements are a short list and each item is a hard constraint. Distinctly acid soil, roughly in the range commonly cited as pH 4.5 to 5.5, and it performs badly above about 6 — the reverse of most crops, and not correctable by liming. Free drainage, because the roots will not tolerate standing water. Warmth, with growth effectively stopping in the low teens Celsius and heat becoming a limit at the other end. Moisture distributed through the growing season rather than delivered in one burst, and humidity, because dry air takes water out of a leaf faster than roots replace it. And, for the small-leaf form, tolerable winters — the northern limit of tea is effectively a small-leaf limit, since the large-leaf variety dies in a hard frost. Assembled, that list describes the humid subtropics and the tropical highlands, which is where nearly all tea is grown.
The soil constraint is the strictest one
Of the requirements, acidity is the least negotiable and the most geographically limiting. Naturally acid soils are the product of high rainfall leaching bases out of a profile over long periods, which is why the tea map overlaps so closely with regions of weathered, leached, high-rainfall ground. A grower elsewhere either finds a pocket of naturally acid soil or acidifies one, and holding a soil below its natural pH indefinitely means fighting the site permanently. This is one of the quiet reasons marginal tea industries stay small: the plant can be grown but the ground has to be maintained against its own tendency. It also explains why some places that look climatically plausible have no tea. A mild humid maritime climate over limestone is not a tea site regardless of how the temperature and rainfall figures read, and soil rather than weather is the first thing to check when someone proposes that a region could grow tea.
Season length, and the yield that follows from it
The second constraint is the length of the growing season, which sets how many harvests a year are possible and therefore what the land can produce. An equatorial field at altitude plucks all year. A field in Shizuoka takes a small number of flushes. A field at the cold margin may take one or two, and the plant spends most of the year dormant. Yield per hectare falls accordingly, and it falls a long way — which means a marginal-climate producer is not competing on volume with anyone and never will be. Everything about how such an industry can work follows from that. It has to sell at a price that a very low yield can support, which means selling on rarity, on place, on hand production, and to a domestic market willing to pay for a local product. That is a viable business and it is a different business from tea growing as practised in the producing countries.
The constraint that actually binds is labour
The plant will grow in more places than tea is grown. What stops an industry appearing is that tea is picked repeatedly through a season by hand or by machine, and that harvesting is the dominant cost in the crop. In a high-wage economy, hand plucking at any commercial scale is not affordable, and machine harvesting demands a field laid out for it — level, uniform, clonal, gentle enough to work across — which a small experimental planting on marginal ground rarely is. So the limiting factor for tea in Europe, North America or Oceania is not usually climate or soil but the cost of getting leaf off the bush. That is worth stating because it reframes the whole subject. Marginal-climate tea growing is not primarily an agronomic experiment about whether the plant survives; that question was answered long ago. It is an economic experiment about whether a crop with a very high labour requirement can be produced in a place where labour is expensive.
The places it happens anyway
Small industries and single estates exist well outside the belt. The Azores produce tea on São Miguel, on volcanic soil in a mild wet Atlantic climate, and have done for a long time. Cornwall has a commercial planting taking advantage of a mild maritime climate and acid ground. The south-eastern United States has a plantation in South Carolina and scattered smaller growers. New Zealand has a commercial planting using Taiwanese cultivars. Several other countries have single estates or research plantings. In every case the operation is small, the tea is expensive, and a substantial part of what is being sold is the fact of local production. There is no dishonesty in that, and it is worth being clear-eyed about what it is: these are not scaled-down versions of Assam or Shizuoka, they are a different kind of enterprise that happens to grow the same plant, and judging them by the standards of a producing-country tea misses what they are for.
What the margins are useful for
Two things, beyond the tea itself. First, cold-margin growing is where cold-hardiness gets tested for real: a plant that has survived several decades of winters in Georgia, Shandong or a northern Japanese prefecture is demonstrably adapted to that place in a way no catalogue description establishes, and breeders in cold origins regard surviving stands as material rather than as heritage. Second, marginal producers are unusually transparent, because they are small, they are selling on story, and they have nothing to gain by vagueness — several of them publish more about their cultivars, spacing and harvest dates than large producers do. For a reader the more useful takeaway may be the negative one. When somebody proposes that tea could be grown somewhere new, the questions that decide it are the soil’s pH, the length of the season and the cost of picking, in that order, and the last of the three is the one that has stopped almost every attempt.