It's a plum tree!

(Caution: spoiler)

About the symbolism of the trees
Only two species of trees have a symbolic value in The Double that are fundamental in complementing the plot's narrative: the pear tree (white flower) and the plum tree (pink flower), both with strong symbolism in Chinese tradition and culture. The pear tree is a symbol of immortality and its fruit symbolizes love and marital fidelity. The plum tree is a symbol of resilience and perseverance in the face of adversity.

The diferences between plum tree and Wintersweet
There is no way to confuse the plum tree with Wintersweet, since the plum tree has pink flowers and Wintersweet has yellow flowers. The plum tree, when it's adult (in up to 5 years, when it starts to bloom) consists of a trunk, and the Wintersweet, which in addition to being slow growing and just a shrub, has invariably yellow flowers (only blooming after 10 to 15 years, from the seed planted, when it reaches maturity).

Confusion in interpretation
There is a confusion about the tree that appears in the final scene of the series (episode 40) due to the misinterpretation of dialogue between XFF and DS regarding his return from the war. A few moments before this scene, XFF appears running in slow motion to the door and she watches the snow falling with a red dress. She appears with her usual hairstyle. Soon after, the scene is cut and we see her at the top of the mountain and we assume that a few years have passed because in this scene she wears a different hairstyle, marking the changes over time, and we also see an adult tree. In the sequence that follows, XFF remembers when she asked DS, at the moment they were planting the small plum tree (with the inscription “planted in March”): “When will you come back?” and DS answers, "The day Wintersweet  blooms will be the day I return in triumph and safety." There are those who read/hear this and interpret it literally as if the tree that appears immediately after this dialogue is a Wintersweet, but it is definitely not a Wintersweet. DS wasn't referring to the little plum tree they just planted, but for the optimistic prediction of the following spring, therefore one year later from that moment, that is, from March when he and XFF planted the small plum tree, to March of the following year, during the blooming of Wintersweet. In other words, DS hoped to return the following year, not when the plum tree was full grown and blooming, as that would take years, but the following spring when 'any' Wintersweet would bloom again.

The plum tree as a way of demarcating time
The time that has passed shows that his prediction was an illusion, as the size of the plum tree indicates that the time was much longer than he predicted, as it was already mature (it begins to bloom in March, after a period of 3 to 5 years. In the scene, the plum tree was at least 3 years old and was probably blooming for the first time). Wintersweet blooms between February and March and it takes 15 years to grow from seed to adult.

The screenwriter's intention
The screentwriter, to eliminate any doubt, let the image speak and communicate for itself, because if DS says he'll return when Wintersweet blooms, what we, watchers, actually see is a plum tree and that the war lasted so many years that the small plum bush that they planted is now adult and blooming. The scene clearly shows a plum tree, with its unique pink flowers, and not a Wintersweet that only exists with yellow flowers. They are traditional trees well known in China (and also in Japan), the objective is precisely to present the plum tree as an adult tree in order to demarcate the long waiting time. 

About XFF's values

XFF's attitude in the scene is almost hopeless, she wears red, a symbol of happiness and prosperity for the bride and groom,  and a white stole (she also appears with SYR with the same outfit). White, in Chinese culture, symbolizes purity and also mourning, which demonstrate the emotional state of the XFF, which remains pure, as she remains faithful to her marriage, and melancholy, due to the enormous gap of time without knowing whether DS is alive or not, but she remains resilient and persevering (plum tree attributes). The scene presents XFF's long wait for DS through the age of the plum tree, a symbol, at that moment, of the almost disbelief that she feels for his return.

About XFF's state of mind

This atmosphere in which she's immersed explains the expression on her face and her gaze when she hears the gallop of a distant knight, she still seems absorbed and immersed in that state of mind, almost resigned to a status quo, fatigued by the long wait and perhaps without greater expectations. We imagine that she always goes to that place where she and DS last performed an act together, it's as if the plum tree keeps his presence there. So she just looks back, still anesthetized, she doesn't immediately recognize that it's DS returning, because he's very far away, only we, watchers, know that the knight is DS, not only because of the pendant that he brings back and is the main symbol of love between them, but also because we recognize his figure, his red clothing that he wears not only for the position he occupies as Duke but also to evoke the traditional color of the bride and groom.

The challange called The Double

The Double is incredibly subtle, the non-explicit narrative is also hermetic, it depends on our attention and patience to unpack it, to extract its perfume.  

I hope I have clarified the plum tree dilemma...

The importance of the plum tree for China

The national flower of the Republic of China was officially designated as the plum blossom by the Executive Yuan of the Republic of China on July 21, 1964. The plum tree can grow for a long time, ancient trees are found throughout China. Huangmei county in Hubei features a 1,600-year-old plum tree from the Jin Dynasty which is still flowering.

Long live plum trees!

(The links below contain all information about the plum tree and Wintersweet)

Plum tree
https://en.m.wikipedia.org/wiki/National_flower_of_the_Republic_of_China

https://www.google.com/search?client=ms-android-samsung-ss&sca_esv=abed81e574b41454&q=plum+tree+blossom+china&udm=2&fbs=AEQNm0AuaLfhdrtx2b9ODfK0pnmi046uB92frSWoVskpBryHTtShVNbk-60xlcGTvYzJ-DKkEMNxV-kfsznyyrX_HVTEig8oh2BhtImtFIiy9jncpIj1ruaX04G2m7JJ8mRigKj2clzPsP9n4KzRmddjrZsHW8PWDbKrAwwzoRqYr43egDcSt-X7yIJMWe4NKS_9xIFAiKNEGrwXaPOco6LV603oVFhlEEkss5bfA_DA9Ix4L0Kax-A&sa=X&ved=2ahUKEwjGs7rb_auIAxWVlZUCHRVgIp8QtKgLegQICRAB&biw=384&bih=729&dpr=2.81

Wintersweet
https://www.thespruce.com/how-to-grow-wintesweet-5091598

http://env.people.com.cn/BIG5/n1/2024/0103/c1010-40151463.html

https://en.m.wikipedia.org/wiki/Chimonanthus_praecox

https://www.google.com/search?q=How+long+does+wintersweet+grow%3F&oq=How+long+does+wintersweet+grow%3F&gs_lcrp=EgZjaHJvbWUyBggAEEUYOTIHCAEQIRigATIHCAIQIRigATIHCAMQIRigAdIBBzkxM2owajeoAhSwAgE&client=ms-android-samsung-ss&sourceid=chrome-mobile&ie=UTF-8

https://www.gardenersworld.com/how-to/grow-plants/how-to-grow-chimonanthus-wintersweet/