Working on a farm in France back in 2011 we painfully hauled gallons upon gallons of water to make sure the fledgling potato plants would survive the abnormally dry April. Waking up one morning it was pouring, and my first thought was, “Thank god we don’t have to do the potatoes today.” Being a longtime city girl that made me laugh, I think many of us, especially those that reside in an urban setting, view weather as a passing joy or inconvenience. It’s easy to forget that the amount of sunlight, temperature, and precipitation have a very real impact on the ecosystem. Each organism has certain temperature, water, and energy requirements to survive, above and below these thresholds an organism like a plant or animal cannot survive.
Climatology 101 tells us that the climate has two primary variables: temperature and precipitation. These two factors interact with the surface topography to create pressure differences, winds, storms, fronts, precipitation, humidity, etc. that create our daily weather, and the overarching long-term patterns we call “climate.” Since the climate is such a complicated and variable factor in tea growth, the next several blog posts will be divided into subcategories including temperature, precipitation, and the possible impacts of climate change.
So, without further ado: Temperature!
While this is gross over simplification, the main factor in determining temperature of a region and the seasonality is the amount of solar energy (insolation) that reaches the surface of the earth. Because the earth is a sphere the general trend is as follows: highest insolation at the equator, and thus generally has higher temperatures, while the closer you get to the Poles the less overall insolation reaches those areas, and is this colder.
The Darjeeling region is located right around 27 degrees latitude, which is approximately the same as to Tampa, Florida, and New Delhi, India. So while, by latitude the region should be quite warm, yet the 2000m altitude due to the Himalayas result is a cooler and wetter climate that those other locations. The mean annual temperature in Darjeeling is 13.5°C, with a mean monthly high in July of 17.8°C, and a low in 6.6°C in January. These averages give us a general feel for temperature ranges, the daily highs and lows will be much greater in actual range.
There is no perfect answer for the optimal growing temperature of a tea plant; this is partially due to the many variations of hybrid between the Assam tea plant and the China tea plants (see previous blog post). As Bhagat, Baruah, and Safique point out in their 2010 paper, “The tea plant grows in a variety of climates and has very wide ecological amplitude. It is therefore very difficult to specify the ideal climate tea requires for good growth especially with regards to most of the meteorological parameters.” However, very generally speaking the best growing conditions range between air temperature ranges between 13 and 30°C.
Soil and Leaf temperature
Not only does air temperature give a maker for the general growing conditions, a 1972 study by M.K.V. Carr looked in greater detail into the leaf temperature and soil temperature’s impact on tea plant growth. These factors are both influenced by the amount of sunlight, and water contained in the soil and air. Carr found that leaves only started growing after their surface reached a temperature of 21°C, however rates of photosynthesis dropped drastically after 35°C as the leaves became damaged by heat. The same study found that root growth only commenced after the soil had reached a temperature of 2O°C, and did not grow at all below 1O°C causing a period of dormancy.