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Eucommia ulmoides (Duzhong), with pungent and sweet flavors, has a neutral and warm nature, possesses descending properties, belongs to Yang, and is non-toxic. It enters the Kidney Meridian, tonifies the middle jiao, strengthens willpower, benefits the Kidney, and replenishes essence. It is particularly effective for lower back pain accompanied by inability to flex or extend, and can also treat dampness and itching of the feet and scrotum, as well as stop frequent urination and nocturnal emission. This herb can serve as the sovereign drug (Jun Yao) and is also adept at serving as an attendant drug (Chen Shi). However, it is somewhat drying; when used together with Prepared Rehmannia Root (Shudi), the dryness and dampness are balanced, naturally avoiding the worry of stirring up internal heat.

One might ask: The Kidney dislikes dryness, yet Eucommia is drying in nature; how can it enter the Kidney to strengthen the waist? I suggest adding Prepared Rehmannia Root, which is more suitable, but this seems to make Prepared Rehmannia Root nourish the Kidney, rather than Eucommia benefiting the Kidney directly. The response is: Tonifying the Kidney does not necessarily require Prepared Rehmannia Root. I use Prepared Rehmannia Root merely to achieve a synergistic effect. Although the Kidney dislikes dryness, if dampness invades the Kidney, the waist becomes heavy and unable to bend or straighten, indicating that the Kidney also dislikes dampness. Eucommia's drying nature dries out pathogenic water in the Kidney, rather than scorching the true Yin fluids of the Kidney. Removing Prepared Rehmannia Root does not hinder the treatment of dryness in the Kidney, and using Prepared Rehmannia Root does not obstruct dampness in the Kidney, because Eucommia can tonify the Kidney on its own, without relying on the assistance of Prepared Rehmannia Root.

One might ask: Eucommia is not a drying herb, yet you claim it is. What is your basis? I reply: Regarding the fibrous strands of Eucommia, it is indeed not a drying herb. However, the "dryness" of Eucommia lies precisely in the resistance of these strands to breaking. Things that are too rigid break easily, while things that are too soft do not break. The fibers of Eucommia break when stir-fried with fire; given their inherent softness, why single out its nature as drying? There is another meaning. Eucommia is damp when unprocessed and becomes dry after stir-frying. Those unbroken fibers, unless stir-fried until they no longer exist, must have been forced into dryness by the heat. Since the Kidney dislikes dryness, administering a drying herb often stirs up fire, which is why I teach people to use it with Prepared Rehmannia Root. As for dampness in the Kidney Meridian, it is not only suitable to be used with Prepared Rehmannia Root, but raw usage is best, and stir-frying should be avoided. If the Kidney already has dampness, adding Prepared Rehmannia Root increases moisture, which counteracts Eucommia. Once stir-fried, it loses its original nature, becoming purely tonifying without dispelling, so the damp pathogen cannot be scattered quickly. Why is Eucommia, which is damp when unroasted, suitable for treating dampness? This is because Eucommia contains both dryness and dampness; here, "damp" does not refer to water qi. Pathogenic dampness transforms upon contact with true water; using it raw preserves its true qi.

One might ask: Eucommia tonifies the Kidney, yet why did Zhang Zhongjing not include it in the Eight-Ingredient Pill with Rehmannia (Ba Wei Wan)? It is unknown whether Eucommia tonifies the fire in the Kidney but disturbs Kidney Qi. If Kidney Qi is disturbed, Cinnamon Twig (Guizhi) and Aconite Root (Fuzi) cannot remain stable in the Kidney palace, fearing the risk of floating away. Therefore, Cinnamon Twig and Aconite Root are used instead of Eucommia. Does this mean Eucommia is absolutely unusable? If there is dampness in the Kidney, it is appropriate to add Eucommia to the Eight-Ingredient Pill, taking advantage of its ability to move and scatter dampness. One should not stubbornly refuse to use it and discard it entirely.

One might ask: Eucommia tonifies the Kidney, yet people commonly use Psoralea Fruit (Pogu Zhi / Buguzhi) to assist it. Is this not too drying? Eucommia achieves great efficacy when combined with Psoralea Fruit. Ancients disliked its excessive dryness and added Walnut Kernel (Hetao Ren) to moisten it, likening the relationship to fish and water. In reality, when Eucommia is combined with Psoralea Fruit, there is no need for Walnut Kernel to moisten. Psoralea Fruit warms and tonifies the Fire of the Gate of Vitality (Ming Men), while Eucommia nourishes and benefits the Water of the Kidney. This creates the beauty of Water and Fire interacting harmoniously (Ji Ji); why is Walnut Kernel's moistening necessary? Although Eucommia benefits from the assistance of Walnut Kernel, it does not hinder its Kidney-tonifying function. However, Eucommia essentially does not rely on Walnut Kernel. Some say Walnut Kernel moistens the dryness of Psoralea Fruit. When Psoralea Fruit is used with other herbs, there is no seen need for Walnut Kernel's assistance. Why is Walnut Kernel added specifically when used with Eucommia? If it were not because of Eucommia, I would not believe it.