If you are deadly serious about tea, you probably know about Tea: Cultivation to Consumption, edited by K.C. Willson and M.N Clifford and published by Chapman and Hall in 1991. This 769 page monolith, a compendium of scholarly articles about all aspects of tea, is far and away the most comprehensive text on tea I have ever seen. I’m going to paraphrase one of the articles, about solar radiation, in order to address some issues about harvesting that were raised in my last post. This is going to sound stupid, because I’m paraphrasing a book I don’t currently have at my disposal. I borrowed it once from a university library halfway across the state, but it’s proved elusive to purchase. If I get my hands on this book again, I’ll fill in the information missing here–like the author’s name.
Photosynthesis is how plants live. Light energy + water + carbon dioxide are converted into carbohydrates + oxygen. You can think of carbohydrates as calories, energy that the plant needs to grow and function. If the tea plant is getting enough of the above ingredients, it produces new shoots and leaves. If not, it stays dormant. (Man, I can relate.) The optimum growing temperature for the tea under study was 86°F. (I believe these characteristics were generalizable to all varietals of tea.) If the temperature dropped below 55°F, the growth rate of tea slowed exponentially. Similarly, growth dropped off precipitously when the temperature got above 95°F.
In my last post, I mentioned that lower winter temperatures in the Northern Hemisphere result when Earth’s axis is tilted away from the sun. In contrast, seasonal temperatures don’t vary as much in regions around the equator, as they’re always receiving relatively direct sunlight. As a result, there are tea-growing areas, notably in central Africa, where the climate spends little to no time out of the 55°F-95°F range, and tea can grow year-round. Traditionally, temperatures in Chinese tea-growing regions don’t stay above 55°F until early April, or qingming, at which point the first flush of new growth appears. Korea is 5° to 15° of latitude to the north, so temperatures surpass that base temperature of 55°F later in April.
So in the article I read, the author posited an equation that predicted the growth rate of tea given the average daily temperature. A universally accepted measurement of tea growth is the “shoot replacement cycle”: this is how long it takes a shoot of plucked tea, let’s say two leaves and a bud, to replace itself. First, the tea plant has to be getting enough water, or the equation wouldn’t hold true. The equation I had written is:
t (T-Tb) = ϑ
where t = time in days, T= mean temperature in °F, and Tb is the base temperature of 55°F, below which tea doesn’t do so great. The resulting product of ϑ is “thermal time,” expressed in day-degrees. I don’t know what the value of ϑ is, but I presume it’s a constant for all tea bushes. Let’s say the value is 300. In this case, if the average temperature is 85°F, 10 (85-55) = 300, so the shoot replacement cycle is 10 days. If the average temperature is only 75°F, 15 (75-55) = 300, and the shoot replacement cycle will take 15 days. If the average temperature is 55°F or below, no value of t is going to satisfy the equation, and you’ve got no shoot replacement. I like to think of thermal time as “man-hours” of labor the sun is putting in. Something’s definitely missing here, because I got the impression from the reading that growth slowed above 86°F, exponentially so above 95°F, and that isn’t reflected in the equation. You’ll just have to buy me the book to figure it out.