Hana wo Rōsureba Kaori Koromo ni Mitsu.
It means that when you go to pick flowers in a green field in spring, your kimono or clothes have sweet scents from the flowers.
The guests can feel a spring atmosphere, which gives us the love of sweet and cute flowers, in the tea room by seeing these words.
It makes you warm and comfortable.
Originally, this poem was made by a Poet, 干良史, from the Tang dynasty, the title is "春山夜月" :spring mountain and moon light, later it was used in a proposal for the training of Zen by Kidō Chigu（虚堂智愚） who is the founder of Rinzai Zen Buddhism（臨済宗）in Southern Song, China.
In fact before this phrase in the poem, there is the following phrase.
Mizu wo Kikusureba Tsuki Te in Ari.
The moon which you can't always catch with your hands because it is in the sky high and far away, but if you scoop the water by hand, the moon is reflected in the water in your hands.
It means that there is a consequence if there is a cause, so you can't get any results just in inaction.
Then, unless you pick flowers and scoop water by hand, the fragrance of flowers doesn't move to you, and the moon can't also be seen in your hands.
Beginning something is very important and useful.