How a tea plant loses water
An evergreen with a large standing leaf area transpires continuously through the stomata on the undersides of its leaves, and the rate depends on temperature, humidity, wind and light rather than on the plant alone. When water is short the plant closes its stomata to conserve it, which also shuts down the intake of carbon dioxide and therefore stops growth — the two cannot be separated, which is why a plant under water stress stops producing shoots regardless of how much sun it is getting. Humidity matters independently of soil moisture: a hot dry wind pulls water out of a leaf faster than roots can replace it even in moist ground, which is why wind exposure and shelter are agronomic variables in their own right. What follows is that the crop responds not to how much rain fell but to whether the plant could hold its stomata open, and a region with a high annual rainfall figure and a three-month dry season has a three-month problem.
What drought does, in order
The sequence is fairly consistent. First shoot extension slows and the plucking round lengthens. Then shoots begin to terminate in dormant buds rather than continuing to grow, and the intake fills with banjhi material that is tougher and less useful. Then mature leaves are shed, starting with the oldest, which reduces the plant’s photosynthetic capacity and therefore its ability to recover. In a severe or prolonged drought there is dieback of branches and, eventually, plant death, with young plantings and recently pruned bushes going first because both have small root systems or depleted reserves. Recovery is not immediate when rain returns, because the plant has to rebuild leaf area before it can rebuild shoots. That lag is why a drought damages the following season as well as the current one, and why yield statistics for a drought year understate the event. A grower loses the crop during the dry period and part of the crop after it.
Waterlogging, which is the opposite problem and worse
Tea roots need oxygen, and saturated soil has none. A field that sits wet develops root disease and dies back in patches, and unlike drought there is no recovery — the plants are dead rather than checked. This is why tea is planted on slopes rather than on the flat bottom land nearby that looks more fertile, why drainage is cut at establishment, and why a low corner of a field that floods will simply be a dead corner for the field’s whole life. It is also a reason the crop is absent from places that appear suitable on rainfall figures alone. The asymmetry is worth stating plainly because it runs against intuition. A tea plant will survive a bad dry season and carry a reduced crop. A tea plant in standing water for a fortnight in warm weather may not survive at all. Too much water in the wrong place is the more dangerous of the two failures.
Why most tea is not irrigated
Tea is grown predominantly in high-rainfall regions, on slopes, often at scale and often by smallholders, and each of those works against irrigation. Slopes are expensive to irrigate, high-rainfall regions have a dry period rather than a dry climate so infrastructure would sit idle most of the year, the crop’s value per hectare in commodity markets does not obviously repay the capital, and smallholders have neither capital nor, frequently, a water right. So the majority of the world’s tea is rain-fed, and where irrigation exists it tends to be on estates in the drier origins, on young plantings during establishment, or in nurseries — where it is universal, because unrooted cuttings cannot be left to chance. The exception that proves the pattern is the nursery. Everyone irrigates the stage where failure is total and immediate, and almost nobody irrigates the stage where failure is partial and delayed, which is a fair description of how capital gets allocated in agriculture generally.
What irrigation changes beyond water
Where irrigation is available it does more than carry a field through a dry spell. It widens the window in which a field can be pruned, because a bush cut back hard can be supported through recovery rather than depending on stored reserves and rain. It makes replanting less of a gamble, since a young field’s first dry season is the moment it is most likely to fail. It allows fertiliser to be applied and taken up on schedule rather than when rain permits. And in seasonal regions it can bring a crop forward at the front of the season, which is worth a great deal where an early premium exists. Each of those is a decision that becomes available rather than an automatic gain. The general point is that irrigation converts a set of weather-dependent operations into scheduled ones, and the value of that is mostly in reduced risk rather than in extra yield.
Water stress and the cup
There is a folk claim that a little water stress improves tea, and it is a partially reasonable one stated far too confidently. Slow growth is genuinely associated with more concentrated material, and mild water limitation slows growth, so the direction is plausible and consistent with what altitude and cool weather do. But drought as growers experience it is not mild, it produces banjhi shoots and coarse leaf, and the resulting material is worse rather than better. The honest position is that a narrow band of mild limitation may do what the claim says and that a dry season does the opposite, and the two are separated by an amount nobody is measuring in a commercial field. TeaHQ’s general position on this class of claim applies: the mechanism running from growing conditions to the cup is growth rate, that mechanism is real, and any specific claim about a specific field’s water status is an inference rather than an observation.