What does "Only the scattered grass and trees fear the beauty's twilight" mean?

The meaning of the poem is: I think of the vanilla trees gradually falling away, and I am afraid that the beauty will slowly age.

This passage is the etymology of "beauty grows old". When you read this sentence, you may have a picture in your mind of a peerless beauty getting older day by day in the autumn wind, thinking about her past glory, feeling sad and unable to look back.

This poem comes from "Li Sao" written by Qu Yuan

The Miao descendant of Emperor Gaoyang came here, and my emperor Kao said Boyong. The photo of Ti Zhen was in Meng Zouxi, but Geng Yinwu came down. The emperor visited Kuiyu for the first time, and Zhaoxi gave Yu a good name: his name was Zhengzexi, and his courtesy name was Lingjun. Fenwu not only has this inner beauty, but also attaches great importance to cultivation. Hu Jiangli and Pi Zhixi were admired by Ren Qiulan. If Miyu is about to fall behind, I'm afraid I won't be able to catch up with him as time goes by. In the morning, the magnolias are blooming, and in the evening, the island is surrounded by wilderness. The sun and the moon are suddenly not flooded, and spring and autumn are the order of the times. But the vegetation is scattered, fearing the beauty's twilight. If you don’t strengthen your body and abandon your filth, why not change this attitude? Let's ride on the horse and gallop, come to me, Daofu, to lead the way!

Let me briefly translate it below:

I am a distant descendant of the ancient emperor Gaoyang, and Boyong is my late and respected father. When Tai Sui was in the Yin year and the Yin month, I was born on the Gengyin day. My father carefully guessed my birth date and gave me the corresponding reputation through divination. Name me Zhengze, and give me the nickname Lingjun. I have many inner beauties and excellent talents. I am wearing the fragrant Jiangli Angelica, and I am wearing strings of autumn orchids with rich fragrance. As time goes by, I'm afraid I won't be able to catch up. Time waits for no one, which is worrying. In the morning I climbed to the top of the mountain to pick magnolias. In the evening I picked magnolias and came to Shuizhou Riverside. Time does not stop for a moment, spring passes and autumn comes, and the four seasons are renewed alternately. Thinking of the vegetation falling and withering in the autumn wind, the worry of getting old arose sadly. If you don't take advantage of the right time to cleanse yourself of dirt, why don't you change your beauty-loving nature? Get on the horse and let go of your hooves. Come on, I am willing to be the pioneer.

Expand your literary knowledge about Qu Yuan and learn about him together:

A poet and politician from Chu State during the Warring States Period. The name is Ping, and the word is Yuan. It is also named Zhengze (going straight on the right path, strict with self-discipline), and the word Lingjun (endowment is good, fair and equal). He is our country's first great patriotic poet and one of our country's greatest romantic poets. He is the founder and representative author of Chu Ci style. He created the tradition of "vanilla beauty". His representative works include "Li Sao", "Nine Songs", "Tianwen", etc. Among them, "Li Sao" is the longest political lyric poem in ancient my country. He served as Zuotu and Sanlu official of Chu State. In domestic affairs, he supported King Huai of Chu, discussed state affairs and dealt with guests, drafted constitutions and reforms, and advocated "beautiful government": "sage kings and virtuous ministers" and "people-oriented". In diplomacy, he advocated "uniting Qi to resist Qin" and sent envoys to Qi twice. Together with Polish Copernicus, British Shakespeare, and Italian Dante, he is also known as one of the "four major cultural celebrities in the world."