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According to historical records, during the Nguyen Dynasty, Thanh tra grapefruit – a renowned specialty of Thua Thien Prefecture – was carefully selected annually to be offered to the King. Stemming from this tradition of tribute, the people of Thuy Bieu region and the local authorities have organized the annual Thanh tra grapefruit festival to honor this famous regional specialty. In 2018, the 6th Thanh tra grapefruit festival expanded to become a provincial festival of Thua Thien Hue, closely associated with the Thanh tra grapefruit tribute ceremony. Notably, starting in 2018, the Thanh tra grapefruit festival featured two new activities held for the first time: an introduction to the process of planting and cultivating Thanh tra grapefruit trees, and the Thanh tra grapefruit tribute ceremony (Thanh tra grapefruit tribute to the King). The "Thanh tra grapefruit tribute ceremony" takes place at the Luong Quan and Nguyet Bieu communal houses, and the "Thanh tra grapefruit tribute ritual" takes place at the Imperial City of Hue (Đại Nội Huế).  

Outwardly, the "Hue" Thanh tra grapefruit differs from other types of pomelos. It is smaller (averaging about 0.7-1 kg/fruit) and pear-shaped with a sunken stem end, while the Phuc Trach pomelos and Tan Trieu sugar orange-leaf pomelos are flat or slightly convex. The color and texture of the "Hue" Thanh tra grapefruit's skin when ripe are greenish-yellow, smooth and shiny, with a distinct light, natural fragrance. The segments of the "Hue" Thanh tra grapefruit are slender, straight, dry, and crisp, with a subtly sweet and slightly sour taste, and without any bitterness or astringency.
Regarding quality, "Hue" Thanh tra grapefruits have a total soluble solids content (Brix) of 9.43 - 11.23 %, a total sugar content of 7.80 - 9.34 %, a total acid content of 0.58 - 0.71 %, and a Vitamin C content of 59.29 - 66.20 mg/100g. 

The unique characteristics and reputation of "Hue" Thanh tra grapefruits are due to the natural geographical conditions and the local people's experience in cultivation. 

Thanh tra grapefruit growing region, in particular, is located in a tropical monsoon area. The high concentration of rainfall from September to November (averaging over 2,700mm) coincides with the Thanh Tra tree's greatest need for water, which contributes to high rates of flower bud differentiation and fruit set for "Hue" Thanh tra grapefruit. During the dry season, especially at the beginning, the relatively high air humidity is a distinctive feature, so supplemental irrigation does not require a large investment of time and effort.
The terrain of the Thanh tra grapefruit growing region in Hue is primarily flat or gently undulating along the Huong, Bo, O Lau, and Truoi river basins, creating ideal conditions for establishing concentrated and easily cultivated Thanh Tra growing areas.
The specific soil characteristics of the geographical area where Thanh tra grapefruits are grown are such that total nutrient levels in the soil range from average to poor, while the readily available nutrient levels are average to relatively high (readily available nutrients are the source of nourishment for Thanh tra grapefruit trees). The unique quality of "Hue" Thanh tra grapefruits is due to the combined effects of various soil factors, with phosphorus and potassium in the soil having a significant impact on the total sugar and vitamin C content, resulting in "Hue" Thanh tra grapefruit that is neither too sweet nor too sour, but has a distinctively delicate, sweet-tangy flavor. Furthermore, most of the soil used for Thanh tra grapefruit cultivation has an acidic to slightly acidic reaction, which is ideal for the trees' absorption of macronutrients and micronutrients from the soil. Notably, the relatively high level of micronutrient boron in the soil helps prevent fruit deformation, keeps the rind from becoming too thick, and contributes to a bright shine and high degree of uniformity.
The experience of cultivating "Hue" Thanh tra grapefruits involves locals selecting the best air-layered branches or grafted plants for propagation. The trees are shaped for the first four years, with branches and fruit thinned to facilitate care and improve fruit quality.
The Thanh tra grapefruit harvest season in Thua Thien Hue is distinct from other pomelo-growing regions. "Hue" Thanh tra grapefruits are harvested from August to September each year, earlier than Phuc Trach pomelos and much earlier than pomelo-growing areas in the North such as Doan Hung pomelos.
The combination of natural elements and production methods has shaped the Thanh tra grapefruit growing regions of Hue for hundreds of years, creating the unique characteristics of "Hue" Thanh tra grapefruit that other regions cannot replicate.
Geographical area includes communes, wards, and towns belonging to Thừa Thiên Huế province:
- Phong Mỹ, Phong Xuân, Phong Sơn, Phong An, Phong Hiền, Phong Thu, Phong Hòa, Phong Bình, Phong Chương, Phong Điền in Phong Điền District;
- Quảng Vinh, Quảng Phú, Quảng An in Quảng Điền District;
- Bình Tiến, Hương Bình, Hương Xuân, Hương Văn, Hương Vân, Tứ Hạ in Hương Trà Town;
- Dương Hòa in Hương Thủy Town;
- Lộc Bổn, Lộc An, Lộc Điền, Lộc Hòa, Xuân Lộc, Lộc Thủy, Lộc Tiến in Phú Lộc District;
- Hương Thọ, Thủy Bằng, Thủy Biều, Hương Long, Hương Hồ, Hương An in Huế City. 

Source: Bảo hộ chỉ dẫn địa lý “Huế” cho sản phẩm quả Thanh trà 
Translator: Nguyễn Hải Phong