Detailed Explanation of Tea Tree Cultivation Techniques and Planting Management Methods
Temperature is a fundamental condition for the life activities of tea trees. It affects the geographical distribution of tea trees and also constrains their growth rate. The influence of temperature on tea trees is mainly manifested in two aspects: air temperature and soil temperature.
Air temperature mainly affects the growth of aboveground parts, while soil temperature mainly affects root growth. However, air temperature and soil temperature are interrelated. As far as air temperature is concerned, tea trees can adapt widely from tropical to temperate zones. But for growth, there are three cardinal temperatures, namely the starting temperature for tea tree growth, the optimum temperature, and the low-limit temperature.
(1) Starting temperature for growth
The average temperature that triggers tea tree bud break is called the starting temperature for growth, biologically known as the minimum temperature. For most tea tree cultivars, the daily average temperature needs to stabilize above 10°C for tea buds to begin sprouting. However, a few cultivars, or due to different ecological environments, have already begun sprouting at below 10°C. For example, Zhejiang's Biyun, Longjing 43, and Jiangxi Wuyuan Zaoya have a starting temperature for bud break of ≥6°C. These belong to the early-sprouting cultivars, and their harvest period can be earlier than other cultivars.
(2) Optimum temperature
After tea buds germinate, when the air temperature continues to rise to 14-16°C, the tea buds gradually unfold tender leaves. The optimum temperature for tea tree growth is between 20-30°C. Within this range, tea shoots accelerate their growth, with an average daily elongation of 1-2 cm or more. In most tea regions of China, from Qingming (early April) to before Shuangjiang (late October), the daily average temperature is between 20-30°C, which is precisely the optimum temperature period for tea tree growth and also the tea harvesting season.
The cumulative value of biological effective temperature (daily average temperature above 10°C) during the tea tree growing season is called effective accumulated temperature. The suitable effective accumulated temperature for tea tree growth is above 4000°C. The annual effective accumulated temperature in China's tea regions is generally between 4000-8000°C. The more effective accumulated temperature, the longer the annual growth period. Due to differences in climatic conditions, the growth periods of tea trees in different tea regions across northern and southern China vary. In most tea regions, the annual growth period of tea trees is about 8-9 months, while the harvestable period is 7-8 months.
(3) Low-limit temperature
In most mountainous areas of China, after December until February of the following year, the average temperature is generally below 10°C. Tea buds stop germinating and enter a winter dormancy state. Sometimes severe low-temperature frost occurs, and tea seedlings, young trees, or cultivars with poor cold resistance may suffer freeze damage. The absolute minimum temperature that tea trees can endure varies depending on cultivar, tree age, organ, cultivation management level, and growing season. For example, when the air temperature drops to -2°C, most tea flowers drop off and die; when the temperature drops to 1-2°C, germinated tea buds will also wither and scorch. However, tea tree branches and shoots have a stronger ability to endure low temperatures. Arbor-type large-leaf varieties can endure around -5°C; shrub-type medium- and small-leaf varieties can endure low temperatures even better, generally around -10°C. If covered by heavy snow, they can endure low-temperature attacks of around minus 15°C. Also, different cultivars of tea trees certainly have different cold resistance, but the same cultivar performs differently under different ecological conditions. For example, Zhenghe Dabaicha can endure -7°C in Fujian, while growing in the southern Anhui tea region it can endure -8 to -10°C. Generally speaking, when temperatures fall below the low-temperature limit that tea trees can "endure," freeze damage occurs. The degree of freeze damage to tea trees is not only directly related to the temperature level but also closely related to the duration of low temperature, wind speed, and freezing time. According to a survey by the Zhejiang Meteorological Bureau in Shengxian, Zhejiang, during the overwintering period of tea trees, when the air temperature drops to around -6°C, with continuous freezing for 6 days and a northwest wind speed of 6-8 m per second, the tender shoots of local tea tree cultivars will suffer freeze damage of varying degrees; when the minimum temperature drops to -8°C with continuous freezing for more than 12 days, it will cause even more severe freeze damage, killing tender tea shoots and turning old leaves yellow. Generally speaking, under certain low-temperature conditions, the longer the low temperature and soil freezing time, combined with dry northwest winds or sudden temperature drops after early spring warming, will all aggravate the degree of freeze damage.
While excessively low temperatures can cause freeze damage to tea trees, excessively high temperatures can also cause heat damage to tea trees, but such opportunities are rare. For example, when the daily average temperature reaches above 35°C, growth will be inhibited. When the daily extreme maximum temperature reaches 39°C, with relatively low rainfall, some tea tree canopy mature leaves show scorch damage and tender shoots wilt. This phenomenon is called tea tree heat damage. Usually, new shoots and tender leaves are more susceptible to this kind of stress damage than older branches.